DICTIONARY OF PHOTOGRAPHY. 
 
 
 By THOMAS SUTTON, B.A. 
 
LONDON : 
 
 WILLIAM STEVENS, PRINTER, 37 , BELL YARD, 
 TEMPLE BAR. 
 
PREFACE. 
 
 This Dictionary of Photography contains a minute account 
 of the principal photographic processes now in use, and a 
 description of the various substances employed by the 
 photographer, together with an explanation of optical terms, 
 the theory of lenses, rules of perspective, &c. No account, 
 however, has been given of such common forms of apparatus 
 as may be seen at every photographic depot in the king- 
 dom, as this would have occupied space unprofitably. My 
 object has been to place in the hands of the practical 
 photographer a useful book, which will assist him in the 
 endeavour to comprehend the optical and chemical prin- 
 ciples of his art, and save him the trouble of referring to 
 the numerous bulky and costly works which I have been 
 myself obliged to consult. In my description of the several 
 Processes I have given those formulae which, while they 
 produce the best results, involve the simplest manipulation 
 and fewest materials, rejecting all such modifications as 
 appear to be unsupported by sound reasoning and conclusive 
 experiments ; in the articles which treat of Optics I have 
 endeavoured to explain clearly the principles of perspective, 
 
VI 
 
 PREFACE. 
 
 and the theories of achromatism, aberration, the production 
 of images, and construction of lenses ; — and in the Chemical 
 articles, avoiding what is hypothetical and uncertain, I 
 have confined myself to the description of the known pro- 
 perties and photographic uses of the various substances 
 treated of. In short my aim has been not so much to 
 produce a work abounding with novelties, hypotheses, and 
 suggestions, as one containing a plain statement of ascer- 
 tained facts, and which may be relied on for accuracy. 
 
 The works which I have chiefly consulted are Dr. Ure's 
 “ Dictionary of Arts, Manufactures, and Mines," the che- 
 mical treatises of Gmelin, Liebig, Thenard, Brande, Faraday, 
 Fownes, Pereira, Redwood, and Griffin ; and the optical 
 works and tutor's manuscripts which form part of the 
 Cambridge course of mathematics. To Brande's admirable 
 “ Manual of Chemistry" I am perhaps more indebted than 
 to any of the other works mentioned, and I make this 
 acknowledgment with pleasure as I received my first lessons 
 in chemistry from that gentleman, at the Royal Institution, 
 some twenty years ago. I would also acknowledge in this 
 place the assistance I have received from the valuable papers 
 on Photographic Chemistry communicated at different 
 times by Mr. Hardwich to the Photographic Society, and 
 , by Messrs. Davanne and Girard to the French Photographic 
 Society. The opinions expressed in the present work with 
 respect to the organic nature of the photographic image in 
 all the negative processes, and the positive processes upon 
 paper, are, I believe, fully supported by the experiments of 
 
PREFACE. 
 
 Vll 
 
 the above named gentlemen, and may be considered as no 
 longer hypothetical, but certainly true. 
 
 I have also to acknowledge the valuable assistance I have 
 received from the gentleman whose name is associated with 
 my own on the Title page, and who is the author of all the 
 chemical articles of the letters A, B, C. 
 
 This is, I believe, the first Photographic Dictionary that 
 has been published in Europe. A similar work was is- 
 sued some years ago in America, but I have not been 
 able to procure a copy of it. 
 
 THOMAS SUTTON. 
 
 St. Brelade, Jersey, 
 August 17 th , 1858 . 
 

 
A 
 
 PHOTOGRAPHIC DICTIONARY. 
 
 A. The termination a, of chemical names, signifies an oxide. 
 Thus, Soda is an oxide of Sodium, Ammonia of Ammonium, and 
 Baryta of Barium. Some oxides, as those of silver, gold, &c., do 
 not take this terminal, a defect which leads to misapprehension. 
 The Nitrate of Silver is the Nitrate of the Oxide of Silver, which 
 would be obvious if it were called Nitrate of Argenta, just as the 
 similar salt of Potassium is called Nitrate of Potassa. 
 
 Aberration. This is a term much used in Optics. When a 
 pencil of light is refracted through a lens, or reflected from a mirror, 
 it happens in certain cases that the directions of the refracted or re- 
 flected rays do not all pass accurately through a focus. This error 
 is called “ aberration,” a term which must not be confounded with 
 “ deviation,” q. v. 
 
 There are many kinds of aberration, but those with which the 
 photographer is most concerned are called “ spherical aberration,” 
 and “ chromatic aberration.” 
 
 I. Spherical Aberration . — This is a term exclusively employed to 
 denote the aberration produced by the reflection or refraction of a 
 pencil at a spherical surface. Thus, in Pig. 1. — 
 
 B 
 
2 
 
 ABE 
 
 Let A B be a single convex lens, and Q A B a pencil of liglit 
 incident upon it, proceeding from a luminous point Q. The pencil, 
 after refraction through the lens, will not form a cone of light in 
 which all the rays come to a common focus, but an effect will take 
 place which it is important clearly to understand. In the first 
 place, the emergent pencil is symmetrical with respect to an axis E S, 
 which axis produced passes through the centre 0 of the posterior 
 spherical surface A S B. The refracted rays which emerge from the 
 immediate neighbourhood of the point S form a small pencil, which 
 may be considered as having a focus F, called the “geometrical 
 focus.” The outer rays of the emergent pencil will cut the line S F 
 at points c, d , b , as shown in the figure ; the distances F c, F d, F b, 
 increasing as the distance from S of the point of emergence of a ray 
 increases. F b is therefore called the aberration of the ray Q, B, F d 
 of the ray Q, D, F c of the ray Q C, and so on. 
 
 When F S is large compared with S A, the aberration F d is pro- 
 portional to the square of S A. 
 
 It is impossible to construct a single lens with spherical surfaces, 
 so that the pencils shall be entirely free from aberration ; but by 
 combining two or more lenses, made of different kinds of glass, 
 or of the same kind of glass, spherical aberration may be to a great 
 extent, although not entirely, corrected. Such compound lenses, or 
 combinations, are said to be “ aplanatic,” q. v. 
 
 There are certain forms of reflecting and refracting surfaces and 
 lenses in which a particular pencil is reflected or refracted without 
 aberration. The only case with which the photographer is concerned 
 is that of the parabolic mirror. All rays which proceed from a 
 luminous point in the focus of the mirror are reflected in directions 
 parallel to its axis. 
 
 The methods usually employed for correcting spherical aberration 
 in photographic lenses are described in the article “ Lens.” 
 
 It is a common error to confound spherical aberration with curva- 
 ture of the image. Curvature of the image may exist where there is 
 no spherical aberration, and vice versa. See “ Lens.” 
 
 The nearest approach to a correct focus which can be obtained 
 with a lens, when a large aperture is employed, is called the “ least 
 circle of aberration.” 
 
 It will be seen in Fig. 2, that if the various rays of a refracted 
 pencil are produced through the axis, there will be a certain posi- 
 tion, m, of a circular area through which they all pass, in which the 
 diameter of that circle will be the least possible. If F a is the 
 aberration of the pencil, the distance of this least circle of aberration 
 from F is three-fourths of F a ; and if the aperture of the lens A B 
 
ACC 
 
 3 
 
 is small compared with its focal length, C E, the diameter of the 
 least circle of aberration is proportional to the cube of the diameter 
 of the part of the lens employed. 
 
 II. Chromatic Aberration . — The light which proceeds from the 
 sun, and most luminous bodies, is found to be heterogeneous, that 
 is, composed of different kinds of light, of different degrees of re- 
 frangibility. If a ray of such light is refracted through a prism or 
 lens, it will be decomposed into its constituent rays ; and if it be a 
 direct pencil passing through a lens, there will be formed a system 
 of emergent pencils, corresponding to the different kinds of light 
 of which the incident pencil is composed. Sunshine is found to 
 be composed of light of seven different colours, viz., red, orange, 
 yellow, green, blue, indigo, violet, arranged in the order of their 
 refrangibility, red being the least, and violet the most refrangible. 
 When, therefore, a pencil of sunshine is refracted through a convex 
 lens, the foci of the coloured pencils are arranged along the axis 
 of the lens in the order of refrangibility. If we designate these 
 foci by the letters r, o , y, g , b , i, v, (r being farthest from, and v 
 nearest to, the lens), the distance between r and v is called the 
 “ chromatic aberration ’ 5 of the pencil. 
 
 It is found that, by combining n lenses made of n different 
 kinds of glass, according to a certain formula, n different coloured 
 foci may be united in the same point on the axis. When two or 
 more foci are thus united, the lens or combination of lenses is said 
 to be “ achromatic , 55 or corrected for chromatic aberration. In such 
 optical instruments as are not intended to be used in photography, 
 it is customary to combine the focus of the green rays with that of 
 the orange ; but in photographic lenses the violet rays should be 
 combined with the yellow. See “ Spectrum . 55 
 
 Accelerators. This name is given to any substances used in 
 photographic processes, with a view to shorten the time of ex- 
 posure, either in the camera or the printing-frame. In the 
 
 B 2 
 
4 
 
 ACC 
 
 daguerreotype, bromine and chlorine, and especially bromine, exer- 
 cise a very decided accelerating action, the time of exposure being 
 reduced from minutes to seconds. They require dilution, and there- 
 fore must be held in solution, or retained by some solid bodies 
 which easily give them off in vapour. Under whatever names the 
 accelerators used in this process may pass, Woolcott solution, Hun- 
 garian solution, chloro-bromide of lime, &c., the active principle is 
 the same in all. When bromine and chlorine are used in their com- 
 binations in the calotype and other processes by solutions, they have 
 no such quickening power. It is probable that the pure iodide is 
 not quickened by any addition. 
 
 The substances which have been recommended to increase sensi- 
 tiveness in these processes, may be divided into five classes. Taking 
 iodide of silver as the basis of all quick formulae, the first class 
 includes the other insoluble salts of silver, such as the bromide, 
 fluoride, &c., which have all in turns been recommended, but with- 
 out good reason. The fact that these salts give a visible image 
 more quickly than the unmixed iodide, has been taken as a proof 
 of greater sensitiveness ; but the truth is, the iodide in its best 
 state, though the last to give out an image to the eye, is the first to 
 receive one capable of development by the usual developers. The 
 second class includes the easily decomposed soluble salts of silver, 
 as the nitrate and acetate. The presence of one or more of these is 
 essential to forcible development, and this is their principal use ; 
 but they are also said to accelerate, in a slight degree, the action of 
 light itself. The third class is composed of alkalis and basic salts. 
 There can be no doubt that acids, in proportion to their oxidizing 
 properties, restrain or neutralize the chemical action of light in the 
 sensitive film. But beyond neutralizing any acid present in the 
 film, alkalis do not appear to modify the active power but rather to 
 exert their own peculiar action in decomposing the sensitive layer 
 itself. Fourth : organic matter of many kinds is said to accelerate, 
 as honey, sugar of milk, &c. Twenty things have been spoken of, 
 but the best experiments prove that, though they add, by their power 
 of combining with reduced silver, to the visibility of the scarcely 
 apparent picture, and to its final vigour, their action is really to 
 retard, and not accelerate. Still, their presence, in proper propor- 
 tion, is essential in all negative processes, and in all cases, except 
 the delicate silver film of the collodion positive, where intensity is 
 not required and where organic matter gives drab tones. The last 
 class consists of bodies having a strong desire for more oxygen, and 
 which therefore are, or may be, used as developers. Gallic acid, used 
 in the calotype and in several procesess, and iodide of iron, which, 
 
ACE 
 
 5 
 
 when the film is formed, becomes nitrate of iron, are supposed to 
 act as accelerators ; but the effect is more to cheat the operator, by 
 carrying on the development simultaneously with the exposure, than 
 to favour the real ictus of the solar ray. 
 
 The truth seems to be, that a certain finely divided, semitrans- 
 parent, pappy and neutral condition of iodide of silver is obtain- 
 able, which with a minute excess of nitrate, in our wet processes, 
 gives the most sensitive surface yet discovered. There are certain 
 mechanical conditions, and conditions of temperature, molecular 
 arrangement, proportions, dryness, optical correctness, and easiness 
 and smartness of manipulation, atmospheric pressure, and other in- 
 fluences, which all affect the impressibility of a chemically prepared 
 layer ; but these will be better treated under the word “ Sensi- 
 tiveness,” q. v . 
 
 Acetates. Compounds of acetic acid and oxides, as acetate of 
 silver, which contains acetic acid, silver, and oxygen. They are 
 generally used either to neutralize free nitric acid in N. S. solution, 
 and substitute acetic acid for it, which interferes less injuriously 
 with the production of the image, or else to form a small quantity of 
 acetate of silver in connection with iodide of silver, and thereby add 
 vigour to the impressions. They are formed by dissolving the 
 oxides in the acid, or by adding acetic acid to the proper carbonates, 
 when carbonic acid is driven off with effervescence. The acetate of 
 soda is most convenient. 
 
 Acetate of Ammonia. N H 4 0, Ac 0 3 . Kept in aqueous 
 solution, and formed by saturating carbonate of ammonia with 
 acetic acid. It is used like the acetates in general, but is not so 
 suitable as acetate of soda, on account of the nitrate of ammonia 
 formed. 
 
 Acetate of Iron. There are two acetates of iron, but one, the 
 peracetate, is not used in Photography. The protacetate, or acetate 
 of the protoxide Ee O, Ac 0 3 has been used to develope both 
 positives and negatives ; and when it is used, 20 grains to the ounce 
 of water, it gives the negatives greater opacity than either the nitrate 
 or sulphate of iron. To make it, mix solution of 139 parts of sul- 
 phate of iron and 190 of acetate of lead, or saturate acetic acid by 
 sulphuret of iron. 
 
 Acetate of Lead. Sugar of lead of commerce, Pb O, Ac 0 3 -(- 
 3 Ii 0 — 190, is sold in crystals of various purity. They are easily 
 
6 
 
 ACE 
 
 purified by re-crystallization. This acetate is produced by digesting 
 litharge, which is oxide of lead, in acetic acid more or less pure, or 
 by exposing plates of lead to fumes of the acid, and dissolving the 
 superficial coating in excess of acid. This and other salts of lead 
 have been added to the N. S. bath, to increase sensitiveness in the 
 collodion process, which they do by setting up partial decomposition 
 and forming basic salts when the bath is old or impure. The film 
 is then so easily reducible that gallic acid will develope it, but the 
 picture will not be clear. Acetate of lead added to the ordinary 
 developers, is also said to increase the density of the negative. 
 
 Acetate of Lime, Ca O, Ac 0 3 . Silky acicular prisms which 
 have been added to the developer, but with no well-defined object. 
 
 Acetate of Potassa, Ko Ac 0 3 . Most conveniently preserved 
 in solution. It may be fused, and thus dried, but it is difficult to 
 keep it from deliquescing. It is used to discover the adulteration 
 of citric acid with tartaric ; for which purpose add it to the saturated 
 solution, when the tartaric will precipitate as bitartrate of potassa. 
 It should be remembered, that it is formed in minute quantity in 
 iodised collodion if the ether has been kept. This is rather bene- 
 ficial for negatives, but would lessen the softness and half tone and 
 injure the colour of positives. 
 
 Acetate of Silver, Ag O Ac 0 3 . A salt of silver very nearly 
 insoluble in water. The little which water will take up has been 
 found very useful in many negative processes, in excluding the 
 stronger acids, and in furnishing oxide of silver to the organic 
 matter present. It thus favours quickness and vigour. In its 
 action on organic matters, it resembles acetate of lead, which 
 organic matter in many forms decomposes, forming insoluble com- 
 pounds with the lead in a basic state. The red compound of 
 organic matter and silver, formed in developing, readily attaches to 
 itself the oxide of the decomposing pyrogallo-nitrate, while the bluer 
 image, which is more free from organic matter, is intensified much 
 less. When the acetate is used to saturation in a N. S. bath for 
 collodion negatives, it even produces such excessive deposition of 
 silver as often to veil the entire negative. The smallest quantity in 
 excess of what is necessary to exclude nitric acid will be found 
 sufficient ; and as its effect is the same in all the processes, it must 
 not be used for collodion positives, of which the deposit should be 
 as delicate as possible. It always requires the presence of some free 
 acetic acid to prevent its causing fog by its alkalinity. Papers for 
 
ACE 
 
 7 
 
 printing by development, or for sun-printing, may be prepared with 
 it in the same way as citrate or chloride of silver papers are pre- 
 pared. To procure it in crystals, mix together solution of 137 parts 
 of crystallized acetate of soda and 170 of nitrate of silver, when the 
 crystals are thrown down in a lamellar form. 
 
 Acetate of Soda. The crystals are Na O, Ac 0 3 , 6 H 0 = 137. 
 This is the most convenient acetate for the photographer, and he 
 may make it by saturating common washing soda (carbonate of 
 soda) with acetic acid, or vinegar, and afterwards crystallizing and 
 recrystallizing. It is important also as the source of pure acetic 
 acid. 
 
 Acetic Acid. Ac 0 8 HO; or C 4 H 3 0 3 , H 0 = 60. This 
 acid is used in the developer in the calotype, wax-paper, albumen 
 and collodion processes, to check the spontaneous decomposition 
 and too rapid action of the developing compound, to keep the lights 
 clean, and also to make the developer flow evenly and bite uni- 
 formly into the film ; it is used in most solutions of N. S. which 
 are intended to excite iodised surfaces on paper, albumen, collodion, 
 and other vehicles, and wherever N. S. is employed it is more or 
 less useful: it is added to gallic acid to lessen its tendency to 
 become mouldy: it has been used as a solvent of xyloidine, to 
 form a film, and is altogether of very extensive and important ser- 
 vice in Photography. It is therefore of moment to get it pure and 
 understand its action ; and it is of the more importance to know 
 what ends it answers, because it is so generally useful, that it has 
 been recommended in cases where it would be injurious, as in the 
 N. S. bath for collodion positives, where all organic matter, whe- 
 ther acid or otherwise, should be avoided, and where nitric is the 
 proper acid ; and also in collodion, which should be preserved in a 
 strictly neutral condition, except in special cases. In order to see the 
 effects it produces, let a person try a collodion negative from a bath 
 made of commercial nitrate of silver, smelling strongly of nitric acid, 
 yet otherwise pure : the resulting image is slowly impressed, feeble 
 in character, and blue in tone, but clear and distinct. The addition 
 of carbonate of soda, until slight alkalinity is produced, makes 
 the picture much redder, much quicker in its formation, with much 
 fog and but little increase of strength. Acetic acid in slight excess 
 of the alkali now shows itself, by restoring clearness entirely, by 
 reproducing slowness and blue tone in a very slight degree, and 
 at the same time by a marked improvement in vigour and beauty : 
 and up to a certain point every drop increases these effects ; but it 
 
8 
 
 ACE 
 
 is remarkable bow little the sensitiveness is impaired. Its effect in 
 development is best studied by comparing two solutions of N. S. 
 mixed with pyrogallic acid, one containing acetic acid, the other 
 containing none, and observing how much the acid retards the 
 blackening which such solutions undergo. 
 
 Other acids exercise an influence similar to, but not identical 
 with, that manifested by acetic. 
 
 Acetic acid is formed, under the name of vinegar, by the ferment- 
 ation of malt liquors, and of wines and spirits : it is also pro- 
 duced in the distillation of wood, when it is called pyroligneous acid. 
 In both cases it is contaminated with organic matters, which colour 
 it, and, in wood vinegar, make it empyreumatic and tarry. If added, 
 in these states, to N. S., the nitrate is discoloured by light, and a 
 black precipitate gradually formed. The pure acid is obtained from 
 these vinegars by saturating them with oxide of lead, carbonate of 
 soda, or similar substances, crystallizing out the pure acetates, 
 and submitting the pure salts to distillation with sulphuric or 
 hydrochloric acid. The acetic is driven off by the stronger acid, 
 a portion of which is apt to go over with it : and when this 
 is the case, the acetic acid,, when added to nitrate of silver, will 
 throw down chloride or other salts of silver, and weaken the solu- 
 tion ; in development the same cause will weaken or even arrest 
 the appearance of the image. It should therefore be tried with 
 N. S., and with chloride of barium, before use. 
 
 The strength of pure acetic acid is very variable, and cannot 
 be determined by its specific gravity. The most constant is the 
 “ glacial,” which below 40° of temperature becomes solid. When 
 it does not solidify, the only means of knowing its strength is by 
 observing the quantity of crystallized carbonate of soda which is 
 necessary to neutralize it. The dilute acetic acid of the London 
 Pharmacopoeia, sold by the druggists, should be such that one fluid 
 ounce saturates 57 grains : the acetic acid fortior, of the Phar- 
 macopoeia, or the acid called Beaufoy’s, should take 390 grains to 
 the fluid ounce ; and the glacial requires 1036 to the ounce, or 129 
 to the fluid drachm. When pure, any of these may be used by 
 calculating from these numbers the equivalent measures. The num- 
 bers 1, 7, and 18, are near enough; but it would be better if a 
 standard acid were used by photographers, easily verified by a 
 standard solution of carbonate of soda. 
 
 The disadvantages attending the employment of this acid are 
 worth attention. As used, it is 50 times as dear as citric; it is 
 liquid and bulky, and inconvenient in travelling ; its fumes are inju- 
 rious to the brass work of the apparatus, and unpleasantly strong,. 
 
ACE 
 
 ACI 
 
 9 
 
 and it is of very uncertain strength. It is apt, by solvent action on 
 the size of paper, to render it absorbent, and to cause the picture 
 formed on it to “ sink” and lose brilliancy. And it is apt, when 
 present with alcohol, as in the N. S. bath and collodion, to form 
 acetic ether, which injures the transparency and strength of the film, 
 mars the sensitiveness of the plate, and impairs the detail of the 
 image. On these accounts citric acid may possibly supersede it. 
 
 Acetic Ether. Eormed occasionally in ether, by air and light, 
 and injurious in collodion, because it in some degree lessens its 
 transparency and injures its tenacity. Its photographic effect re- 
 sembles that of acetic acid : and it decomposes the contained iodide. 
 
 Aceto-iodide of Iron. Solution of iodide of iron (q. v .), with 
 acetic acid added to lessen its injurious effects. 
 
 Aceto-nitrate of Silver. A mixture of acetic acid with solu- 
 tion of N. S. : the proportions are given with the processes. When 
 the solutions are concentrated, acetic acid precipitates nitrate of 
 silver, but not when they are dilute, if pure. The reason why 
 the weaker acid, acetic, here displaces the stronger, is the sparing 
 solubility of acetate of silver. 
 
 Achromatic. See “ Aberration.” 
 
 Acids. The uses of acids for special purposes, not actino-chemi- 
 cal, as in the making of pyroxyline, cleaning vessels, removing the 
 size of paper, &c., are specified under the proper heads. Their 
 action in photo-chemistry is what concerns us here. Acids are 
 substances sour to the taste, which change vegetable blues to reds, 
 and which unite with alkalis or other bases in such a way as to 
 neutralize each other’s properties. They are of two kinds, chemi- 
 cally considered : oxy-acids or acid oxides, and hydracids or com- 
 pounds of non-metallic substances, and some compound radicals 
 with hydrogen. The hydracids decompose the soluble silver salts, 
 and therefore completely bar the development of photogenic images, 
 while the decomposition of the acids also prevents our studying 
 their effects, on the formation itself of the image by light. The 
 oxyacids as a class exert in a greater or less degree a retarding 
 effect on the rapidity of formation of the image, which is veiy re- 
 markable; this is in proportion to their oxidising power. The 
 gallic and pyrogallic acids exert, on the contrary, a powerful reducing 
 or deoxidizing influence, which has made of them our most useful 
 
10 
 
 ACT 
 
 developers. But while gallic has very feeble acid properties, pyro- 
 gallic is not strictly an acid at all : and both act more like sugar 
 and other neutral organic substances, than like true acids. 
 
 Of the oxyacids, those which produce insoluble salts of silver 
 when present in the film or the N. S. bath, retard the production of 
 an invisible image by the liberation of free nitric acid, and if they 
 are added to the developer, they will partially or entirely annul its 
 action: they are the oxalic, phosphoric, sulphurous, and others. 
 Those which give salts of silver more or less soluble, vary in their 
 effects according to their oxidising power. The vegetable acids, as 
 acetic, citric, malic, and tartaric, are the lowest in the scale ; the 
 mineral acids, perchloric, sulphuric, &c., are next, and nitric is the 
 strongest. One twentieth of a minim of nitric acid in the N. S. 
 bath, will perceptibly injure its sensibility, and will have a still 
 more decided effect in weakening the developed image, and keeping 
 the reduced silver pure and free from organic matter. The organic 
 acids, however, affect the sensitiveness but little, while they give 
 colour and intensity in a considerable degree. These effects are 
 most strongly perceived when the acids are added to the exciting 
 bath ; but a larger quantity added to the developing solutions have a 
 corresponding influence. The difference observed in various kinds 
 of collodion, ether, paper and its sizing, and other material, may be 
 always traced to the presence of acid bodies. In all negative pro- 
 cesses, it will be judged essential, in consequence of these established 
 reactions, to remove every trace of nitric acid, and make use of a 
 vegetable acid to preserve clearness. The acetic, malic, and suc- 
 cinic, act much alike, but the acetic is rather the best negative 
 acid ; and all may be used in tolerable quantities without very rapid 
 difference in the results. Citric acid must be used much more 
 carefully, for while even 5 or 6 minims of acetic may be added to 
 each fl. ounce of the N. S. bath without causing vastly more change 
 in it than 1 minim, citric must be added by proportion at least 20 
 times as small, and each addition will increase the effect per- 
 ceptibly. 
 
 The consequences of employing acids in the fixing, toning and 
 other solutions, with the effects of acids in reacting on finished 
 photographs, will be discussed under the proper words. 
 
 Actinism. (Greek atcnr, a ray). A ray of light, whether 
 proceeding from a heavenly or terrestrial body, is found, in general, 
 to possess three properties, viz., the luminous, the calorific, and the 
 actinic. The actinic is that chemical property of the ray, which 
 produces important chemical changes in many substances submitted 
 
ACT 
 
 AEE 
 
 11 
 
 to its action, and on which the whole art of photography is founded. 
 When a ray of light is decomposed by refraction through a prism 
 into its constituent colours, it is found that actinism exists chiefly 
 among the violet, and scarcely at all among the yellow rays. 
 Hence the importance of combining the violet or actinic focus of a 
 photographic lens with the yellow or luminous focus : for the actinic 
 rays produce the photographic picture,* while the luminous rays 
 render the image visible on the focussing screen. 
 
 Actinograph. A name given by Sir John Herschel to his 
 “ self-registering photometer.” 
 
 Adhesion, Attraction of. A remarkable instance of this 
 kind of attraction occurs in the intimate union which takes place 
 when a collodion picture film is transferred to gutta percha, leather, 
 paper, waxed cloth, and other surfaces. The collodion appears to 
 enter into the pores of the material in such a way that it cannot be 
 rubbed up, and it is very difficult to detect its presence. Air 
 adheres to paper and metal surfaces so closely, as often to interfere 
 with their being easily coated or wetted. 
 
 Aerial Perspective. The earth’s atmosphere contains, be- 
 side air, aqueous and other vapours partially condensed, and also 
 solid particles which scatter light, and thus render themselves more 
 or less visible. The air is of a blue colour. Hence it follows 
 that objects are viewed through a medium which imparts to them 
 more or less of its own blue local tint, according to their distance, 
 and covers them with more or less of indistinctness. These effects 
 are called “ aerial perspective,” and it is erroneous to suppose that 
 photography cannot faithfully reproduce them. On the contrary, it 
 is frequently found to exaggerate them, and the reproach under 
 which it has lain, of representing nature like a hard and distinct map 
 upon one plane, has originated in the timid practice of photo- 
 graphers, in avoiding the very class of objects which they should 
 aspire to copy, and in confining their practice out of doors to those 
 cases in which the unusual clearness and distinctness of distant 
 objects seemed to invite the camera. But every day, as the real 
 capabilities of landscape photography are better understood, the 
 photographer should become more ambitious, and seek for those 
 very studies, which, in the infancy of the art, were esteemed im- 
 practicable. Skies, moving water, animate objects, mists, haze and 
 all atmospheric effects, should now engage his earnest attention. 
 It will be observed, that aerial perspective is increased in proportion 
 
12 
 
 M8C 
 
 ALB 
 
 as the vaporous and other particles in the atmosphere are more 
 strongly illuminated, and is therefore greatest in the direction of the 
 source of light. 
 
 zEscultne. A white powder, obtained by deposition from a 
 strong alcoholic solution of horse-chesnut bark. A mere trace, dis- 
 solved in spring water, communicates to it a blue opalescence ; and 
 the effects of its solutions on luminous and actinic rays are instruc- 
 tive and curious. It has the property of rendering the chemical 
 rays visible ; and this has been taken advantage of where there is a 
 great distance between the chemical and visual foci of a lens, to find 
 the place of the chemical image. Other bodies have the like pro- 
 perties. See “ Fluorescence.” 
 
 Affinity. That power of attraction which causes dissimilar 
 chemical substances to combine. It is wrong to reason as if the 
 chemistry of photographic operations was always in accordance with 
 the usual affinities of bodies, because light, like electricity, will over- 
 come, and even sometimes reverse the order of the strongest of them. 
 
 Agate Burnisher. Positive prints on plain paper are some- 
 times “ mealy ” and deficient in vigour. In such cases, it is thought 
 by some persons an improvement to impart a smoothness and glaze 
 to the surface of the paper by rubbing it all over with a polished 
 piece of agate. In this operation the paper should be laid face 
 upwards on a slab of plate glass, marble, or other hard polished 
 substance. 
 
 Air-pump Process. A process in which, the paper being pre- 
 pared in vacuo, was found to be more readily impregnated with the 
 solutions employed, but which was not sufficiently practical to be 
 generally adopted. 
 
 Albumen. This, in photography, means the white of an egg, 
 which is a pure form of it. It is used on paper for both negatives 
 and positives, and on glass for negatives and dia-positives, as a 
 vehicle for the sensitive silver salts. Por negatives on paper, albu- 
 men is not often used, for the same reason that collodion negatives 
 are seldom transferred to paper ; that is, because the advantages 
 of the albumen and collodion are then partly lost. It is not so 
 sensitive as collodion, from its more homy nature and its being dry ; 
 but, for the same reasons, it will keep longer when sensitized. 
 From its entering into combination with nitrate of silver, it gives a 
 
ALB 
 
 13 
 
 blacker image than collodion or gelatine, and one more inclined to 
 become yellow : the image, from its organic character, is also more 
 soluble in hypo and other solvents of the silver salts, and is there- 
 fore reddened and weakened more in fixing. The blackness of the 
 image makes it the best material for transparencies on glass ; but its 
 tendency in negatives is to give hard cutting pictures, and it is more 
 troublesome than collodion in the manipulation. When applied to 
 paper for printing, it gives the power of rendering the details of the 
 negative with extreme sharpness arid great transparency in the 
 shadows; but the smoothness is scarcely artistic; and when the 
 glazed surface is perceived by the eye, it becomes offensive. That 
 veil which sometimes covers pictures on plain paper is never seen on 
 albumen; but, on the other hand, plain paper gives good tones 
 with greater ease and certainty. 
 
 Albumen retards the action in the camera or printing-frame, but 
 increases the beauty and vigour of the image. It combines, like ,other 
 forms of organic matter, with bases and basic salts, which are then 
 called albuminates. Thus, in the paper processes it unites with and 
 is coagulated and rendered insoluble by the N, S., and the surface 
 becomes a layer of chloride and albuminate of silver, which does not 
 require any application of heat, as some suppose. It is this combi- 
 nation chemically with the silver salt which reddens the tone of the 
 reduced silver ; for those organic matters which have no combining 
 have also no colorific power. 
 
 Albumen is, before coagulation, soluble in water, and is most fre- 
 quently more or less diluted with water in all the processes, to 
 increase its fluidity. The entire destruction of the minute membra- 
 nous cells in which it exists in the egg, either by mechanical 
 violence, as in whipping it, or by the addition of a small portion of 
 alkali, which dissolves them, is necessary to the obtaining of a 
 uniformly clean layer, and the alkalinity may be neutralized after 
 the application of it, to the glass or paper, by acetic acid in the excit- 
 ing solution. 
 
 Albumen Processes. The albumen processes now in use are 
 as follow : — v 
 
 1st. The positive printing process on albumenized paper. 
 
 2nd. The negative albumen process on glass. 
 
 3rd. The dia-positive process on albumenized glass (see 
 “ Dia-positive ”). 
 
 I. Albumenized-paper Printing Processes. 
 
 There are two processes to be described under this head — a sun- 
 printing process and a development-printing process. 
 
 * 
 
ALB 
 
 n 
 
 1. The Sun-printing Process. 
 
 To prepare the Albumen . — Take a sufficient number of fresh- 
 laid fowls’ eggs, each of which contains nearly an ounce of albu- 
 men. Break each egg on the edge of a cup, and collect the 
 white, or albumen, carefully rejecting the germ and yelk. Put all 
 these whites together into a large basin, and add an equal quantity 
 of clean rain water, and 6 grains of common salt to every fluid 
 ounce of the mixture. Then beat the whole up to a very stilf froth 
 with a bunch of quills. Allow it to settle till the next day, and 
 decant the liquid, which is then fit for use. 
 
 The albumen, before being beaten up, is contained in small mem- 
 branous cells, which are broken in the process of beating, and the 
 albumen liberated. This membranous tissue is highly soluble in an 
 alkali, and the addition of a few drops of ammonia 1 6 the albumen 
 has been recommended by M. Davanne, as a means of rendering it 
 more fluid. The ammonia, being volatile, mostly escapes from the 
 albumenized paper while drying. 
 
 The proportion of water to the albumen may be varied according 
 to the nature of the subject to be printed. More water gives less 
 glaze, and vice-versa. 
 
 To Albumenize the Paper. — Pilter the salted albumen through 
 two folds of fine muslin, previously damped, into a shallow bath, to 
 the depth of at least a quarter of an inch. Turn back a corner of 
 the paper, and place the face of it on the liquid, leaving it in contact 
 for about a minute. Then hang it up to dry, by pinning the dry 
 corner to the edge of a shelf. Unless the membranous cells which 
 contain the albumen are thoroughly broken up, it is very liable in 
 drying to form streaks and waves on the paper. 
 
 The best paper to employ is that which has the smoothest and 
 finest surface. The paper manufactured in France by Messrs. 
 Marion, and Canson Freres, is generally preferred. 
 
 To excite the Paper. — Float the albumenized side on a 60 
 grain N. S. bath, for two minutes, and hang it up to dry in the 
 dark. 
 
 This is the full strength of the bath. A weak negative requires 
 a strong silver bath, apd a short exposure ; a strong negative a 
 weaker silver bath, and a longer exposure. 
 
 The Exposure to Light . — Expose in the pressure-frame either 
 to sunshine or diffused light (the latter is .best for feeble negatives 
 abounding in half-tone), until the print is somewhat darker than, 
 you wish it to be ultimately. 
 
ALB 
 
 15 
 
 To fix and tone the Print . — Immerse it at once in a fixing and 
 toning batli made thus : Dissolve eight ounces of hypo- sulphite of 
 soda in eight ounces of clean rain water. In a separate vessel 
 dissolve eight grains of chloride of gold in eight ounces of distilled 
 water. Add the latter solution to the former in small quantities 
 at a, time, stirring well after each addition. The mixture will 
 assume a milky colour. This is the fixing and toning bath. The 
 print is to remain in it about half an hour, or until the desired 
 tint is produced. 
 
 The fixing and toning bath may be used several times, and several 
 prints may be placed in it at a time ; but it should be thrown away 
 as soon as it becomes dark-coloured, or tones feebly. 
 
 To remove the redundant Chemicals from the Paper . — This 
 is effected by copious washing, and long soaking in water changed 
 several times. Some persons use warm, or even boiling water, in 
 the final operation. The print is then pressed between clean 
 cloths, and hung up to dry. The chemistry of the process is now 
 completed. 
 
 The edges should be trimmed, and the print mounted as soon as 
 possible, as handling it with the fingers is very likely to produce 
 faded spots and stains. See “ Mounting.” 
 
 2. The development-printing Process. 
 
 The particulars of this process will be found under the head of 
 “ Printing by Development.” The paper must be albumenized in the 
 same way as for sun-printing, and the remaining operations are 
 identical with those for printing by development on plain paper. 
 It may be as well, however, in this process to omit the ammonia in 
 the albumen bath, as free alkali in the paper is likely to occasion 
 discoloration in the process of development. 
 
 II. Albumen-Negative Process on Glass. 
 
 To prepare the Albumen . — Collect in a basin the whites of a 
 number of eggs, carefully separating the germ, and all portions of 
 yelk. To each ounce of this albumen add one dram of distilled water, 
 in which is dissolved 6 grains of iodide of potassium ; also to every 
 5 ounces of the mixture add one drop of ammonia. Beat the whole 
 to a stiff froth with a bunch of quills, and allow the liquid to settle 
 till the following day. 
 
 To albumenize the Plate . — The glass plate must first be cleaned 
 
16 
 
 ALB 
 
 very thoroughly, and polished with a cambric handkerchief just 
 before use. (On this head see the article “ Cleaning.”) Attach to 
 the under side of it a gutta percha plate-holder, having a wooden 
 handle a foot long. Then breathe on the plate, and, holding it 
 horizontally in the left hand, pour upon the centre of it a sufficient 
 quantity of the albumen from the basin to cover it, allowing the 
 albumen to filter through an opening in the dry froth or crust. 
 Make the albumen flow backwards and forwards over the plate three 
 or four times, and then let it all run off into a separate basin, from 
 which it must be carefully filtered before being used a second time. 
 Tn coating the plate, be particularly careful to prevent air-bubbles 
 from forming upon it. Next, take the handle of the plate-holder 
 between your hands, and, with the plate in a vertical position, spin it 
 round quickly for a minute or so, in order to drive the albumen to 
 the edges by centrifugal force. This done, remove the excess of 
 albumen from the edges by means of a pipette ( see “ Pipette”), and 
 dry the plate before a clear fire, keeping it rotating all the time by 
 means of the handle, as before directed. When dry it is ready for 
 the next operation. Albumenized plates maybe put away in a plate- 
 box, and kept for a considerable time without deterioration in a dry 
 place. Care must be taken, in the operation of albumenizing the 
 plate, that no particles of dust adhere to it. 
 
 To excite the Plate . — Place it on a dipper, and immerse it 
 quickly and without hesitation in a vertical bath of aceto-nitrate of 
 silver, made £fms : Distilled water 1 ounce, nitrate of silver 50 grains, 
 glacial acetic acid 1 dram. Leave it in the bath for a couple of 
 minutes, then wash it well in clean water, and lastly in distilled 
 water, and set it up to dry. When dry put it away in the plate-box 
 until ready for use in the camera. It^may be preserved in a 
 sensitive state for several days. Some persons add a few drops of a 
 solution of iodide of potassium to a 'new nitrate bath, and filter it on 
 the following day, in order to saturate it with iodide of silver. 
 When this is done a new bath is not so liable to attack the iodide of 
 silver in the film. 
 
 The Exposure . — Albumenized plates, from which the excess of 
 free nitrate of silver has been removed by washing, are, whether used 
 in a dry or wet state, extremely insensitive to light ; but, when only 
 slightly washed, exposed at once, and developed with a strong 
 developer, a much shorter exposure is sufficient. This should be 
 timed solely with reference to the shadows, the lights being left to 
 take care of themselves. When the camera is properly constructed, 
 so as to prevent stray light from falling on the plate, it is hardly 
 possible to over-expose a dry, washed, albumenized plate. 
 
ALB 
 
 17 
 
 To develop the Image. — First, immerse the plate in distilled 
 water ; then place it in a levelling-stand, and pour over it a saturated 
 solution of gallic acid, to which a few drops of aceto-nitrate of silver 
 have been added. The development occupies about twenty minutes. 
 
 To fix the Picture . — Wash the plate in rain water, and pour 
 over it a nearly saturated solution of hyposulphite of soda. This 
 will quickly remove the yellow iodide of silver from the film. Then 
 wash the plate well under a tap, and dry it before the fire. 
 
 The negative may be varnished with any good varnish, ( see “ Var- 
 nish, 5 ’) but this is not always done. 
 
 III. Albumen — Dja-positive Process on Glass. 
 
 Positives obtained by this process are intended to be viewed by 
 transmitted light. The manipulation is so nearly identical with that 
 of the albumen-negative process, described in the foregoing article, 
 that it is only necessary to point out the difference between them. 
 
 The negative to be copied is placed either in a copying camera, 
 ( see “ Copying Camera,”) or in direct contact with the sensitive plate 
 ki a pressure-frame. In the latter case the plate must be used dry, 
 and the exposure to diffused daylight, or artificial light, only occupies 
 a fewlseconds ; in the former case, the plate may be either dry or wet, 
 and the exposure is considerably longer. The wet process is the 
 least troublesome, and yields the best results, because the operations 
 of exciting, exposing, and developing, may then siicceed each other 
 at once, and less time is allowed for a combination take place 
 between the silver and albumen, which causes the lights of the 
 picture to assume a yellow tint. The development is also a much 
 quicker operation in the wet process, more nitrate of silver being 
 allowed to remain on the plate, and for this reason also the lights 
 are less likely to assume a yellow tint. 
 
 The difference between this and the negative process consists 
 chiefly in the employment of a gold toning-bath, in order to vary 
 and improve the tint of the finished picture. Thus, before fixing 
 the picture, pour over it a little of the following solution : — 1 grain 
 of sel d’or, 20 drops of hydrochloric acid, and two ounces of distilled 
 water. Allow this to remain on the plate a short time, until the 
 shadows assume a purple tint ; then wash it off, and fix as before. 
 
 Dia-positives on glass should be viewed with the plain side of 
 the glass next the eye ; and against the film the rough side of a 
 finely ground glass should be placed, the two glasses being bound 
 together at the edges with a strip of tape or paper pasted over 
 them. In this way the print is protected from injury, and has a 
 proper semi-transparent background. 
 
 c 
 
18 
 
 ALB 
 
 ALC 
 
 The chief use of this process is for printing transparent slides for 
 the stereoscope. In this operation it must be remembered that the 
 picture taken from the right station must be viewed by the right 
 eye, and vice-versa ; and also that the objects in the view must 
 not be reversed as regards right and left. It may therefore be 
 necessary to place the negative in the copying frame with its back to 
 the lens. Matters of this kind must be carefully considered by the 
 operator ; and his ingenuity will suggest the proper way of proceed- 
 ing in every case. No general rules need be laid down in this place. 
 
 Albuminate of Bakyta. Strictly speaking, this should be a 
 salt of albuminic acid and oxide of barium. It is said to be formed 
 when albumen papers are salted with chloride of barium ; but the 
 compounds formed by albumen and other such substances with 
 bases and basic salts, require to be further examined. An inquiry 
 into their exact composition would throw much light on photo- 
 graphic chemistry. 
 
 Albuminate of Lime. A compound analogous to the pre- 
 ceding. 
 
 Albuminate of Silver. The white precipitate which falls 
 when albumen is added to a solution of nitrate of silver is called by 
 this name. It would imply that it is a neutral salt of albuminic 
 acid and protoxide of silver ; but there is not sufficient proof that 
 the silver exists in it in the state of protoxide in combination with a 
 normal acid body. Albumen is alkaline, and an animal substance 
 of a very complex nature, containing even inorganic salts essential 
 to its constitution. It therefore undergoes decomposition in con- 
 tact with nitrate of silver, and all that can be said is, that the pre- 
 cipitate contains organic matter and some oxide of silver, of which 
 the former acts the part of an acid, and the latter of a base. 
 
 Albuminate of Suboxide of Silver. The red compound 
 formed when light reddens the albuminate of silver, and containing 
 less oxygen than it. 
 
 Alcohates. Crystallized salts, which include alcohol in their 
 composition, in the place of water of crystallization. Without 
 forming a true alcohate, alcohol sometimes modifies the forms of 
 crystals deposited from its solution, as in the case of the collodion 
 iodizing solution, made with nearly absolute alcohol, saturated with 
 iodide of potassium. Instead of pure cubes, the forms are then 
 
 
ALC 
 
 19 
 
 more or less acicular, shooting in directions at right angles to one 
 another. 
 
 Alcohol. On this important substance the remarks interesting 
 to a photographer will be, its special use in the collodion process, 
 its effect as organic matter on the photogenic image, and its assist- 
 ance in developing. Pure alcohol as organic matter is nearly inert 
 in photolytic processes, but when decomposed or oxidized with 
 ether, by long use in the exciting bath, organic principles are formed 
 which act like strong acids, and their effect is too injurious to be 
 attributed to the formation of acetic acid. , Such a bath, if evapo- 
 rated to dryness, will show that the organic matter is in combina- 
 tion with the nitrate of silver ; for on fusion the salt froths violently 
 as long as any organic matter is undecomposed. Added to the 
 collodion developers, it has been found very beneficial in making 
 them flow evenly and quickly over the plate ; but it has not been 
 tried as it deserves in the development of paper negatives. When 
 it is added to the solution of gallic acid, it is easy from its greater 
 solvent power to get the proper strength, and there is less tendency 
 to the formation of crystals and stains, than when concentrated 
 aqueous solutions are employed. The spirit has greater power also 
 of penetrating into the wax and size; but this property, though 
 useful in negatives, forbids its use in paper positives, where surface 
 vigour is requisite. 
 
 The most important use of alcohol is in collodion, and attention 
 must be paid to its strength and purity. 
 
 Absolute alcohol of S. G. 796 or 75*25 above proof, is very 
 difficult to keep in use, because, from its strong attraction for water, 
 it rapidly absorbs it from the air. But this is not necessary to the 
 photographer. Absolute alcohol is not the best for collodion, either 
 as respects the physical or chemical qualities of the article. When 
 ' iodide of potassium (the most generally useful iodide) is used in 
 collodion, it is so little soluble in alcohol + ether, that if the strongest 
 spirits are employed, the iodide is precipitated in the film before 
 going into the N. S. bath. The consequence is seen both in the 
 appearance of the film before going into the camera, and in the 
 developed picture. The film before exposure appears by trans- 
 mitted light semi-opaque, bluish in colour, and by reflection dim 
 yellow, like bad iodized paper, because the salt, as in paper, is not in 
 solution when the nitrate attacks it, but when the iodide is properly 
 formed in a film containing sufficient water, it remains more or less 
 in a state of semi- solution, as in jelly, and the colour is bright 
 yellow, perfectly transparent by transmitted light, and of a blue 
 
 c 2 
 
20 
 
 ALD 
 
 ALK 
 
 cast by reflection. The same want of transparency is ‘observed 
 with the same colours, in whatever way the iodide of potassium be- 
 comes excessive. In the picture the result is a sensitiveness scarcely 
 superior to that of albumen on glass, and very feeble reduction. Alco- 
 hol of S. Gr. 845 is very good for mixing with ether in making the 
 plain collodion, and *810 in the iodizing solution. The water 
 being in the ethereal mixture counteracts its tendency to precipitate 
 the salt from the iodizer. The strength of pure alcohol is obtained 
 by its specific gravity ; as it occurs in commerce it is generally from 
 S. G. 820 to 850. What is called “proof spirit” is alcohol of 
 S. Gr. 920. By spirit 60 degrees over proof, a strength which is 
 very commonly sold, is understood a spirit 100 measures of which, 
 added to 60 measures of water, will form proof spirit. Its S. Gr. is 
 880. Spirit of 50 degrees over proof needs only 50 measures of 
 water to 100 of spirit to bring it down to proof, and so on. In the 
 Trench formulae, spirit of so many degrees is often spoken of. This 
 refers to Baume’s hydrometer, for comparisons of which with the 
 S. Gr. and with proof spirit, see “Tables of Weights and Mea- 
 sures.” 
 
 With respect to the purity of alcohol, it is necessary to be very 
 careful in collodion for the positive process. All alcohol is apt to 
 contain organic matter, fusil oil, grain oil, &c., which, though in 
 small quantities, will exercise a more decided photographic effect, 
 prejudicial or otherwise, than a large quantity of the alcohol itself. 
 In negative processes these impurities may be beneficial, but not 
 where pure metallic silver is to be produced. In all cases alcohol 
 from the grape seems preferable to that from grain or roots. The 
 last, when not purified, often leaves on distillation, a red residue in 
 the retort, very offensive to the smell. The purity of alcohol is a 
 much more important matter than its strength. 
 
 Aldehyde, (Ah short for alcohol, dehyde for dehydrogenatus.) 
 Alcohol, from which 2 atoms of hydrogen are removed. It is found 
 sometimes in acetic acid, and it probably exists in minute quantities 
 in ether and alcohol, which from age and decomposition in the N. S. 
 bath have been partially oxidized. It is a very powerfully deoxi- 
 dizing agent, and has been used to improve the tone of collodion 
 positive pictures, by removing organic matter from the picture, and 
 reducing it more completely to the metallic state. 
 
 Alkaline Collodion. N. S. bath, toning bath, &c. See 
 “ Collodion,” &c. 
 
 Alkalis. Certain oxides of metals and compound radicals which 
 
ALL 
 
 ALT 
 
 21 
 
 are of an acrid taste, which change vegetable blues to greens, and 
 whose properties are masked or neutralized by their union with 
 acids. Their actinic effects are opposed also to those of the acids. 
 As acids, when free in the sensitive surfaces, have a retarding power 
 over the formation of the photogenic image, so alkalis, by neutralizing 
 the acid, appear to possess a quickening influence ; but when an excess 
 of alkali is present, it precipitates oxide of silver by its proper force, 
 without the access of light. Acids preserve the lights of photographs 
 clean, under the action of the developer, by preventing spontaneous 
 reduction of the gallo -nitrate, &c., and often by obliterating the faint 
 impression of light up to a certain point ; alkalis produce universal 
 blackening of the silver surface. In the hyposulphite bath, acids 
 liberate sulphur, and produce changes wdiich blacken the prints 
 immersed in it, and then make them turn yellow and fade ; alkalis 
 favour the stability of the bath and of the prints, but redden without 
 weakening it. On the contrary, acids check the toning power of 
 sel d’or and increase its stability, while alkalis decompose and render 
 it violent. 
 
 Potassa and soda are often called alkalis propel* and fixed alkalis, 
 because they are bodies which possess alkaline properties in the 
 highest degree, and because they are not apt to fly off like ammonia, 
 which has received the name of volatile alkali. But metallic 
 protoxides in general have the alkaline quality of neutralizing acids, 
 and when soluble, of affecting vegetable colours. AN. S. bath may 
 have its free acid neutralized as well by oxide of silver as by potassa. 
 
 Alkaline salts, carbonates, chlorides, &c., are the carbonates, &c., 
 of the alkali. In some of these, as in nitrate of soda, the alkali is 
 completly neutralized by the acid, but in the carbonates and others 
 there still remains some alkaline reaction. The most generally 
 useful alkaline substance for the photographer is carbonate of soda, 
 and will be found to answer in almost all cases where acids are to 
 be got rid of. 
 
 Allotropic, (tfXXoc another, rpoirog form). When the same 
 substance exists in two or more forms having different properties, 
 but still chemically the same, as albumen in the liquid and coagulated 
 state, the unusual form is said to be allotropic. Light produces 
 allotropic forms in some bodies, and certain temperatures do the 
 same with others. Those who suppose the action of light to consist 
 in a molecular disturbance of the sensitive surface, imagine it by 
 actinism to produce these allotropic conditions of bodies. 
 
 Alternation or Luminous Action. It is asserted by some 
 
22 
 
 ALU 
 
 AMM 
 
 that there is a periodical recurrence of two opposite effects in the 
 action of light ; that the first impression is negative as to light and 
 shade, then positive and afterwards negative again, and so on at 
 regular intervals. Light undoubtedly, under certain conditions, 
 appears to exercise both a negative and positive effect as to reduction, 
 but in some instances we know the conditions for the two effects are 
 different, and may be varied to produce either result at will ; and this 
 is probably the case always, though the difference is so subtle as to 
 escape our observation. That the effects recur, or are repeated 
 regularly, is not established by any experiments. See “ Lever se 
 Action of Light.” 
 
 Alum. Ko, So 3 ; AL 2 , 0 3 , 3 So 3 ; 24 Ho. A double sulphate 
 of potassa and alumina, having an acid reaction. It has been 
 recommended for the removal of hyposulphite of soda from paper 
 pictures, without abstracting the size. If the acidity of alum has no 
 injurious effect on the print, which is doubtful, it may be used with 
 advantage. It is often used with resin in sizing papers, and that 
 will account for the superior cleanness of some papers under 
 development. 
 
 Ambek,. A fossil resin found on some sea coasts, and also in 
 seams of coal; it is used in making amber varnish. See “Varnish.” 
 
 Ambbotype, Collodion positives are sometimes called “ Ambro- 
 types,” in America. 
 
 Amethyst. This and other precious stones, as the opal, are found, 
 under prolonged exposure to light, to lose their sparkle. 
 
 Ammonia. N H 4 0, or N H 3 . This gas is always used by the 
 photographer in aqueous solution, called “liquid ammonia.” It 
 dissolves chloride of silver, but not iodide; hence it is used to 
 separate one from the other, and also to fix prints on chloride 
 papers. It is applied after bichloride of mercury, to intensify weak 
 pictures, and it is used in the preparation of ammoniacal oxide and 
 ammonia-nitrate of silver, and has been added to collodion to render 
 it more sensitive ; but it decomposes the pyroxyline, rendering it 
 rotten, and the collodion thin, and also gradually impairs the condi- 
 tion of the N, S. bath. It is useful in preparing positive prints 
 for toning with sel d’or, and, as a strong detergent, is often made into 
 a paste with tripoli, for cleaning glass plates. As far as possible its 
 
AMM 
 
 23 
 
 presence in any sensitive surface is to be avoided, except’ where nitric 
 acid may be present also in excess. 
 
 Gaseous ammonia is evolved when two parts of dry quicklime are 
 mixed with one part of chloride of ammonium in a retort, and a 
 gentle heat applied. Water takes up 6 or 700 hundred times its 
 volume of this gas, and the saturated solution is of the S. G. 875. 
 The druggists’ liquor ammonise of the London Pharmacopoeia is 
 S. G. 960. The liquor am. fortior is S. G. 882, and is reduced to 
 the other, by adding 2 ounces water to each fl. ounce. Of these three 
 solutions the first contains 33^ per cent, by weight of ammonia, the 
 second only 10, the third nearly 30. The solution absorbs carbonic 
 acid from the air, with the formation of carbonate of ammonia. 
 
 Ammonia, Acetate of, Aurate of Aureole, &c. See “ Ace- 
 tate,” &c. 
 
 Ammoniacal Oxtde of Silver, sometimes called Ammoniuret 
 of Silver. It is a solution of oxide of silver in liquid ammonia, 
 (see “ Ammonia,”) and is used in printing. See “ Printing.” 
 
 Ammonio-chromate of Copper. Chromate of copper dis- 
 solved in liquid ammonia. The solution is green, and papers pre- 
 pared with it are bleached by light. When the shadows, under a 
 negative, are sufficiently whitened, either copper or silver compounds 
 may be reduced upon them by developing with carbonate of soda or 
 nitrate of silver. 
 
 Ammonio-citrate of Iron. This neutral double salt of citrate 
 of peroxide of iron, and citrate of ammonia, dissolved in water, and 
 mixed with solution of ferro-cyanide of potassium, gives a purple 
 solution nearly black. Papers prepared with this are bleached in 
 the light, but darken again in the dark. Salts of cyanogen, and 
 salts of iron, are both subject to actinolyitc action ; but the iron is 
 here the one attacked. This paper may be used to take evanescent 
 pictures of many objects successively. The ammonio-citrate of iron 
 itself, is a beautiful pharmaceutical preparation. It is a salt of the 
 peroxide of iron, which is partially reduced by the chemical rays to 
 protoxide. Papers prepared with a solution of it, change by insola- 
 tion from a bright yellow to an ochrey grey, and may be developed 
 before the picture becomes visible by ferridcyahide of potassium, 
 (red prussiate of potash), which proves the reduction to the state of 
 protoxide, by the formation of prussian blue. It is known to all 
 photographers that the protosalts of iron, as the protonitrate, the 
 protacetate, the protocitrate, &c. are developers, and their formation 
 
24 
 
 AMM 
 
 ANH 
 
 by light is a curious instance in which the usual order of operations 
 in taking a picture is reversed. First, the paper is prepared with an 
 iron persalt, which, by solar action, becomes a developer in certain 
 parts only of the paper, and the picture is then produced by the 
 action of this local developer on any salts applied uniformly to the 
 paper which it is capable of deoxidizing. Bichromate of potass, 
 nitrate of silver, and many others may be thus applied, and appear 
 to develope the print themselves, but in truth only supply material 
 to the invisible developer, which pre-existed on the surface. 
 
 Ammonio-nitrate of Silver. Chemists give this name to a 
 white substance composed of 100 parts of nitrate of silver, and 29 J 
 of ammonia ; but in photography it is either a solution containing 
 oxide of silver, nitrate of ammonia, and ammonia, or oxide of silver 
 and nitrate of ammonia alone. It is used in “ printing,” q. v. 
 
 Ammonio-tartrate of Iron. A compound behaving like the 
 ammonio-citrate. 
 
 Ammonium ; N H 4 . A hypothetical substance not yet isolated. 
 
 Ammonium, Chloride of, Phosphuret of, &c. See 
 “ Chloride,” &c. 
 
 Amphitype. (Greek, on both sides*). A process dis- 
 
 covered some years ago by Sir John Herschell, in which light pro- 
 duces either a positive or negative. A sheet of paper is first prepared 
 with a solution, either of ferro-tartrate, or ferro-citrate of protoxide, 
 or peroxide of mercury, and then with a solution of ammonio-tartrate, 
 or ammonio-citrate of iron, the latter solution being in excess. On 
 exposure to light in the camera, a negative is produced of more or 
 less vigour, and of a very rich brown tint when the paper contains 
 a salt of lead. It gradually fades in the dark, but may be restored 
 as a black positive, by immersing it in a solution of nitrate of mercury, 
 and ironing it with a very hot iron. 
 
 Angular Aperture. The angular aperture of a lens is the 
 angle which its diameter subtends at its principal focus. 
 
 Anhydrous. Many substances in chemistry have such a strong 
 affinity for one or more atoms of water, that they seem incapable of 
 existing without these atoms, or a base of some kind as a substitute. 
 When the water is replaced by a base, or is otherwise entirely 
 
ANI 
 
 APP 
 
 25 
 
 removed, the substance is anhydrous, (a not, and vcwp water). 
 Glacial acetic acid contains one atom of water, which, when it com- 
 bines with oxides or other bases, it liberates. 
 
 Animal Charcoal. The product obtained by the carboniza- 
 tion of muscle, horn, blood, bones, and other animal matters. It is 
 distinguished from vegetable charcoal by its lustre and sponginess. 
 It is used in photography to remove colour and organic impurities 
 from solutions of nitrate of silver, &c. In commerce it contains 
 phosphates and other salts derived from the animal organism, which, 
 when it is added to N. S. solutions, produce a yellow precipitate of 
 phosphate of silver, and this often to such a degree as greatly to 
 weaken the solution. Nitrate baths have been so weakened by it as 
 to cause the iodide of silver entirely to peel off the plate on attempt- 
 ing to excite it. When purified it often contains hydrochloric acid, 
 which precipitates white chloride of silver. It is called in trade 
 ivory black, bone black, &c., and its impurities are always such, that 
 it is better to employ kaolin. 
 
 Animal Oil of Dipped. Has been employed as a solvent of 
 bitumen in M. Niepce’s processes. See “ Bitumen.” It is rectified oil 
 of hartshorn, and is itself discoloured by solar light. 
 
 Animal Substances. The animal matters used in photo- 
 graphy are gelatine, albumen, gluten, isinglass, and a few others ; 
 they are of service both in forming transparent films, and in them- 
 selves combining with silver compounds for the production of images. 
 They are substances with which English papers are sized, and hence 
 pictures on English papers are redder in tone than those on the 
 Erench and German papers sized with starch. See “ Organic 
 Matter.” 
 
 Aperture. See “ Lens.” 
 
 Aplanatic. (Greek, tt \ avau) to cause to wander). In optics, a 
 lens is said to be aplanatic when it has been corrected for spherical 
 aberration. 
 
 Apparent Direction, Distance and Magnitude. See 
 “ Stereoscope.” 
 
 Application of Chemical Substances to Paper and 
 Other Surfaces. This pari of photographic manipulation is one 
 
26 
 
 AQU 
 
 ABG 
 
 of some importance. Tlie production of uniform films by means of 
 vapours is very simple in execution, but it is doubtful in such cases 
 as the daguerreotype, what is the exact composition of the sub- 
 stances formed. Their continual change of colour is an evidence 
 that they are not definite, not to speak of the amazing difference in 
 their photographic properties. In the production of films of true 
 chemical compounds, if pure salts are to be examined, it is necessary 
 they should be precipitated on glass or other surfaces absolutely 
 inert, for if collodion or other matters be present, no argument can 
 be built upon the assumption of their purity. The pure salts 
 themselves must settle upon the glass and be carefully drained, or 
 experiments may be tried upon them while still in suspension. In 
 other cases, sensitive surfaces are formed directly on paper or 
 gelatinous beds by immersion and floating, by glass rod, by spatula, 
 or by brush. When a cream of the substance is to be applied, the 
 spatula must be used, but the plan is only useful in experiments, as 
 the coating is too uneven for the production of fine results. When 
 a little variation in the quantity or strength of the salts is of no 
 moment, floating and immersion may be resorted to, but these plans 
 quickly lessen the quantity of the solution, and when double de- 
 composition occurs, they also soon reduce its strength, and the pre- 
 pared films are not constant in their actinic properties. The glass 
 rod and brush are the most exact methods of manipulation, and of 
 these the rod is more correct, the brush more convenient. All 
 photographic formulae should, if possible, be so constructed that the 
 solutions, paper, &c., should continue unvarying. 
 
 Aqua Portis. Nitric acid. 
 
 Aqua Begia. Nitro-hydrochloric acid, q. v. 
 
 Aqueous Ammonia. Liquid ammonia. See “Ammonia.” 
 
 Aqueous Chlorine. Solution of chlorine gas in water. 
 
 Aqueous Nitric Acid. The strongest nitric acid contains an 
 atom of water; but in combination, as in nitrate of silver, it is 
 anhydrous. 
 
 Areometer. An instrument much used on the continent for 
 measuring the specific gravities of liquids. 
 
 Argento-cyanide of Potassium. A solution of cyanide of 
 
AES 
 
 AET 
 
 27 
 
 silver in cyanide of potassium, used in electroplating and in silver- 
 ing copperplates for the daguerreotype. 
 
 ArsenIc Sulphuret or Cobalt. A compound of arseniuret 
 and sulphuret of cobalt, which is partly decomposed by light. 
 
 Arsenious Acid. As 0 3 . This has been tried instead of 
 acetic in the developer, but it prevents blackening for a time only. 
 
 Artificial Light. The light produced by electricity and 
 combustion is found to be more or less actinic. The photographer 
 may therefore avail himself of this property of various artificial kinds 
 of light, to work at night, or when diffused daylight is too feeble for 
 his purpose. Of all kinds of artificial light, that produced by elec- 
 tricity appears to be the most actinic. The most convenient mode 
 of employing electric light is by means of a galvanic battery, having- 
 wires tipped with charcoal points brought into close proximity. 
 The light produced in this way is extremely intense and highly 
 actinic. A burning jet of the mixed gases oxygen and hydrogen, 
 brought to bear upon a ball of lime, is perhaps the next in intensity. 
 The oxy-calcium light, produced by passing a jet of oxygen into the 
 flame of a spirit lamp, and causing it to act upon a lime ball, is also 
 very intense. Common gas burners and argand lamps, aided by a 
 reflector, may also be used by the photographer for certain purposes, 
 and even the light of a tallow candle is sometimes dangerously actinic. 
 Various chemical compositions have been employed for producing, 
 by their rapid combustion, an intense actinic light, acting during a 
 few seconds only. By mixing together 4 parts of chlorate of potass, 
 2 parts of sulphur, and 1 part of sugar, all in fine powder, and 
 igniting it, a very intense light is produced. 
 
 The effect of any artificial light is greatly increased by placing it 
 in the focus of a parabolic reflector. 
 
 Artificial Parchment. If a sheet of blotting-paper is im- 
 mersed for a second or two in a mixture of 2 parts sulphuric acid 
 and 1 part water, and then rinsed in several changes of water, an 
 extraordinary change is produced in it, apparently without altering 
 its chemical composition. It becomes tough, strong, semi-trans- 
 parent, and very difficult to tear, even when wet; resembling in 
 appearance common parchment, and possessing many of its proper- 
 ties. When dry it contracts and becomes brittle and easily cracked. 
 If a piece of calico or linen is submitted to the same treatment, it is 
 similarly acted on and rendered waterproof, and semi-transparent 
 
28 
 
 ASP 
 
 AZO 
 
 while wet. On drying, however, it contracts and cracks like the 
 blotting-paper. 
 
 Artificial parchment has been used with some success in making 
 a pyroxyline which is said to give a fluid and good collodion. No 
 other use has yet been made of it in photography. 
 
 Asphaltum. This is an indurated bitumen, found in Judea, the 
 West Indies, South America, and other places. It varies in quality, 
 but may be purified by boiling it in water, when the pure asphalt 
 melts and floats upon the surface, while the impurities subside. It 
 is the basis of most black varnishes, being dissolved in naphtha, 
 turpentine, &c., with oil, gutta percha, india-rubber, &c., to give it 
 toughness. See “ Yarnish.” 
 
 Astro-photography. A convenient name for the application of 
 photography to the delineation of solar spots, the moon’s disc, the 
 planets and constellations. 
 
 Atom. This word is applied by those who adopt the atomic 
 theory, to the minute ultimate particles beyond which they suppose 
 matter cannot be divided. In chemistry it is often synonymous 
 with “ chemical equivalent,” q. v. 
 
 Atomic Theory. The theory which supposes all bodies to be 
 composed of indivisible particles, every substance being an aggregate 
 of atoms all of the same form and weight. 
 
 Aurate of Ammonia. This may be formed if ammonia is 
 added to a strong solution of chloride of gold, with a view to neu- 
 tralize the acid. It is fulminating gold, and must be dealt with 
 cautiously. 
 
 Avoirdupois Weight. See “ Tables of Weights and Mea- 
 sures ” at the end of the volume. 
 
 Axis. A line with respect to which a lens or piece of apparatus 
 has its parts symmetrically situated. 
 
 Azotates. The Trench name for nitrates. 
 
 Azote. Nitrogen. 
 
 Azotic Acid. Nitric Acid. 
 
BAC 
 
 BAS 
 
 29 
 
 Background. In taking portraits, it is generally necessary to 
 place a background behind the sitter. This is made by stretching a 
 sheet of canvas, previously wetted, on a deal frame, and painting it 
 of an appropriate colour in distemper. The canvas should be in 
 one piece, and not less than eight feet square. When nailed on to 
 the edges of the frame in a wet state, it contracts on drying, and 
 becomes perfectly flat and tight. The water in which it is wetted 
 should be strongly sized : it will then be ready for painting on, when 
 dry. Oil colour is objectionable, from its imparting a glaze to the 
 surface. The colour should be perfectly dead and opaque, and of a 
 neutral tint, made by mixing black, white, and red, in the propor- 
 tion which is thought most desirable. As a general rule, the back- 
 ground should be a shade darker than the middle tints of the picture, 
 but in vignetted portraits it may be a shade lighter. A shaded 
 background is a great improvement to a portrait when judiciously 
 done, but it involves so much extra trouble, that few professional 
 portraitists have attempted it, as a rule. Painted backgrounds in 
 which peeps of distant scenery, bits of balustrade, columns, curtains, 
 &c., are introduced, are decidedly in bad taste. A very light back- 
 ground is also in general an offence against good taste, particularly 
 when the figure is very dark, and the outline hard and sharp. 
 
 Barium. A metal : used only in combination with chlorine, &c. 
 
 Barium, Chloride of, Iodide of &c. See “ Chloride,” &c. 
 
 Baryta. The protoxide of barium. This oxide is precipitated 
 from its solutions by many organic matters, forming either neutral 
 or basic organic salts. These exist, in combination with similar 
 salts of silver, in the presence of chloride and nitrate of silver, in 
 many printing formulae, and modify the action of the actinic ray, 
 and the nature of the reduced substance. Hence the colour of the 
 photograph is different, according as the salt first used was one of 
 barium or some other metal. Barium appears to have considerable 
 colorific power, and therefore chloride of barium is often recom- 
 mended as superior to chloride of sodium in the salting of paper. 
 
 Baryta, Acetate of, Albuminate of, &c. See “ Acetate,” &c. 
 
 Barytes. Synonymous with baryta, but not equally correct. 
 
 Base and Basic. In chemistry, that element of a compound 
 which most resembles hydrogen, and is most unlike oxygen in its 
 chemical and electro -chemical relations, is called the base, and in 
 
30 
 
 BAS 
 
 substances composed of two simpler compounds, that compound 
 which is like hydrogen is still the base. In electrolysis, the base 
 travels to the cathode or negative metal, and is therefore called 
 electro-positive ; while the acid or other body with which the base 
 was combined, goes to the anode or positive pole, and is called 
 electro-negative. The base travels in the direction of the electric 
 current, and the acid against the current. The same relation 
 appears to exist between these anions and cathions, in actino or 
 photo-chemistry ; for bases are eminently favourable to actinic 
 action, while acids are strongly opposed to it. Alkalis are very 
 strongly basic, and exert a remarkable power as accelerators, while 
 acids are strongly retardative. Of compounds in which the acid 
 and base completely neutralize each other, the action upon bodies 
 undergoing actinolysis is neutral also ; hence nitrate of potassa may 
 exists in the nitrate of silver solutions in considerable quantity, and 
 yet give no indication of its presence. When a compound not 
 neutral, contains both acid and base, it is said to be acid or basic 
 according as the acid or basic element is in excess, and retards or 
 accelerates as a pure acid or alkali would do. These facts have not 
 been attended to sufficiently, and hence the discordant opinions of 
 different photographers respecting the useful or injurious properties 
 of the same substance. It should be carefully noted that every 
 compound, whose effects are to be tried, is strictly neutral, or allow- 
 ances made for its want of neutrality. 
 
 Basic Nitrite of Silver. Nitrite of silver is a neutral 
 compound of nitrous acid and oxide of silver ; but when the oxide of 
 silver is in greater quantity than is required to neutralize the acid, 
 the salt becomes basic, is alkaline to test paper, and acts as an alkali 
 in N. S. solutions. Bused nitrate of silver often contains this nitrite. 
 It then is so strongly acted on by developers, as to give a picture 
 quickly with a weaker developer than would otherwise be necessary ; 
 for gallic acid will develope collodion with its assistance, and with the 
 ordinary pyrogallic the picture is formed instantly, and with great 
 force. Crystallized nitrite of silver is sometimes acid, and acts in an 
 opposite manner. Basic salts of lead are sometimes added to nitrate 
 of silver solutions, with the view of forming this basic compound of 
 silver. Its effect in giving intensity to the image is not always 
 observed, but is most apparent in those cases where there is a com- 
 plete absence of those organic or other matters which usually have a 
 like influence. 
 
 Basic Oxides. Some compounds of oxygen with bodies^ have 
 
BAS 
 
 BEN 
 
 31 
 
 acid properties, and others are neutral, but the protoxides of the 
 metals, and some others, are more or less basic. Water is a neutral 
 oxide of hydrogen. 
 
 Basic Persulphate of Iron. The red powder which is gra- 
 dually deposited in solutions of the protosulphate used as a developer 
 is a basic persulphate; that is, it contains sulphuric acid and 
 peroxide of iron in neutralizing proportions, together with peroxide 
 in addition. Now, as the solution was originally without excess of 
 sulphuric acid, it follows that as more of the base has precipitated 
 in this red powder than of the acid, the solution after this decomposition 
 must contain too much acid. This is the reason of its being weaker 
 than at first, and if you neutralize the acid its energy will be 
 restored. 
 
 Bath. This name is given to any photographical solution, in 
 which plates or papers are immersed, or on which they are floated in 
 any process. It is also given to the vessels in which such solutions 
 are contained. The principal baths are nitrate of silver, the fixing, 
 the toning, and the washing baths, and are treated in their order. 
 The vessels have been constructed of various forms, horizontal and 
 vertical, and of various subtances, glass, porcelain, gutta percha, &c. 
 The porcelain baths are not made of material suitable for resisting 
 the action of the chemicals, or they would be the best, because they 
 are opaque, and are easily procured of any size. Gutta percha is 
 most generally in use, but it is apt to communicate and to receive 
 impurities. Glass, if it could be cast of the required forms and 
 sizes, is undoubtedly to be preferred to anything else, notwith- 
 standing its brittleness. The cemented glass vessels come to pieces. 
 
 Bath. Aceto-nitrate bath, fixing bath, &c. See “ Aceto-nitrate.” 
 
 Beaufoy’s Acetic Acid. Commercial acetic acid, sold by 
 druggists under that name. It should be of the same strength of 
 tbe strongest acid of the London Pharmacopoeia. 
 
 Bellows Camera. See “ Cameras.” 
 
 Benzine. Another name of “benzole/’ q. v. 
 
 Benzoate of Silver. Ag O C 14 PI 0 3 . When formed by 
 double decomposition, this is a white curdy precipitate. It dis- 
 solves in boiling water, and falls in crystalline scales as the solution 
 
32 
 
 BEN 
 
 BIC 
 
 cools. Like other organic salts of silver, it becomes brown under 
 the action of light, and may be used as the basis of a printing- 
 process. 
 
 Benzoic Acid. Obtained from benzoin by sublimation or by 
 neutralizing the acid which exists ready formed in the benzoin with 
 alkalis ; it occurs as acicular crystals, or a white crystalline mass. 
 Liebig found bitter-almond oil having the composition C 14 H 6 0 2 to 
 be converted by oxidizement in the air into benzoic acid and water, 
 which was combined with the acid. He therefore assumed the oil 
 to be a hydruret of a compound radical, which he termed Benzoyle 
 or Benzule C 14 H 6 0 2 . This, taking another atom of oxygen, 
 became benzoic acid, while the hydrogen, taking another atom, 
 became water. This acid is decomposed by chlorine in the sun’s 
 rays. Of the 5 atoms of hydrogen which the acid contains, three 
 can be successively replaced by chlorine, giving rise to three new 
 acids, which so closely resemble each other that they can only be 
 distinguished by destructive analysis. 
 
 Benzole. This must not be confounded with benzoyle or ben- 
 zule. It is a limpid, colourless liquid, S. Gr. 86, and is obtained by 
 decomposing benzoic acid by slaked lime. Its composition is C 12 
 H 6 , and when acted on by the sun’s rays in the presence of chlorine, 
 it takes 6 atoms of chlorine, becoming a crystalline solid with the ; 
 formula C 13 H Cl . Its use in photography is as a solvent of bitu- 
 men in Niepce’s processes, and as a solvent of certain gums for mak- 
 ing a clear varnish. See “ Varnish.” 
 
 Berlin Ware. This is a kind of pottery marked with a blue 
 stamp, and of such a quality as to resist the action of nitrate of 
 silver, even when fused in it. Ordinary ware will not do for this 
 purpose, and the porcelain dishes used in photography are altogether 
 very inferior. 
 
 Bi, prefix. See “ Nomenclature.’’ 
 
 Bicarbonate of Soda. Na 0, 2 C 0 2 , HO. The salt called by 
 this name in commerce is often a sequicarbonate. Both are used to 
 form with citric acid, citrate of soda ; and this, to produce citrate 
 of silver in printing, q. v. The bicarbonate is so called, because it 
 contains 2 atoms of carbonic acid to one of soda. 
 
 Bichloride of Mercury. Hg Cl 2 . Commonly called 
 
 
BIC 
 
 33 
 
 “ corrosive sublimate,” and formed thus : — 21bs. of mercury are 
 boiled with 30 oz. of sulphuric acid to dryness; it is then rubbed 
 to powder with 41bs. of chloride of sodium, and the mixture exposed 
 in a proper vessel to a red heat. Corrosive sublimate rises and sul- 
 phate of soda remains. A solution of this in water or ether is 
 decomposed by light ; one atom of the chlorine becomes hydrochloric 
 acid, and chloride of mercury or calomel is deposited, being inso- 
 luble. The solution itself is also of use in photography. A process 
 for whitening collodion positives (which see) has been contrived with 
 its assistance ; and either alone or with other salts it will deepen the 
 tone of a positive to a degree sufficient to allow of its use as a nega- 
 tive. Similar reactions accompany its employment on paper 
 positives ; for the picture is first whitened, and then either reap- 
 pears of itself, or is reproduced by ammonia or other solvents of 
 chloride of silver. By this process collodion pictures are deepened 
 but paper ones are weakened, and both these effects have been taken 
 advantage of. In the processes for photographing the solar spec- 
 trum, a subchloride of silver is always had recourse to; one 
 means of producing which is the immersion of a silver plate in the 
 aqueous solution of this salt. Its whitening power on reduced silver 
 may be usefully applied to decolorizing stains on the hands or linen 
 and to removing the yellow colour of the whites in over-toned 
 positive prints. 
 
 Bichloride of Platinum. Pt. Cl 2 . This perchloride of 
 platinum is formed by dissolving the metal in nitro-hydrochlorie 
 acid, and evaporating to dryness. The dark brown residue forms a 
 deep yellow solution in water, which, when quite neutral, and added 
 to lime water, gives a copious white precipitate in the sun’s rays. 
 It has been proposed to determine by the weight of this the actinic 
 power of the light at the time. The aqueous solution has been tried 
 as an etching liquid in engraving photographs on steel plates ; and 
 as a toning agent for paper positives instead of chloride of gold. 
 
 Bichromate of Potassa. KO, 2 Cr 0 3 . This salt is largely 
 manufactured for calico-printers. Its solution should not be brought 
 into contact with the skin, as it sometimes causes sores difficult to 
 heal. One ounce in ten of cold water forms a saturated solution ; 
 it is much more soluble in hot water. It easily gives up one atom 
 of oxygen to deoxidizing bodies, becoming neutral chromate and 
 binoxide of chromium. The binoxide has a tendency to be reduced 
 still further to sesquioxide. When the chromic acid is set free from 
 the potassa by sulphuric acid, it becomes a strong oxidizer, by the 
 
 D 
 
34 
 
 BIC 
 
 BIN 
 
 facility with which it parts with oxygen ; and such a mixture of 
 bichromate and sulphuric acid will determine whether a print has 
 been toned with gold or sulphur, by the greater resistance which 
 gold offers to its oxidizing influence. Light will also act upon the 
 bichromate in the presence of organic matter, and precipitate insolu- 
 ble oxide of chromium in combination with that matter. This 
 property has been taken advantage of as the basis of several photo- 
 graphic processes, and the chemical action is strictly analogous to 
 that of the use of nitrate of silver alone on paper. A visible image 
 is impressed on the paper, on which reduced silver and other metals 
 (iron, &c.) may be afterwards precipitated by suitable developing 
 solutions. When protosulphate' of iron and gallic acid are employed, 
 the picture, or its chemical composition, resembles writing ink. The 
 iron and other metallic solutions are sometimes presented, with the 
 bichromate in the paper, to the action of light, and the variety of 
 modifications appears to be infinite. The insoluble compounds of 
 chromium, formed by light, and the action of the light itself, have 
 both been used in the art of dyeing, the chromium forming a 
 mordant on the textile fabric, in parts exposed to light through a 
 perforated pattern, on which mordant the colours are subsequently 
 applied ; or the colours may be put on with the bichromate, and 
 subsequently washed out from the parts not acted on by the light, 
 just as in a similar photographic process on paper. Papers prepared 
 with bichromate of potassa and nitrate of silver have also been 
 found to give images varying in colour from red to green, and blue. 
 We believe, before the publication of the talbotype and daguerreotype 
 processes, if not before that of Niepce’s, fixed pictures were taken by 
 Mr. Mongo Ponton, by means of this salt, and the process published. 
 See “Printing.” 
 
 Biconcave. See “ Lens.” 
 
 Biconvex. See “ Lens.” 
 
 Bin, prefix. See “Nomenclature.” 
 
 Binocular Vision. Vision, when both eyes are employed. 
 The term is used in contradistinction to monocular vision, in 
 which only one eye is employed. 
 
 The effects produced by binocular vision are very remarkable. The 
 subject will be found discussed at some length in the article 
 “ Vision,” q. v . 
 
BIN 
 
 BLA 
 
 35 
 
 Binocular Perspective Portraits. This term has been 
 applied to photographic portraits produced in a camera which is 
 made to revolve in a horizontal plane through an angle, its axis 
 being always directed to the same part of the sitter. It was sup- 
 posed that in this way a single portrait could be obtained, which 
 would exhibit the same effects of solidity as duplicate pictures viewed 
 in the stereoscope. This notion is, however, erroneous, as will be 
 found explained in the article on the “ Theory of the Stereoscope.” 
 
 Binoxide of Nitrogen. N 0 3 . This gaseous compound has 
 been recommended on account of its reducing properties, as a useful 
 addition to the ordinary developers, with a view to increase their 
 energy. This is a mistake ; when passed into gallic or pyrogallic 
 acid it is decomposed, and when it is absorbed by sulphate of iron, 
 it acts more as an acid in moderating its energy and whitening the 
 deposit, than as an accelerator. Old solutions of sulphate of iron 
 which contain it, or are said to contain it, are not so energetic as 
 freshly made ones. Though they give better colour and middle 
 tints, they require longer exposure, and nitric acid is more convenient 
 than the binoxide, and will produce similar effect. 
 
 Bistre Tints. See “ Toning.” 
 
 Bisulphate of Potass. K 0, 2 S 0 3 . This salt is formed in 
 the process for making pyroxyline with sulphuric acid and nitre, 
 and adheres with such force to the fibres as to require extra good 
 washing. As it has an acid reaction, its presence in collodion would 
 be injurious. One part of it mixed with 4 parts of sulphate of 
 iron has been used as a developer. Its acidity is here the useful 
 quality, and citric acid would be still more useful and convenient. 
 
 Bitumen. Mineral pitch or tar, supposed to be formed in the 
 earth from the decomposition of vegetable substances. The semi- 
 fluid sorts are distinguished as naphtha, rock-oil, petroleum, or 
 Barbadoes tar; the solid bitumen is distinguished as asphaltum, 
 Jews’ pitch, &c. See “Asphalt, Photo-engraving, Photo-litho- 
 graphy.” 
 
 Black Glass. See “ Glass.” 
 
 Black Japan. See “ Japan.” 
 
 Black Sensitive Paper. Papers have been prepared with certain 
 
 D 2 
 
36 
 
 BLA 
 
 BLE 
 
 black chemical compounds, and also with mere mixtures of a black 
 or very dark colour, and sensitive to actinic impressions. In some 
 the lights are bleached into existence by the light itself ; in others 
 they are developed by some solvent or chemical process afterwards. 
 They, may be used for the production of either positives or negatives. 
 
 Black Tone of Prints, “Toning.” 
 
 Black Tarnish. See “ Varnish.” 
 
 Blacking for the Inside of Cameras, &c. Of all things 
 which have been tried for covering the inside of lens tubes, cameras, 
 and other pieces of apparatus, nothing has been found so suitable as 
 black cotton velvet. Tubes are sometimes blackened by a solution 
 of bichloride of platinum in water, which precipitates platinum as a 
 smooth black layer over the metal on which it is brushed. Some- 
 times bone black or ivory black is ground up with water, and a 
 little glue to a proper consistence, and applied as water colour to 
 the surface. But both the colour and texture of black velvet are 
 infinitely superior for this purpose to either platinum or charcoal ; 
 the pile of the velvet acting as so many little wells, into which the 
 light enters and is lost. Nothing takes a longer exposure than this, 
 if it be desired to photograph it. 
 
 Bleaching. Light has a very powerful effect in bleaching 
 linens, woollen, and other fabrics from which the colour is capable 
 of being discharged, though its effects in this way have not received 
 the attention which they demand. The action of light has often 
 been described as a deoxidizing action, but this is a very loose and 
 incorrect statement of facts. There is no doubt that its effects on 
 some bodies are thus correctly described, but it is equally certain 
 that it exerts on other bodies the powers of an oxidizer ; the oxygen 
 which it removes from one class of substances it causes to combine 
 with another class. Its effects, for instance, on organic compounds, 
 are many of them deoxidizing, but are of an opposite character on 
 most organic matters. Nascent oxygen is found in chemical opera- 
 tions to have a powerful bleaching influence, and it is probable that 
 in those cases where light is applied as a bleaching agent, its 
 specific action is to ozonify the oxygen of the air in contact with the 
 coloured material, and thereby cause it to unite with and destroy 
 the dye. Chlorine and some of its compounds are strong bleachers, 
 but it is found that they are nearly or quite inert if no moisture is 
 present : the chlorine decomposes tlie water, uniting with its hydro- 
 
BLI 
 
 37 
 
 gen to form hydrochloric acid, and liberating oxygen, which is the 
 real bleaching element. Now, the decomposition of water by chlo- 
 rine in the sun’s light lies at the foundation of photography, and 
 hence the superior efficacy of chlorine and light together over chlo- 
 rine alone. In many cases the chlorine itself is combined with 
 organic matter by the sun’s power, just as oxygen is ; and it should 
 be remembered that chlorine, bromine, iodine, and oxygen are, as to 
 their electro- chemical and actino-chemical properties, bodies of the 
 same class. If light is a deoxidizer, it is also a deodizer and de- 
 chloridizer, and if under certain conditions light becomes an oxidizer, 
 it may also in analogous cases become an iodizer. Thus, in photo- 
 graphy we not only reduce iodide of silver, but we also, by changing 
 the conditions, form it. Amongst “ Miscellaneous Processes ” will 
 be found some where reduced and blackened silver salts are again 
 compounded with chlorine, &c., and bleached. That effect, which 
 has been called the reverse action of light, where the usual positive 
 and negative effects are reversed, is probably an analogous chemical 
 change. The substances used in the arts in bleaching photographic 
 and other papers, and otherwise preparing them for the market, are 
 often injurious; this fact should be attended to, and at the same 
 time photographers will do a kindness to bleachers, dyers, and 
 others, if they can make their art practically influential in the arts in 
 general. 
 
 Blistering. Blistering of the film is a defect which sometimes 
 occurs in albumen processes on glass. It happens on spots where the 
 albumen does not adhere tightly to the glass. The albumen film, like 
 every other film which is capable of imbibing moisture, expands on 
 being wetted. Now should any part of the film be non-adherent to 
 the glass while the surrounding part adheres tightly, the non-adher- 
 ing part will expand on being wetted, and raise itself into a blister. 
 The proper remedy for this defect appears to be, first, to clean the 
 glass with ammonia or soda instead of alcohol or nitric acid ; and 
 secondly, to add a little ammonia to the albumen. The reason of 
 this will be evident from the following considerations. Albumen is 
 a sticky substance, and will adhere to glass like any other sticky 
 substance, provided its surface next the glass be not coagulated , 
 Should, therefore, any trace of nitric acid or alcohol remain on the 
 glass, it would be likely to coagulate the albumen, and render the 
 film non-adherent in those parts, while the presence of an alkali, 
 either in the glass or the albumen would tend to prevent coagula- 
 tion, and the albumen would adhere to the glass when dry, in the 
 same way as any other sticky varnish. In all the albumen processes 
 
38 
 
 BLO 
 
 BLU 
 
 on glass, the nitrate bath is strongly acid, and therefore an alkali 
 may be added to the albumen with perfect safety. 
 
 The cases of blistering which occur in the albumenized collodion 
 process on glass may be traced to the same cause. The film of 
 collodion is not sufficiently adherent to the glass in those parts where 
 blistering occurs, to prevent the swelling of the albumen from 
 forming a blister. In this process the collodion film should be as 
 porous and adhesive as is consistent with the fulfilment of other 
 conditions ; the nitrate bath in which the collodion is excited should 
 be either neutral or slightly alkaline, and the excess of nitrate of 
 silver should be thoroughly washed out of the collodion film. 
 
 Blotting Paper, See “ Filtering Paper.” 
 
 Blue Glass. The question whether blue or white glass is the best 
 for the portrait-room, has been frequently discussed. Blue glass 
 increases the time of exposure more or less, according to the depth 
 and shade of colour, but not to such an extent as to constitute a 
 practical objection to its use. On the other hand, it acts beneficially 
 in modifying the glare of light which is sometimes injudiciously 
 admitted into the glass room, and in this way relieves the eye of the 
 sitter from a painful annoyance, which causes an unpleasent ex- 
 pression. Blue glass stained with cobalt is said to be very perma- 
 nent in colour. White glass containing manganese is acted on by 
 light, and becomes of a reddish tint, which is very objectionable. 
 When the glass room has a good aspect, and collodion positives only 
 are required to be taken in it, glass slightly tinted with blue may be 
 very advantageously employed. 
 
 Blue Meniscus. It has been proposed to use a blue meniscus 
 instead of an achromatic lens for taking views, partly on the ground of 
 economy, and partly because it was believed that a better picture 
 might be produced. But these supposed advantages of a blue lens 
 have not yet been demonstrated practically ; on the contrary, a blue 
 lens appears to have the disadvantage of increasing the time of 
 exposure, and of yielding an inferior picture to that produced by 
 an achromatic lens. This might have been expected; for an achro- 
 matic lens not only combines two of the coloured foci, but com- 
 presses together, so to speak, all the other coloured foci, some of 
 which are more or less actinic; while a blue lens merely stops 
 certain coloured rays from passing through it, but leaves the foci 
 of those which do pass through very widely separated. It seems 
 impossible to obtain a satisfactory picture with either a colourless or 
 
BLU 
 
 BBO 
 
 39 
 
 blue meniscus, in consequence of their having no single actinic focus 
 in which the principal actinic rays are collected. Besides this objec- 
 tion, no single lens can have its spherical aberration diminished or 
 corrected when its focal length and the radius of its anterior surface 
 are given quantities ; as happens in the case of the landscape lens. 
 
 Blue Bays. See “ Spectrum/’ 
 
 Body. This term is applied to substances, in allusion to the 
 quantity of stuff or substance which they contain ; in meaning it 
 is opposed to thinness, weakness, transparency, and flimsiness. 
 Hence we have papers without body and with body ; body colour in 
 opposition to transparent colours ; and photographs or pictures with 
 no body, in opposition to those which have plenty of reduced 
 material and of vigour. Beside the starch or glue sizing, inorganic 
 substances, as china, clay, &c. are often added to papers to give to 
 them body; for their effects in photography, see “Paper.” The 
 amount of body in a collodion film is also a matter of importance. 
 
 Boracic Acid. B 0 3 . This acid has been used in the propor- 
 tion of 10 grains to the ounce of sulphate of iron developer, instead 
 of acetic or nitric, and that to produce negatives on collodion. It 
 has also been employed so to modify the dark subchloride of silver, 
 which is impressible by the solar spectrum in such a way as to give 
 the prismatic colours, that its power of rendering the green rays is much 
 increased. This application was suggested by the quality it has of 
 producing a very peculiar green colour in the flame of alcohol. 
 
 Borate of Lead. Pb O B 0 3 . Has been tried as a basis in 
 paper on which nitrate of silver was applied, but without apparent 
 effect on the reduction of the nitrate in the light. 
 
 Borate of Silver. Ag O, B 0 3 . Formed in paper by applica- 
 tions of borate of soda (borax) and nitrate of silver, and capable of 
 being printed on. 
 
 Breadth. This term is used by artists to denote the proper 
 balance and relative value that should exist between the lights and 
 shades of a picture. It is used in contradistinction to the term 
 “ spottiness ” of effect — a fault which may be illustrated by referring 
 to the appearance of the squares of a chess-board. 
 
 Bromate of Potassa. K 0, Br 0 3 . An impurity occurring in 
 
40 
 
 BEO 
 
 bromide of potassium, probably not injurious, and easily converted 
 into bromide by fusion at a red beat. 
 
 Bromide of Aluminium. Obtained by dissolving gelatinous 
 alumina in hydrobromic acid, and evaporating to dryness ; and when 
 used in collodion resembles the corresponding ammonium compound. 
 
 . Bromide of Ammonium. N H 4 Br. This has been used in 
 the preparation of bromided collodion, in the proportion of 4 grains 
 to the ounce, and with a 60 grain N. S. bath ; and has been said to 
 possess the property of giving an intense image, even in feeble light, 
 and of representing greens and yellows with more correctness than 
 the iodides. These properties will be discussed under “Bromide of 
 Silver,” and “ Bromides,” 
 
 Bromide of Arsenic. As.. Br 3 . This is a ter-bromide, and is 
 formed directly by the action of bromine on arsenic. It has been 
 added to collodion as a preserver and as an accelerator, but it is 
 very improbable that it can be both.. In the collodion film it will 
 be decomposed with the formation of hydriodic acid and arsenious 
 acid (common arsenic), and the accelerating power, if any, will be 
 due to the arsenious acid, which will favour the development, for it 
 sometimes has even an alkaline reaction. 
 
 Bromide of Cadmium. Cd. Br. Occurs in acicular crystals 
 or nacreous scales, and is formed by mixing bromine and filings of 
 cadmium in water ; the acicular crystals contain water, but when 
 sublimed, it condenses in the anhydrous pearly scales. It is pre- 
 ferred by many to the other bromides* for the same reasons as the 
 corresponding iodides, viz., its stability, solubility, and purity. 
 
 Bromide of Calcium. Ca. Br. Obtained by digesting 
 hydrate of lime in a solution of protobromide of iron, by heating 
 lime in bromine vapour (not bromide of lime, q. v.) } or by dissolv- 
 ing carbonate of lime in hydrobromic acid. It is white and deli- 
 quescent, and its aqueous solution yields silky hydrated crystals. 
 Collodion, prepared with this bromide, is said to improve with age. 
 It is not so good as bromide of potassium for preparing sensitive 
 papers for keeping, because the nitrate of lime formed is deliquescent, 
 and keeps the paper moist, and therefore more prone to deterio- 
 ration. 
 
 Bromide of Gold. Au.. Br 3 . Aqueous bromine dissolves gold, 
 
mo 
 
 41 
 
 and forms a dark grey bromide. This has been added to collodion 
 containing no iodide, and treated in the visual way. The film thus 
 prepared would contain auro-bromide of silver, and was found to 
 be instantaneously impressed, but apt to fog completely over the 
 plate. 
 
 Bromide of Iodine. Crystals of iodine absorb bromine, and 
 form first a solid compound of one atom of each element, but after- 
 wards become liquid, when the compound contains two atoms of bro- 
 mine to one of iodine. This compound is soluble in water, and the 
 solution is employed as an accelerator in the daguerreotype process. 
 
 Bromide of Iron. Be. Br. Used in collodion, but, to be use- 
 fid, must be used in its pure state, and soon after mixing, as it 
 rapidly decomposes the collodion. Nor must the sensitive plate 
 be kept long before development, since the nitrate of iron, which it 
 then contains, would produce irregular action. If carefully em- 
 ployed. the bromide of iron assists development sufficiently to make 
 a very sensitive surface. 
 
 Bromide of Lime. This, in consequence of its solidity and 
 anhydrous condition, is the most convenient accelerator in the 
 daguerreotype process. It neither gives off aqueous vapour to be 
 condensed on the plate, nor is so awkward to manipulate as the 
 liquid accelerators. In forming it, the bromine is placed in a 
 capsule a little above good hydrate of lime in a close flask. It is 
 necessary to keep the whole as cool as possible, or more or less 
 bromide of calcium will be formed, which would be useless. The 
 bromide of lime is of a beautiful scarlet colour, the calcium salt 
 white. If much heat occurs, the whole of the bromine may go to 
 form this white substance. The proper compound is an oxi-broinide, 
 easily giving off the one atom of bromine and becoming oxide ; the 
 other gives up no bromine except to strong acids. 
 
 Bromide of Magnesium. Mg. Be. A salt occurring in sea 
 water, and the source of bromine. 
 
 Bromide of Mercury. Has been recommended for collodion, 
 but the protobromide is insoluble. And the perbromide, containing 
 two atoms of bromine, would liberate nitric acid in the N. S. bath 
 and film, and therefore produce a very insensitive and feeble film. 
 
 Bromide of Nickel. Ni. Br. A deliquescent salt of a whitish 
 
42 
 
 BEO 
 
 colour. It becomes oxidized by tbe air when in solution, and turns 
 red. It is not so good as the pure alkaline bromide for photo- 
 graphic purposes. 
 
 Bromide of Potassium. K. Br. When pure, this is the most 
 convenient bromide to select, when a sensitive surface containing 
 bromide of silver is to be formed by double decomposition. Its 
 usual impurities are carbonate of potassa and chloride of potassium, 
 derived from the carbonate used in its manufacture. The first of 
 these, from its alkaline nature, will be injurious if in great excess ; 
 but it is easy to get rid of it by adding bromine to the aqueous 
 solution of the bromide, until a little colour is produced, and after- 
 wards evaporating and fusing. Bromide of potassium has been of 
 service on some occasions, in partially fixing negative and positive 
 proofs, which it does by forming, with the unreduced and insoluble 
 silver salt, a soluble double salt of silver and potassium. Other 
 bromides, and many iodides, chlorides, and cyanides, act in the 
 same way, and even nitrate of silver. These double salts are all 
 partially decomposed by the smallest dilution of their conceiftrated 
 solutions with reprecipitation of the insoluble silver salt, and their 
 solvent power increases wonderfully with their degree of concentra- 
 tion. They must, consequently, be used in large quantities, when 
 water is to be afterwards freely used in their removal. Hypo- 
 sulphite of soda comes under the same rule, and cannot be advan- 
 tageously had recourse to except this law be remembered. The 
 principal use of this and the other bromides (see “Bromide”), in 
 photography, has been, in the collodion and paper processes, to 
 form a mixture of bromide of silver with iodide in the sensitive 
 surfaces, and it has been occasionally employed alone, without 
 any iodide. Tor the advantages sought for in this mixture, see 
 “ Bromide of Silver,” in the next article. 
 
 Bromide of Silver. Ag. Br. An insoluble salt of silver, of 
 a yellowish colour, and thrown down whenever bromine in solution, 
 or any soluble bromide, is added to nitrate of silver. It is soluble 
 in concentrated solutions of nitrate of silver, and of various iodides, 
 bromides, and analogous salts, and to a very minute extent in their 
 dilute solutions. Like other insoluble silver salts, it becomes darker 
 in colour in the sun’s light, when nitrate of silver, or other soluble 
 salt of that metal, is present ; and, like them, it is capable of receiv- 
 ing a latent impression by light, capable of development. When 
 mixed with iodide of silver, it has been supposed by some to give 
 greater sensitiveness, to increase the density of the image, and to 
 
BKO 
 
 43 
 
 produce a better rendering of objects which give off green rays. 
 All these points have been disputed by others, and the opinions as 
 to the accelerating, intensifying, and colour-rendering power of this 
 substance are very discordant. The difference of sentiment in per- 
 sons who have tried pictures with and without it, arises from want 
 of proper care in the conducting of the experiments, and especially 
 from want of observation as to the purity and neutrality of the 
 solutions employed, and as to the modifying effects of organic 
 matter of different kinds. Nor is any mistake more common than 
 to say one process is more sensitive than another, merely because, 
 with the same exposure to light, that one has given a stronger or 
 blacker colour in the shadows than the other. This difference may 
 exist, and yet the weaker picture may have been more completely 
 impressed by the actinic influence ; for the true test of sensitiveness 
 is the complete reproduction of the detail in the darkest shadows, 
 and not the vigour of the blacks themselves. When chloride, bromide, 
 and iodide of silver are precipitated in a pure state from nitrate of 
 silver in excess, and exposed to light, the chloride is the first to 
 appear discoloured, next the bromide, end lastly the iodide, and when 
 organic matter is present, the discoloration of all is more decided, 
 and the difference between them is still more marked than before. 
 There is much less difference between the iodide and bromide, than 
 between either and the chloride. When they are acted on by a 
 developer in the presence of organic matter, it is still found that such 
 matter has more effect in the coloration of chloride than of either 
 of the others, and that there is still a difference between the bromide 
 and iodide. But as to their impressibility by light, for the produc- 
 tion of a non-apparent image, the order of these three, in similar 
 neutral condition, is reversed, for the iodide is most quickly impressed ; 
 though, in the process of development, the organic matter has less 
 power of colouring and giving vigour to the image which it has 
 received, than in the case of the bromide, and a fortiori of the 
 chloride. In cases, therefore, where organic matter is present, to 
 affect the result, the addition of bromide, and still more of chloride 
 of silver, in small quantity, will give greater strength to the image ; 
 but, at the same time, the sensitiveness is also decreased in a slight 
 degree. But where organic matter does not interfere to a perceptible 
 extent, as in some collodion processes, no improvement will be pro- 
 duced in the sensitiveness of the film, and but little additional inten- 
 sity given to the picture, by the use of bromides in the preparation. 
 
 The greater sensitiveness to green rays which bromide of silver 
 was for a long time thought to enjoy, and for which it was much 
 Recommended in the photographing of foliage, is not in ordinary 
 
44 
 
 BEO 
 
 practice found to exist. The coloured rays, to which iodide of 
 silver is not sensitive, but which affect bromide, are a very incon- 
 siderable portion of those which are reflected by green objects in 
 daylight, and scarcely affect the result. But as they exist to a 
 greater proportional degree in artificial light of various kinds, the 
 admixture of bromide of silver will, when they constitute the illumi- 
 nating medium, be found beneficial. 
 
 In thus recommending the use of iodide of silver alone in general 
 operations, we speak only of average cases ; for there may occur 
 opportunities where the properties of bromide and chloride of silver 
 may be advantageously had recourse to; and different kinds of 
 organic matter introduced into the formulae, will no doubt produce 
 corresponding modifications in the results. It is not impossible that 
 bromide of silver may also, by its lengthening the time of exposure, 
 sometimes usefully affect the image imprinted on the iodide with 
 which it is combined. 
 
 Bromide of Sodium. Na. Br. A salt very closely resembling 
 the bromide of potassium. 
 
 Bromide of Zinc. Zn. Br. Is formed by passing bromine 
 vapour over heated zinc fused at a red heat, and is very deliquescent. 
 From its slightly acid properties it soon acts upon collodion. 
 
 Bromides. The most useful salts of this class have been 
 specified above. In the selection of the most suitable for the purpose 
 in hand, attention must be paid especially, 1st, to their freedom 
 from carbonates and oxides ; 2nd, to their stability in crystals and 
 in their proper solutions ; 3rd, to the nature of the nitrate which 
 they produce in the N. S. bath ; 4th, to their degree of solubility 
 in ether and alcohol, when they are employed in collodion. The 
 bromides of the metals cadmium, zinc, &c., are generally the purest 
 and most stable salts ; those of the alkaline and earthy bases are 
 more apt to contain alkaline matters. Bromide of potassium is not 
 easily soluble in ether and alcohol, and therefore in collodion is apt 
 to be partially precipitated in fine particles, which produce trans- 
 parent circular spots in the picture, and to produce, by its little * 
 solubility, a film of a misty appearance insensitive to light. The 
 nitrates of the alkalis introduced by their bromides into the exciting 
 bath are perfectly neutral, but those of the metals are slightly acid, 
 though not injuriously. The nitrate of zinc is deliquescent, and 
 helps, when formed in the collodion film, to keep it moist. Nitrate 
 of ammonia is often prejudicial, and therefore, bromide of ammonium, 
 
BEO 
 
 BUC 
 
 45 
 
 a very unstable compound in collodion, is generally to be avoided. 
 Bromide of iron is also prejudicial, from the decomposition which 
 the nitrate of iron sets up in the nitrate of silver, on the film and in 
 the bath. The choice lies, for most purposes, between the salts of 
 potassium and cadmium. 
 
 Bromine. (Bpwfiog a stench). A deep reddish brown liquid 
 of a disagreeable odour. In its chemical relations it resembles 
 oxygen, but is a less powerful body, and therefore many bromides 
 will at common, and still more so at high temperatures, absorb 
 oxygen. The displaced bromine then partly unites with the salt, 
 to form bibromide and oxibromide of a red colour, and partly 
 escapes. Its vapour is extremely poisonous ; a single drop of the 
 liquid on the beak of a bird, instantly kills it. Its aqueous solution 
 is used to give off bromine vapour in the daguerreotype process, but 
 is not so good for this purpose as the bromide of lime. Bromine 
 has been added to collodion, but it has no advantage over iodine, 
 since the effect of both is the same, viz., to liberate nitric acid in 
 the bath and film. 
 
 Bromo-iodide of Lime. A mixture of bromide and iodide of 
 lime. 
 
 Bronze in Photographic Printing. In many processes, 
 both of sun-printing and development, the darkest parts of the im- 
 pression assume after a time an olive green colour, lighter than the 
 dark brown which immediately preceded it. This has been called 
 bronze, and its production bronzing. It occurs when the paper is 
 rich in silver salts and organic matter, and especially when there is 
 abundance of free nitrate and the exposure is prolonged. With 
 very vigorous negatives to print from, this may be excessive, but 
 there are means of preventing and also of remedying the extreme 
 action. The paper must be rendered less sensitive, or the bronzing 
 can be removed by chloride of gold and other means. See 
 “ Printing.” 
 
 Brown Collodion, Gallic Acid, Hyposulphite of Soda, 
 Nitrate of Silver, &c. See “ Collodion,” &c. 
 
 Brunswick Black. Por the composition of this black varnish, 
 see “ Varnish.” 
 
 Buckle’s Brush. The little article which passes by this name 
 
46 
 
 BUB 
 
 BUB 
 
 is made by pulling out some fine cotton wool by the fingers, which 
 is then placed in a hook formed at the end of an annealed copper or 
 silver wire, and drawn tightly into a suitable glass tube to serve as 
 a handle. It may be used on albumen or paper surfaces, and is, in 
 many respects, the most convenient method of applying chemical 
 solutions. The solutions, when it is used, remain constant in 
 their action, and the papers are more uniformly of the same strength ; 
 camel-hair and other brushes are immediately destroyed by nitrate 
 of silver ; this brush may be hourly renewed. 
 
 Buff, and Buffing. A buff is used in the daguerreotype pro- 
 cess, for polishing the silver plate, and in the glass processes, for 
 polishing the glass plate. In the former process, it may be either a 
 hand-buff or a buffing-lathe. The hand-buff is made of a piece of 
 deal about sixteen inches long and five inches wide, having a handle 
 at one end, and covered with velveteen, the fine ribs of which are 
 laid across the buff. The buffing-lathe is a wheel in the shape of 
 the frustrum of a cone, the axis of which is the axis of rotation, 
 covered with wash-leather or doe-skin, and turned by the foot. 
 Hand-buffs are generally kept in pairs, face to face, to prevent 
 particles of dust from settling on the velveteen. The hand-buff used 
 for polishing glass plates, has a handle on the back, like a blacking 
 brush, the front being padded, and covered with wash-leather. 
 
 To use the buffing-lathe, the wheel is put into rapid motion by 
 means of the treadle worked by the foot, and the plate held against 
 the leather. In using the hand-buff, the operator puts an old kid 
 glove on the right hand, takes the handle of the buff in the left, 
 rests the end of it on the edge of a table, and lays the face of the 
 plate on the velveteen ; then, putting the fingers of the right hand 
 on the back of the plate, he rubs it briskly backwards and forwards 
 until it has received a sufficient polish ; the plate being turned 
 occasionally diagonally, and cross ways, on the buff, and consider- 
 able pressure being employed. 
 
 A glass plate should first be rubbed dry with a cambric handker- 
 chief, then laid on a pad of paper, and rubbed in every direction 
 with the buff, until the dew from the breath condenses on it in an 
 even sheet, without exhibiting marks or patches. 
 
 The plate, whether metal or glass, should be polished immediately 
 before use, and the clean surface should on no account be touched 
 with the fingers. 
 
 Burning-in. It has been attempted to use photography for 
 ornamental purposes, and also to give increased permanence to 
 
BUB 
 
 CAL 
 
 4f 
 
 photographs, by burning them into glass and ceramic ware. Very 
 little has yet been done in this direction, as the silver which forms 
 the images of an ordinary photograph turns yellow in the furnace, 
 and photographic processes with other metals which yield good 
 colours after toning are at present in an imperfect state. It is 
 possible, however, to apply the arts of photo-lithography, and photo- 
 galvanography, to the purpose of ornamenting ceramic ware, as 
 these processes yield prints in common inks of various colours, which 
 may be treated at the potteries in the ordinary way in which ink 
 patterns are burnt in. 
 
 It is impossible to attach too much importance to the burning-in. 
 processes of photography, since the permanence of a pure photograph 
 is more likely to be secured in this than in any other way. 
 
 Burnishing Photographs. See “ Agate ” and “ Mounting.” 
 
 Burnt-up. A familiar phrase applied to photographs, when, 
 having been very much over-exposed to light, they have a weak 
 misty appearance, and a faint reddish colour. This effect is also 
 sometimes improperly called solarization. 
 
 Cadmium. This metal in appearance much resembles tin, but in its 
 chemical qualities it still more nearly resembles zinc, in the ores of 
 which it is found Zinc is the only metal more electro-positive than 
 cadmium, and therefore the only one which will precipitate it from 
 its solution in the metallic state. It is a scarce metal. Air and 
 moisture scarcely act upon it except it is heated, and this quality 
 makes it valuable in photography, because its salts in collodion are 
 extremely stable, and in the crystalline state they have little tendency 
 to deliquescence and oxidizement. 
 
 Cadmium, Bromide op, Iodide of, &c. See “ Bromide of. 
 Cadmium,” &c. 
 
 Calcium. The metal of which lime is the oxide ; but little has 
 been ascertained of its properties by actual experiment. Its com- 
 pounds with the halogens are generally deliquescent, and prone to 
 oxidizement. 
 
 Calcium, Bromide of. See “ Bromides.” 
 
 Calomel. Hg. Cl. Protochloride of mercury. This salt is 
 fonned in the collodion positive process, when bichloride of mercury 
 
48 
 
 CAL 
 
 in solution is poured over the picture. Ammoniacal gas blackens it, 
 but heat restores the colour, and calomel remains unchanged ; liquid 
 ammonia decomposes it, and black suboxide of mercury is one of the 
 results. Some chemists, who take 100 instead of 200 as the chem- 
 ical equivalent of mercury, call this ^‘-chloride of mercury or Hg 2 Cl. 
 
 Calorific Rays. The heat rays of the spectrum. See “ Spec- 
 trum.” These rays, which reside principally at and beyond the red 
 end of the spectrum, bear a strong analogy in some of their proper- 
 ties to the luminous and actinic rays. They proceed in straight 
 lines, can be reflected from mirrors, refracted through lenses of rock 
 salt, collected into foci, and polarized by various substances. 
 
 Heat is an important agent in photography, in assisting many of 
 the chemical reactions which take place ; but the peculiar action of 
 the calorific rays, when combined with the actinic in the focus of 
 a lens, appears to be too feeble to affect the result. It has been 
 proposed to separate as much as possible the calorific from the 
 actinic focus of a photographic lens, in order to reduce or prevent 
 any injurious action which might be due to the concentration of the 
 calorific rays emitted by bodies; but as glass has the property of 
 stopping, to a great extent, the heat rays emitted by terrestrial 
 bodies (though not from the sun direct), this precaution would 
 scarcely have any practical advantage. The effects of heat on the 
 various sensitive chemicals used in photography have not yet been 
 correctly ascertained by experiment. As a general rule, photogra- 
 phic operations are conducted with the greatest certainty and unifor- 
 mity at a temperature of about 60° Fahrenheit. 
 
 Calotype Process. (Gr. kci\oq, beautiful, tvttoq, an impres- 
 sion.) Under this head are included certain processes for producing 
 negatives on paper, which are identical in principle, and differ only 
 in the manipulation. 
 
 The principle of the calotype process is as follows : — 1st. To pre- 
 pare and expose in the camera a sheet of taper rendered sensitive 
 by being impregnated with a mixture of iodide of silver, nitrate of 
 silver, and organic matter. 2nd. To develop the picture by means 
 of gallo-nitrate of silver or some other similar developer. 
 
 The chief difference between the various processes which are 
 classed under the term calotype, consists in the mode of rendering 
 the paper sensitive, that is in the order in which the ingredients are 
 introduced, the mode of development being nearly the same in all, 
 and merely involving a difference in the strength of the developer 
 and the mode of applying it. 
 
CAL 
 
 49 
 
 A sheet of sensitive calotype paper contains a large quantity 
 (comparatively speaking) of iodide of silver, a small quantity of free 
 nitrate of silver, a small quantity of organic matter, and a sufficient 
 quantity of an organic acid to keep the lights of the picture clean 
 during the development. 
 
 It will be seen that in one of the methods about to be described 
 the paper itself contains sufficient organic matter ; but it will be 
 observed that in this process the paper undergoes complete immer- 
 sion in a strong bath of nitrate of silver. On the other hand, in one 
 of the other methods it will be shown that when the organic matter 
 in the paper has no opportunity of coming into contact with a strong 
 bath of nitrate of silver, it becomes necessary to add organic matter 
 of an energetic kind to the exciting solution. Bearing these facts 
 in mind, the principle of the various calotype processes will be 
 clearly understood. 
 
 The calotype processes may be classed under two heads. The first 
 class comprises two processes in which the paper is first coated 
 with insensitive iodide of silver, and then excited by a weak solution 
 of acidified gallo -nitrate of silver; the second class, those processes 
 in which the paper is first coated with an alkaline iodide, and then 
 excited by immersion in a strong bath of nitrate of silver, the excess 
 of free nitrate being subsequently removed by washing. Among 
 the latter processes, one will be selected for minute description, which 
 appears to be, of all the calotype processes, the simplest in manipula- 
 tion and most scientific in principle. In the processes of Class I., 
 the paper is iodized with iodide of silver ; in those of Class II., with 
 a soluble alkaline iodide. The first kind of iodized paper may, for 
 convenience, be called “ argento-iodized ” paper, the second kind 
 simply “ iodized 55 paper. 
 
 Calotype Processes. Cla-ss I. 
 
 There are two methods which come under this head. In the 
 first, argento-iodized paper is prepared by what is called the “ double 
 wash in the second, by the “ single wash. 55 In all other respects 
 the two methods are identical. 
 
 To argento-iodize the Paper by the Double Wash. — Ploat the face 
 of the paper on a bath containing 20 grains of nitrate of silver to 
 the ounce of distilled water. Let it remain a minute or two on the 
 bath, then hang it up to dry. Next immerse it in a solution con- 
 taining 25 grains of iodide of potassium to the ounce of distilled 
 water. Let it remain a minute or two in this solution, the exact 
 time depending on the kind of paper employed, and requiring to be 
 
 E 
 
50 
 
 CAL 
 
 ascertained by experiment. If too short a time is allowed, the 
 whole of the nitrate of silver is not decomposed, and the paper 
 darkens in the light ; if, on the contrary, too long a time is allowed, 
 the iodide of potassium in the bath dissolves the iodide of silver in 
 the paper. The time of sojourn in the iodide bath is therefore 
 rather critical. On removing the paper from the bath, let it drain, 
 and then immerse it in a pan of water, in which it must be allowed 
 to soak, the water being changed several times, until the whole of 
 the free iodide of potassium is reifioved. This soaking operation is 
 rather troublesome, and the texture of the paper is decidedly injured 
 by it. Should any excess of iodide of potassium remain in any 
 part of the paper, it would decompose the weak exciting solution 
 and produce insensitive iodide of silver, and consequently a white 
 patch in the negative. When the paper has been sufficiently washed, 
 hang it up to dry. It is of a yellow primrose colour. When dry, 
 it may be kept for use in a portfolio. It is not sensitive to light. 
 
 To argento-iodize the Paper by the Single Wash . — Lay the paper 
 on a board with a piece of blotting paper under it, and brush over 
 it a solution called “ double-iodide ” i.e., a solution of iodide of 
 silver in iodide of potassium ; (see “ Double-Iodide.”) The best kind 
 of brush is a large round one of camel’s hair, bound with string or 
 silver wire. Apply the solution copiously both longitudinally and 
 transversely, inclining the board, and keeping a flowing edge. Hang 
 up the paper to dry in a room having a pure atmosphere, free from 
 sulphurous and acid vapours, which bleach the reddish tint that the 
 paper ought to assume. When very nearly or even quite dry, 
 immerse the paper in a pan of water as before, in order to remove 
 completely the excess of iodide of potassium which it contains. 
 Two or more papers should not be soaked in the same pan, but 
 each paper should have a separate pan. Wlien sufficiently washed — 
 an operation which requires several hours — dry the paper and put it 
 away for use. 
 
 Argento-iodized paper may be kept for some weeks without losing 
 its good qualities. Some persons even affirm that it may be kept 
 indefinitely in a dry place. It is said, on good authority, to be 
 much improved by exposing it for an hour or two to strong sun- 
 shine. When this is done, it should not be excited immediately 
 after insolation, but kept for some days previously in the dark, since 
 it has been found that paper absorbs light in sufficient quantity to 
 react on a sensitive surface placed in contact with it in the dark. 
 
 In comparing the two methods of iodizing the paper which have 
 been described, it will be seen that the first is the most economical, 
 but at the same time the least certain. A considerable quantity of 
 
CAL 
 
 51 
 
 iodide of potassium is wasted in the second mode of iodizing, and 
 the washing operation is rather more tedious. 
 
 The paper having been iodized by either of these methods, the 
 remaining operations are the same, and are as follow : — 
 
 To excite the Paper . — Make two solutions, one a saturated solu- 
 tion of gallic acid in cold distilled water, which call solution A ; the 
 other, by dissolving 50 grains of nitrate of silver in 1 ounce of 
 distilled water, and adding 1 dram of glacial acetic acid, which call 
 solution B. Immediately before use, mix, in a chemically clean 
 measure, 1 ounce of distilled water, 1 5 drops of solution B, and 
 about as many drops of solution A, the number of the latter de- 
 pending on the temperature and kind of paper employed. 
 
 Lay the paper on a board with a piece of blotting-paper beneath, 
 and apply this mixture copiously to it with a clean Buckle’s brush. 
 Hold up the paper to drain for a minute, then blot off the surface 
 moisture with clean blotting paper, and put the sensitive paper into 
 the dark slide. 
 
 The Exposure . — The average time of exposure, with a good light, 
 a lens of fifteen inches focus, and a half-inch stop, is seven minutes. 
 
 To develop the Picture . — Lay it on a board, and brush over it, 
 with a clean Buckle’s brush, a mixture composed of 3 parts of so- 
 lution A, and 2 parts of solution B. The picture, the darker parts 
 of which are at first faintly visible, soon comes out of a fiery red 
 tint. At this stage of the development it is necessary to check it, 
 in order to obtain dense blacks instead of feeble reds in the darker 
 parts of the negative. To accomplish this, brush over the picture, 
 and complete the development with a solution of gallic acid alone. 
 
 ' Under this treatment the reds soon darken and intensify, and 
 become eventually opaque blacks. The entire development should 
 occupy about twenty minutes. It is an excellent plan, after having 
 brushed on the gallic acid, to lay the paper face downwards on a 
 horizontal glass slab, on which a quantity of gallic acid solution has 
 been previously spread. 
 
 To fix the Picture . — When the details are fully out, and the blacks 
 of the proper intensity, wash the negative with water, and then im- 
 merse it in a solution containing 1 part of hypo-sulphite of soda 
 to 4 parts of water. Let it remain until the whole of the yellow 
 iodide of silver is removed from the paper. Then wash and soak in 
 water for several hours, changing the water several times, in order 
 to remove the whole of the hypo from the paper. Lastly, hang it 
 up to dry. 
 
 The negative is now finished, and may be waxed at any convenient 
 time. See “ Waxing.” 
 
 E 2 
 
52 
 
 CAL 
 
 There are one or two important points to be observed in this 
 process : — - 
 
 If the iodized 'paper is excited with a weak solution of aceto- 
 nitrate of silver containing no admixture of gallic acid, it is quite as 
 sensitive, or even more sensitive than before, but the negative is 
 devoid of density, and the dark portions are grey, feeble, and me- 
 tallic, like the dark parts of a collodion positive on glass when 
 viewed by transmitted light. This sufficiently shows the importance 
 of introducing organic matter with the exciting solution. But the 
 above effect does not take place equally with all kinds of paper. 
 There is a coarse spongy kind of Whatman’s paper, sized perhaps in 
 a peculiar way, in which the gallic acid may be advantageously 
 omitted in the exciting solution. If, on the other hand, too much 
 gallic acid is added to this solution, the paper is liable to become 
 brown all over, particularly in hot weather. The process has there- 
 fore its drawbacks and uncertainties, particularly at that season of 
 the year when photographic tourists are most in want of a good 
 process. A good test of the proper state of the sensitive paper, is 
 to take a strip into the light. If it darkens instantly to a cold grey 
 tint, incapable of being intensified by the further action of light, 
 the negatives will probably be grey and feeble ; but if it darkens 
 to a redder tint, which gets still darker by continuing the exposure, 
 there is sufficient organic matter present to give a good intense 
 picture. 
 
 Calotype Processes. Class II. 
 
 In these processes, the operations consist in first impregnating 
 the paper with an alkaline iodide ; next, rendering it sensitive by 
 immersion in a strong bath of aceto-nitrate of silver, and developing 
 with gallo -nitrate of silver rather weaker in silver than in the fore- 
 going process. It is not found necessary in this process to intro- 
 duce organic matter into the paper, as that which the paper itself 
 contains, in the form of size, is in general sufficient ; but should 
 that not be the case, the effects and their remedy are obvious 
 enough. With respect to the introduction of organic matter, it 
 appears that an extremely minute quantity, in combination with 
 sensitive iodide of silver, is capable of producing a very marked 
 effect, and that the effect of organic matter is not increased by over- 
 loading the paper with it. This remark is very important, as it 
 shows how wrong in principle are those processes in which a large 
 quantity of organic matter is introduced. 
 
 To iodize the Paper . — Immerse a number of sheets, one at a time, 
 in a solution of iodide of potassium containing 20 grains of the 
 
CAL 
 
 53 
 
 salt to the ounce of distilled water. Let them remain together in 
 the bath for at least a couple of hours, then hang up to dry. When 
 dry, the papers assume a reddish or even deep purple tint, according 
 to the quantity of starch which they contain. When dry, put them 
 away in a portfolio until wanted. They may be kept for a few 
 weeks in a dry place without deterioration. 
 
 To excite the Paper . — Immerse it in a bath of acceto-nitrate of 
 silver, containing 35 grains of nitrate of silver, and two scruples of 
 glacial acetic acid to the ounce of water. First, float the face of the 
 paper for a few seconds on the bath, then immerse it entirely. 
 Leave it about three minutes, and then remove it into a dish of dis- 
 tilled water. Einse it well in this, then press it between blotting- 
 paper and put it into the slide, with its face to the front shutter. 
 
 It may be well, when the nitrate bath is first made, to saturate 
 it with iodide of silver, and filter. 
 
 The Exposure . — This is about the same as in the former process. 
 
 To develop the Picture . — The paper will scarcely exhibit any trace 
 of an image. Immerse it in a solution of gallic acid, to which a few 
 drops of aceto-nitrate have been added. The picture should begin 
 to appear of a red and not a gray tint, which should darken in the 
 course of the development to an intense black in the darkest parts. 
 The dish should be chemically clean, and the developer should 
 remain colourless throughout ; the whites of the picture will not 
 then be discoloured, and the development will proceed much more 
 quickly. 
 
 If the picture comes out grey and metallic, exhibiting all the 
 detail, and having but little intensity, it indicates a want of organic 
 matter in the sensitive film. To remedy this, add a little citrate of 
 soda to the iodizing solution, and substitute a little chloride of 
 sodium for the iodide of potassium. This will entirely alter the 
 character of the picture, increasing the density of the blacks, and also 
 the necessary time of exposure. Sugar, honey, gelatine, and sub- 
 stances of this kind added in large quantity to the iodizing solution 
 produce but little effect. Serum of milk contains some organic 
 salts, which are decomposed and form organic salts of silver, and 
 this substance acts much more powerfully than those before men- 
 tioned, which require to undergo decomposition before they act with 
 effect. 
 
 As a rule, it may be considered that organic salts which at once 
 decompose nitrate of silver and form an insoluble organic compound 
 with oxide of silver, act most powerfully in producing the effects 
 due to organic matter ; and that, next to these come such organic 
 substances, (gallic acid, for instance,) as, when added to nitrate of 
 
54 
 
 CAL 
 
 silver, form a mixture which is immediately decomposed by light, 
 and an insoluble organic compound of silver produced. 
 
 To fix the Picture . — Proceed as described in the former process. 
 
 Having now described the simplest form of the second class of 
 processes, it remains to indicate the Various modifications which it 
 has assumed. 
 
 The first and most important of these consists in waxing the paper 
 before iodizing it. This modification has received the name of the 
 
 Waxed-paper Process. 
 
 % 
 
 Wax being nearly inert as regards its power of combining with 
 silver and forming an organic compound of that metal, its presence 
 does not modify the character of the negative in any appreciable 
 way as regards density ; nor does it appear that any other good is 
 derived from the introduction of wax in the first stage of the process, 
 than stiffening the paper, and rendering it less liable to be torn in 
 the various operations. 
 
 The greasy impurities contained in all commercial samples of wax 
 cause it to repel water, so that there is some difficulty in getting a 
 sheet of waxed paper to imbibe the solutions. This may be over- 
 come by dissolving a few shreds of gelatine in the iodizing solution, 
 and by adding a little alcohol to the gallic acid. In all other 
 respects the waxed-paper process is the same as that previously 
 described. 
 
 The other processes of Class II. are merely modifications of the 
 waxed-paper process in its simplest and best form, as described 
 above. They consist in adding various organic substances, such as 
 rice-water, gelatine, honey, &c., and also bromides, cyanides, fluo- 
 rides, &c., to the iodizing solution. Most of these organic sub- 
 stances are nearly, if not absolutely inert, and the salts added are 
 of very questionable utility. Under these circumstances, and as the 
 above modifications are quite empirical, if not demonstrably inju- 
 dicious, it would not only be useless, but even mischievous, to 
 describe them at length. A word or two, however, may be said 
 on the subject of substituting iodide of cadmium for iodide of 
 potassium in the paper processes. It is a tolerably good salt to 
 employ when pure, and the negative is free from the white spots 
 which sometimes occur when crystals of iodide of potassium 
 remain undecomposed. But the nitrate of cadmium introduced 
 into the nitrate bath has an acid reaction which resembles in a 
 
CAL 
 
 55 
 
 slight degree the reaction of free nitric acid. This renders the 
 paper less sensitive, increases the difficulties of bringing out the 
 details of the shadows, and diminishes the density of the blacks. 
 On the whole, iodide of potassium is decidedly the best salt to 
 employ. 
 
 Before concluding an article which embraces a description of the 
 most important modes at present known of obtaining negatives on 
 paper, it may be well to discuss the points in which these processes 
 are inferior to the analogous ones on glass. 
 
 The principal fault of a paper negative consists in the fact, that 
 a sheet of paper, waxed in the ordinary way, is not evenly trans- 
 parent when held against the light, but exhibits a granular appear- 
 ance, and sometimes the marks of the wire frame used in its 
 manufacture. 
 
 Before a paper negative can be expected to rival one on glass, 
 this radical defect must be overcome. Now, it will be found that 
 if a sheet of paper is dipped for a minute or two in linseed oil and 
 dried, it will exhibit precisely the same defects as a sheet of waxed 
 paper ; but if left in the oil for a week, so as to imbibe it thoroughly, 
 it will appear as evenly transparent as could possibly be desired. 
 It is obvious, therefore, that the radical fault of a paper negative 
 consists in the paper not thoroughly imbibing the wax, which would 
 evidently be a work, not of minutes but of hours, or even days. 
 Fortunately, however, a sheet of paper may have its power of 
 imbibing wax greatly increased by a very simple process. If one 
 half of a sheet of paper is immersed in a mixture containing equal 
 parts of muriatic acid and water, and then rinsed thoroughly in 
 several changes of water, and dried, it is found, in waxing the whole 
 sheet, that the half which has been treated in the manner described, 
 imbibes the wax much more readily than the other half, and presents 
 a beautifully even appearance. It does not appear that the treat- 
 ment with acid has any bad effects in the subsequent operations, 
 but the matter has not yet been sufficiently tested. 
 
 A finished negative on plain paper may be treated with muriatic 
 acid with perfect safety, since it does not attack the image, unless 
 very strong. In this case, about one part of acid to ten parts of 
 water will be strong enough. This treatment may therefore pre- 
 cede the waxing, and the negative will be improved by it. 
 
 Another frequent defect of paper negatives is a want of half tone, 
 too great intensity in the blacks, and too little detail in the shadows. 
 In such cases the fault may generally be corrected by leaving out 
 organic matter, diminishing the acidity of the nitrate bath, and 
 increasing the time of exposure. 
 
56 
 
 CAL 
 
 CAM 
 
 Lastly, the paper processes are very insensitive, when compared 
 with wet collodion. This may proceed principally from the great 
 quantity of acid used in the aceto-nitrate bath in paper work, as 
 compared with the neutral-nitrate bath in collodion work. When a 
 collodion plate is excited in a bath of citro-nitrate sufficiently 
 acidified with citric acid for a paper negative, the excess of nitrate 
 of silver washed off, exposed wet, and then developed with gallo- 
 nitrate, it is very little more sensitive than a piece of paper treated 
 in the same way. On the other hand, if a paper negative is excited 
 in a nearly neutral-nitrate bath, exposed wet, and developed with 
 the pyrogallic developer used for collodion, it blackens all over. 
 A paper negative, therefore, requires more acid to keep the lights 
 clean, and will not bear so energetic a developer as a collodion 
 negative. This is the principal reason of the want of sensitiveness 
 of paper as compared with collodion. There are other causes which 
 tend to increase the sensitiveness of the collodion film ; but these are 
 secondary. 
 
 One more remark : — Acetic acid, although it appears to act ex- 
 ceedingly well in the paper processes, is both costly and inconvenient 
 to the tourist. Citric acid has been proposed as a substitute. It 
 has not yet, however, been thoroughly tested ; still, there is great 
 reason to believe that it will answer exceedingly well. Acetic acid 
 has the property of penetrating the paper more readily than citric, 
 and that would perhaps be found an advantage in preventing the 
 formation of a precipitate which sometimes occurs in the interior of 
 the paper, and which may be traced to deficiency of acid. This 
 defect is more liable to occur in foreign paper containing a free 
 alkali in the size, than in English paper sized, with alum, and which 
 has consequently an acid reaction. 
 
 . 
 
 Camera Obscura. (Latin, “ Camera,” a chamber). A dark 
 box, in which the real image formed by a convex lens is received 
 on a focussing screen. A photographic camera is one in which a 
 sensitive photographic tablet can be inserted to receive the image. 
 
 As might have been expected, there are many forms of photo- 
 graphic camera, and most of them fulfil to a certain extent the 
 required conditions, but none appear to satisfy completely all the 
 conditions of a really good instrument. We shall therefore, in the 
 present article, first describe briefly some of the common forms of 
 the camera, and point out their defects ; and then suggest such an 
 improvement upon them as would be free from the same objections. 
 
 The Folding annera . — The ^simplest form of photographic camera. 
 
CAM 
 
 57 
 
 is that called the “ Folding Camera.” In this instrument, the sides 
 are made of two longitudinal pieces hinged together, and also to the 
 top and bottom, so as to fold between the top and bottom when the 
 front of the instrument is removed. This is a very convenient 
 arrangement as regards portability. The front of the folding camera 
 has a double slider for carrying the lens, so that the lens can be 
 either raised or lowered when the camera is in its usual position, or 
 on its side for taking upright views. The length of this camera is 
 invariable, and the picture is focussed by means of a sliding adjust- 
 ment of the brass work which carries the lens, the lens being mounted 
 in a tube which slides in a jacket. This form of camera answers 
 tolerably well for views, when it is not required to work within a few 
 degrees of the sun ; but when it is required to take skies, or work 
 in the face of a strong light, the folding camera allows too much 
 diffused light to fall on the picture. This defect cannot be remedied 
 without completely altering its form. 
 
 It would be a great improvement to the folding camera, if pro- 
 vision were made for inserting a diaphragm at about one third of the 
 distance between the lens and the picture, for the purpose of cutting 
 off the light which falls on the sides of the camera, and is thence 
 reflected on the picture. The picture formed by the lens is circular 
 and not oblong, so that four segments of what would be an 
 illuminated circular disc, are thrown on the sides of the camera, and 
 a great deal of light is thus reflected on the picture, the whole of 
 which would be intercepted by the proposed diaphragm. These 
 segments of light will be perceived on looking into the camera, with 
 the head and shoulders covered with the focussing cloth. The light 
 reflected in this way has no doubt caused many failures, for which 
 the chemicals have had to bear the blame. 
 
 The common folding camera is therefore a useful and portable 
 but not a comprehensive instrument, and it is faulty in a particular 
 which it is very important to remedy. 
 
 The Expanding Camera . — This camera may be used either for 
 portraits or views. It is composed of two parts ; the front part 
 rigid and carrying the lens, the after part sliding in the front part, 
 and carrying the dark slide and focussing screen. This camera may 
 be brought to within a comparatively short distance of the object 
 to be copied, and the focus of the lens proportionally lengthened. 
 It should be made to take square pictures of the largest dimensions 
 which the lens will cover ; there will then be no necessity for placing 
 it on its side when it is required to take an upright picture. The 
 
58 
 
 CAM 
 
 same kind of diaphragm should be employed as was recommended 
 in the folding camera. 
 
 In every expanding camera, the end which carries the slide should 
 be provided with adjustments for the purpose of placing the plane 
 of the picture at various angles of inclination with respect to the 
 axis of the lens. These adjustments are so extremely useful in 
 enabling the operator to overcome difficulties which are continually 
 occurring in practice, that no camera can be considered perfect 
 without them. 
 
 No provision is made in the common expanding camera for 
 intercepting a vast quantity of diffused light, which is likely to 
 ^ enter when a lens of large aperture is used, or when the instrument 
 is brought to face a strong light. An arrangement which will be 
 described presently, will be found to remedy this defect. The 
 expanding camera can be focussed either by means of the sliding 
 body, or the sliding tube which carries the lens. 
 
 The Expanding and Folding Camera . — This is merely an expand- 
 ing camera, the front and after bodies of which are made to fold. 
 Its merit is, that it can be packed in a smaller compass than the 
 common expanding camera. 
 
 These are the principal forms of camera which are found in 
 the hands of the professional photographer; but a great number 
 of other kinds have been invented by ingenious persons, and are to 
 be found principally among the paraphernalia of eccentric amateprs. 
 Some of these may be briefly mentioned. 
 
 Archer's Camera is a box provided with arm-holes and india- 
 rubber sleeves in the sides, and a piece of yellow glass on the top. 
 All the manipulation of the collodion process is performed within 
 the camera, and no dark tent is required. 
 
 The Bellows Camera is an expanding one, in which the front 
 and after bodies are connected by an expanding cover, resembling 
 the bellows of an accordeon. 
 
 Some other forms of camera have cloth sides connecting the lens 
 with the frame which carries the dark slide, and which blow about 
 in the wind. 
 
 Other kinds have been invented, which are remarkable for their 
 extreme portability, being made of a number of hoops of gutta- 
 percha, which are inflated, and covered with a cloth. One of these 
 
CAM 
 
 59 
 
 bears the name of the Pocket Camera, and, armed with this inge- 
 nious apparatus and a walking-stick tripod, the “ pestilent fag and 
 toil,” as it has been called, of out-of-door photography, may be 
 avoided. But these instruments are generally found to have a 
 screw loose when required for use. 
 
 The bare mention of such contrivances as these will suffice, as 
 they possess no real practical merit, and do not remedy any one of 
 the radical faults of the common apparatus. It is not our object 
 in this work to record and perpetuate matters which experience has 
 shown to possess little or no value, but rather to suggest, if possible, 
 improvements on established forms and methods. Enough, then, 
 has been said about the photographic camera as it is. We will® 
 proceed at once to discuss its weak points, and, if possible, to pro- 
 vide a remedy for them. 
 
 The radical fault of all the present forms of camera is, that they 
 allow the image to be diluted with light that is extraneous to it, and 
 which gets in and finds its way to the sensitive surface, thereby pro- 
 ducing fog on the shadows and general discoloration, and rendering 
 it next to impossible to obtain a clean picture with sufficient 
 exposure to bring out all the details. Now, the way in which dif- 
 fused light enters, and the course it takes from the lens or win- 
 dow in front to its destination on the sensitive tablet may be easily 
 exhibited by the aid of a diagram, and the remedy pointed out. 
 
 If the camera is well made and light-tight, the only way in which 
 diffused light can enter it is by the lens. Let us see, then, in what 
 way the construction of the lens is likely to produce this evil. 
 
 Eig. 3 represents an ordinary portrait lens mounted in a tube : 
 the anterior and posterior lenses being of the same size, and the 
 inside of the tube being blackened with chloride of platinum, which 
 merely changes the colour of the brass without destroying its polish. 
 This is the way in which portrait lenses are generally made and 
 sold. 
 
60 
 
 CAM 
 
 Now observe what follows : — 
 
 Q, is the origin of an oblique pencil. Bays from it cover the 
 whole of the anterior surface of the first lens, and the pencil, after 
 refraction, follows the course indicated by tlte figure. One half of 
 it is incident on the upper part of the posterior lens, and finds its way 
 to the focus F, but the other half falls on the inside of the tube, and 
 is reflected as shown by the arrows. These reflected rays, together 
 with those from all the other oblique pencils that are similarly cir- 
 cumstanced, pass through the posterior lens, and produce a large 
 circular disc of light in the centre of the picture. In this arrange- 
 ment, not only has the central part of the picture more light in con- 
 sequence of the direct incidence of the central pencils, but one half 
 of the rays of the most oblique pencils is lost, and a portion of those 
 that are reflected from their true course are added to the already too 
 luminous centre of the field. 
 
 There are two modes by which this very serious evil may be 
 remedied. One is objectionable, as it involves an error in principle. 
 It consists in placing a stop midway between the lenses, and thus 
 preventing the reflected rays from the inside of the tube from 
 entering the camera. But this plan not only cuts off a part of 
 every oblique pencil, but it compels the part that is available to pass 
 through the outside of the front lens, whereas every pencil should 
 pass centrically through that lens. The other plan is correct in 
 principle, and will be understood by referring to Fig. 4. 
 
 In this arrangement of the lenses, the posterior lens is larger 
 than the anterior, and they are not mounted in a tube, but in 
 wooden partitions; no reflected light can, therefore, by any pos- 
 sibility enter, and none of the rays from the oblique pencils are cut 
 off. Every pencil passes centrically through the front lens, and 
 excentrically through the posterior lens, as it ought in order to 
 get the maximum flatness of field. 
 
CAM 
 
 61 
 
 But even this arrangement is not sufficient to produce a clear 
 picture when the lenses are exposed to a strong light, the bright 
 part of the sky, for instance. Diffused light exists in the atmo- 
 sphere, and if we wish to see distant objects clearly, we must look 
 at them through a tube. The stars, for instance, are visible at 
 noon-day from the bottom of a well. Bearing this principle in 
 mind, an important addition should be made to every camera, in 
 the shape of a long darknened tube in front, as shown in Big. 5, 
 which is a section of a camera, with all the proposed improvements 
 introduced. 
 
 A camera constructed on this principle would be equally suitable 
 for views or portraits, because its lens would give as flat a field as 
 an ordinary view lens, and splendid definition might be obtained by 
 using a stop immediately in front of the front lens. By removing 
 the stop, and working in a strong light with a full aperture, it 
 would be suitable for taking instantaneous pictures. It was with a 
 lens of this kind that M. Flacheron produced his magnificent views 
 of Rome some years ago, and it is surprising that this form of lens 
 should have gone out of fashion, and been supplanted by one in 
 which everything is sacrificed to the central pencils. The great 
 merit of a photograph is to be equally good in every part, and not 
 to have the edges of the field sacrificed to the centre, a defect which* 
 in views of landscapes or architecture, is perfectly unbearable. 
 
 Should it be necessary, when using the above form of camera, to 
 turn its axis upwards, the slide must be always kept strictly vertical 
 by means of the arrangement for that purpose, otherwise, all the 
 vertical lines would appear to have a vanishing point in the zenith. 
 The camera and slide should be square, and the plates large enough 
 to take in the whole of the circular picture that is formed by the 
 lens. Circular views are extremely natural and pleasing, particu- 
 larly when softened off into darkness at the edges. It is a circular 
 picture which is always formed on the retina, and a view bounded 
 by straight lines at right angles always conveys to the mind the idea 
 of looking at things through a rectangular hole in a box, and inter- 
 
 
62 
 
 CAM 
 
 feres with the idea of space and freedom of range. There is cer- 
 tainly something unnatural and disagreeable about it. Large 
 circidar pictures are remarkably fine when viewed in the reflecting 
 stereoscope. 
 
 Copying Camera . — This camera is used for obtaining copies of 
 photographs or prints, either by transmitted or reflected light, of a 
 different size from the original. When the copy is to be less than one 
 half the linear dimensions of the original, the ordinary camera, with 
 a portrait or view lens, may be employed ; but when the copy is to be 
 nearly as large, or even larger, than the original, some ihodifications 
 must be introduced in the form of the ordinary camera, and also in 
 the lens. 
 
 The principle to be borne in mind in copying an object on a dif- 
 ferent scale is this — that the linear dimension of the copy bears to 
 the corresponding linear dimension of the original the same ratio 
 that the distance of the copy bears to the distance of the original 
 from the lens. Tor instance, let C be a certain linear dimension of 
 the copy, and 0 the corresponding linear dimension of the original, 
 and let TJ be the distance of the original from the lens, and Y the 
 equivalent focal length of the lens corresponding to the distance U ; 
 then C : O : : V : U. 
 
 Hence we arrive at the following important conclusion, viz., that 
 so long as the ratio Y : U remains constant, U may be increased as 
 much as you please. Now, as the principal defects of photographic 
 lenses proceed from the obliquity of the lateral pencils, and as the 
 obliquity of these pencils is diminished in proportion as TJ is in- 
 creased, it is evident that the lens should not be brought so near to 
 the object to be copied as to introduce very oblique pencils, but that 
 it should be placed at a distance from it equal to at least three times 
 the longest dimension of the object to be copied, and be of sufficient 
 length of focus to give an image of the required size. 
 
 In ordinary photographic work, pencils having an obliquity of 
 from 17° to 20° are introduced, and the lens has to be so con- 
 structed as to meet this difficulty ; but the form of lens best calcu- 
 lated for giving a flat field when pencils of great obliquity occur, is 
 not that which at the same time gives the greatest distinctness of 
 focus of the central pencils ; that is to say, if the central pencils 
 alone had to be corrected in the best possible way for spherical 
 aberration, the form of lens which this condition would impose would 
 not be such as to satisfy at the same time the conditions necessary 
 for flatness of field when very oblique pencils are introduced. When, 
 therefore, the necessity for obviating the defects due to obliquity is 
 
 
CAM 
 
 63 
 
 in some measure removed, the form of the lens may be so modified 
 as to remedy more perfectly the defects of central pencils. In the 
 lens of a copying camera, therefore, the posterior lens need not be 
 much larger than the anterior ; in fact, a single achromatic lens, like 
 the object-glass of a telescope, may be employed. But this subject 
 has not yet received the attention which its importance deserves, 
 and the best possible form of lens for the copying camera remains 
 yet to be investigated. 
 
 In copying a small photograph or print on a larger scale, the 
 projecting front of the camera should be continued until it nearly 
 touches the picture, and this should be illuminated as strongly as 
 possible, either by the sun, or strong diffused daylight. When 
 artificial light is employed (the oxycalcium light, for instance), it may 
 be brought very near to the end of the camera, and its light con- 
 centrated on the picture by means of a convex lens, as shown in 
 Fig. 6. 
 
 'Whenever the light is sufficiently intense to allow of a stop being 
 used, it should certainly be introduced as a remedy for the unavoid- 
 able defects of the lens. When an achromatic convex lens is used, 
 like that in Fig. 6, the stop should be placed immediately in contact 
 with it, either in front or behind. 
 
 Small bas-reliefs may be copied in this way very successfully, by 
 throwing the light obliquely on them, and using a reflector to 
 diminish the intensity of the shadows. It is generally found difficult 
 m practice to place the plane of the sensitive surface accurately 
 parallel to the plane of the picture to be copied, and unless this is 
 done, the lines of the copy are distorted. Where the original is 
 small, it may be placed on a holder connected with the bottom of 
 
 
64 
 
 CAM 
 
 the camera, as shown in Pig. 6 ; but, if this is not possible, the end 
 of the sliding part of the camera should be fitted with the arrange- 
 ment employed in the ordinary camera, for allowing the plane of 
 the picture to be inclined at any small angle to the axis of the lens. 
 By means of these adjustments, any distortion of the image on the 
 ground glass may be easily remedied. 
 
 When the picture to be copied is transparent, it should, if pos- 
 sible, be inserted within the camera, the front of which should be 
 sufficiently lengthened to receive it, and the light which is transmitted 
 through it should either proceed from the sky, or a large white 
 illuminated disc, or an artificial light placed behind a condenser. 
 When practicable, the sky should always be used as a luminous 
 background, in preference to any other kind of light. This will be 
 fully explained in the article on “ Micro-Photography.” When the 
 sky is used as the source of transmitted light, the instrument may be 
 mounted on a stand, in such a way as to turn on an axis, to permit 
 of its being directed to any part of the heavens. When the copying 
 camera is placed with its axis vertical, the free nitrate on a sensitive 
 plate does not drain towards the lower part, and the picture is con- 
 sequently more uniform in density. Whenever it is practicable, 
 therefore, to point the instrument towards the zenith, this should 
 certainly be done. 
 
 Having now explained the principle of the construction of a 
 copying camera, it will be unnecessary to enter further into detail, as 
 the operator will find no difficulty in modifying his arrangements to 
 suit any particular case. 
 
 Solar Camera . — This is a form of copying camera, in which 
 the sun’s rays are transmitted through a transparent negative. It 
 only differs from the copying camera in which artificial light is 
 employed, in the form of the condenser necessarj^or modifying the 
 course of parallel rays, and preventing the formation of an image of 
 the sun between the lens and the sensitive surface. This form of 
 condenser will be described in the article on “ Micro-Photography.” 
 
 Stereoscopic Camera . — The stereoscopic camera is employed for 
 taking duplicate pictures of objects, from different stations, to be 
 viewed in the stereoscope. 
 
 There are two forms of stereoscopic camera. The first arrange- 
 ment merely relates to the camera stand, which is so contrived as to 
 allow the camera to be shifted from one end of it to the other, and 
 at the same time have its axis always directed towards the same 
 point. In this way the two pictures are taken separately, in an 
 
CAM 
 
 65 
 
 ordinary single camera, and the contrivance, which merely affects the 
 stand, should be called by some appropriate name. 
 
 Since the above mode of taking stereoscopic pictures involves an 
 error of principle, which will be pointed out, it becomes unnecessary 
 to describe the various modes by which an operation that is errone- 
 ous in principle is rendered more or less convenient in practice. It 
 will be sufficient to point out clearly in what this error consists, and 
 then to pass on to the description of the other form of stereoscopic 
 camera which is scientifically correct. 
 
 In taking stereoscopic pictures, the axis of the camera in its 
 different positions should strictly preserve its parallelism, however 
 wide apart the stations may be ; otherwise, there will be more or 
 less difficulty in properly combining the images seen in the stereo- 
 scope. This will be evident from the following considerations : — 
 
 In Fig. 7, let A B be a horizontal 
 line above the level of the eye, (the 
 
 parapet" of a high building, for in- B 
 
 stance,) and let L It be stations 
 from which stereoscopic views are 
 taken. Let the axis of the camera 
 be directed, in both cases, towards 
 the same point C. Then, the line 
 A B in the pictures will not be hori- 
 zontal, but will tend to a vanishing 
 point. In the picture taken from L, 
 
 (the left station,) the vanishing point 
 will be on the right-hand side beyond 
 B, and the line A B will be inclined, 
 as shown in the figure beneath L. In the picture taken from B, 
 (the right station,) the line A B will incline towards the left side, 
 and tend to a vanishing point beyond A, as shown in the figure 
 beneath B. When, therefore, the two pictures are mounted and 
 placed in the stereoscope, the points A, A, will not be oh the 
 same horizontal line, and it will be found impossible to combine 
 either the two A’s or the two B’s, so as to produce stereoscopic 
 effect, without turning the head, so as to bring, from the points 
 A, A, the right eye downwards and the left eye up, and vice versa , 
 for the points B, B. In other words, it will be impossible to com- 
 bine either the two A’s or the two B’s, with the eyes on their 
 normal horizontal line. 
 
 For, suppose we call B the right eye and L the left, and consider 
 the line L B which joins the eyes to be horizontal; then, if we 
 join the optic axes L A, B A, and produce them, it is evident that 
 
 F 
 
66 
 
 CAM 
 
 they cannot possibly intersect in a common point, unless the two 
 points A A are in the same horizontal line ; for, otherwise, the 
 lines B A, L A, will not lie in a plane, and straight lines which do 
 not lie in the same plane, cannot possibly meet in a point. 
 
 The convergency of the two directions of the axis of a camera, 
 when taking duplicate views for the stereoscope, is therefore shown 
 to be wrong in principle ; and if the difficulty in uniting the images 
 is not perceived in practice, it may perhaps be that the error is in 
 most cases trifling in amount, and the head, by a series of small 
 motions, brings the eyes into a succession of proper positions for 
 uniting the several points of the images. 
 
 The scientifically correct form of stereoscopic camera is that in 
 which the axis of the instrument always preserves its parallelism, so 
 that the two pictures are taken on the same plane. When the 
 pictures are small, and the stations near together, this may be very 
 conveniently effected by combining two cameras in one, and taking 
 both pictures on the same plate ; and a great merit of this arrange- 
 ment is, that both pictures can be taken at once. In fact, so great 
 an advantage is this, that no objects which move rapidly can 
 possibly be taken in any other way. The double instrument, called 
 the “ double-lens stereoscopic camera,” (or erroneously, by some 
 persons, the “binocular camera,”) is therefore, in every respect, the 
 most scientific instrument to employ for obtaining stereoscopic 
 pictures, 
 
 A good form of the instrument, for taking the small pictures for 
 the lenticular or cosmorama stereoscope, is shown in Figs. 8 and 9. 
 
 Fig, 8, scarcely requires explanation. When a stop is used, it 
 should be put immediately in front of the first lens. Without a 
 stop, instantaneous pictures could be taken in a good light. The 
 tubes which carry the lenses can be moved backwards and forwards 
 in focussing, by means of a rack and pinion in the sides of the 
 camera. Either tube may be moved independently of the other. 
 The front of the camera has two circular openings to admit the 
 pencils which produce the images. These are covered and un- 
 covered by the revolution of a circular plate, (see Fig. 9,) which has 
 two corresponding openings, and turns about an axis in the centre 
 of the front of the camera. At the bottom of this revolving plate 
 is a heavy handle, which, by its weight, keeps the openings in the 
 plate in a vertical position, and the holes in the camera covered. 
 By a rapid semi-revolution of this plate, a nearly instantaneous 
 exposure may be given to the pictures. 
 
 It only remains to add a few words with respect to the principal 
 dimensions of the camera. 
 
CAM 
 
 67 
 
 The equivalent focal length of the lenses should be five inches, 
 and they should be five inches from centre to centre. This distance 
 between the stations is a mean which will suit a very large class of 
 subjects, including portraits and views, in which there are near as 
 well as distant objects. The pictures are circular, four inches in 
 diameter, and five inches from centre to centre. The positive prints, 
 when cut and trimmed, are reduced to the usual size for the ordinary 
 
 Tig. 
 
 stereoscope, or, when exhibited as circular pictures, are suitable for 
 the large cosmoramic stereoscope, the lenses of which are quarter 
 lenses cut from a large one five inches in diameter. The glass 
 plates suitable for this camera should be ten inches long by five 
 inches wide. The slide should be a non-reversing slide, in which 
 either the coated side, or the back of the plate, may be presented to 
 the lens. 
 
 f 2 
 
68 
 
 CAM 
 
 CAN 
 
 Camphor. The camphors are a species of concrete essential oils. 
 They are essential oils which have become more or less oxidized and 
 solidified. Light has great power in ozonizing oxygen in the 
 presence of essential oils, and the oxygen thus changed, oxidizes 
 either the oil or other substances, which may also be at the same 
 time in contact with it. It is the presence of this form of oxygen, 
 which gives to the essential oils their odour ; for if non-oxygenized 
 oils be distilled off quicklime in vacuo, the product is so far 
 inodorous, that oil of lemons can scarcely be distinguished from oil 
 of turpentine. Hence the effect of light in increasing the perfume 
 of odorous bodies. The camphors pass by further oxygenation 
 into organic acids, which will combine with oxides of silver or other 
 metals, and these changes illustrate the effects of organic matters in 
 photo-chemistry. 
 
 Common camphor is the produce of the camphor laurel of Japan 
 and China. It has been added to gallic acid, to serum, to albumen, 
 and other organic solutions, to preserve them from decomposition ; 
 to gallic acid when used in developing albumen negatives, to prevent 
 “ dulling ” of the film, and to the salting solutions for paper, to 
 produce richness of effect. Its anti-putrefactive power is probably 
 due to its absorbing the oxygen which would otherwise act on the 
 gallic acid or other matter. It is soluble in 1000 parts of water, 
 but alcohol of S. Gr. 806 , will dissolve more than its own weight. 
 
 Canada Balsam. This is a turpentine obtained from the 
 Abies balsamea , which grows in Canada and the state of Maine ; it 
 is used to cement the lenses which form an achromatic combination, 
 in order to diminish the reflections at the inner surfaces. Its 
 colour is not found appreciably to stop the actinic rays when thus 
 employed, though in photographing microscopic objects mounted 
 in it, its antiactinism is very considerable. See “ Transparencies.” 
 
 Cane Sugar. The sugar obtained from the sugar cane. A 
 small piece added to each ounce of the nitrate of silver solution in 
 printing positives, is said to have the effect of giving a smooth 
 velvety surface to the picture, and also of allowing the paper to be 
 kept ; for though, like other papers, it changes after a time to a yellow 
 colour, this entirely disappears in the hypo-fixing bath. Sugar 
 forms a combination with the nitrate of silver in excess, which keeps 
 it on the surface, and is probably decolorized with greater facility 
 than the compound which the nitrate forms with the ordinary paper 
 sizing in the substance of the paper. It renders the paper sticky, 
 and in our hands appeared likely to spot the negative and contract 
 
CAN 
 
 CAE 
 
 69 
 
 dust, without increasing the surface vigour of the impression. See 
 “ Sugar, and Preservative Processes.” 
 
 Canson’s Paper. See “ Paper.” 
 
 Canton’s Phosphorus. Made by calcining oyster-shells in the 
 open fire for half an hour, after which, the whitest pieces are mixed 
 with one-third of their weight of flour of sulphur, and heated red 
 hot in a closely-covered crucible for an hour. The phosphorescent 
 appearance, which this assumes in the dark is much exalted by 
 exposure to the sun’s rays, and this is the case with many phospho- 
 rescent bodies. This effect is independent of heat. See “ Solar 
 Phosphori, and Light.” 
 
 Caoutchouc. Common india-rubber, the inspissated milky 
 juice of certain trees growing in America and the East Indies. 
 Various plate-holders, and other pieces of apparatus, are made in 
 part or wholly of this material ; and if it is to be brought into 
 contact with nitrate of silver solutions, it should be remembered 
 that the very flexible and elastic variety, called vulcanized india- 
 rubber, contains sulphur, imparted to it in its manufacture. It is 
 soluble in ether, chloroform, benzole, and some volatile and fixed 
 oils. Naphtha also dissolves it, with the aid of heat. In some brittle 
 black varnishes, where these solvents are resorted to, caoutchouc, in 
 small quantity, is a useful ingredient. 
 
 Cap of the Lens. The brass, or, what is better, pasteboard 
 cover, which is used to cover and uncover the lens at the time of 
 exposing the sensitive plate to light. 
 
 Capsules. Small shallow basins made of berlin ware, plati- 
 num, &c., for evaporations, solutions, &c. 
 
 Caramel. Burnt sugar. It is soluble in water, and has been 
 tried instead of honey, &c., in preservative processes. It was found 
 to lessen the sensitiveness, but was less inclined to crystallization. 
 
 Carbon. An elementary substance appearing in a great variety 
 of forms, of which the diamond is the purest and charcoal the most 
 common. A stick of charcoal put into a solution of nitrate of silver 
 and placed in the sunlight, will revive pure white silver around and 
 adhering to itself. It has hence been said, that carbon, with the 
 aid of light, can reduce silver in solution to the metallic state ; but 
 
70 
 
 CAE 
 
 the reduction is due rather to the peculiar physical properties of 
 carbon in certain forms, than to an ordinary chemical reaction. 
 This effect of carbon rather resembles the catalytic action of spongy 
 platinum. 
 
 Carbonate of Ammonia. There are several definite hydrated 
 carbonates of ammonia, but the usual commercial carbonate is a 
 sesquicarbonate containing three atoms of acid to two of ammonia. 
 By exposure to air, it becomes a bicarbonate. It is added to iodide 
 of iron to make iodide of ammonium, but this is not the best method. 
 It sometimes occurs in collodion iodized with this ammonium 
 compound, and then its action in producing nitrate of ammonia in 
 the bath must be attended to. 
 
 Carbonate of Baryta. Ba. 0, C 0 2 . An insoluble salt formed 
 when soluble baryta compounds are precipitated by alkaline car- 
 bonates. Used by photographers in making iodide and bromide of 
 barium. 
 
 Carbonate of Lime. Ca. O, C 0 2 = 50. The most abundant 
 compound in nature, comprising the marbles, limestones, and other 
 rocks. Calcareous spar and statuary marble are used in the labora- 
 tory as the purest varieties. This salt will answer many of the 
 demands of photography, as far as regards carbonates ; as, for in- 
 stance, for neutralizing the nitrate of silver, or hyposulphite of soda 
 bath, or other acid solutions, and for estimating the strength of the 
 acids employed as measured by the alkali that is necessary to satu- 
 rate them. It is sometimes found in small specks in the surface of 
 glass plates, and is best removed by nitric acid. It is an impurity 
 in animal charcoal, and in many hard waters. It is insoluble in 
 water. 
 
 Carbonate of Potassa. K O, C 0 2 = 70. This is a deliques- 
 cent, strongly alkaline, fusible salt. Its strong attraction for water 
 renders it useful in dehydrating ether and alcohol, and it is other- 
 wise of service in cleansing glass plates, as an antacid, in forming 
 acetates, in forming carbonate of silver, and in reducing chloride of 
 silver to the metallic state. It often exists as an impurity in bro- 
 mides, iodides, chlorides, cyanides, and other salts of potassium, and 
 is very generally employed in their manufacture. The carbonate of 
 commerce is obtained from the ashes of trees and vegetables burned 
 for the purpose, and is contaminated with chlorides and sulphates, 
 which may be in a great measure got rid of by dissolving the crude 
 
CAE 
 
 71 
 
 carbonate in water, and evaporating till tlie solution is of tbe S. G. 152, 
 when most of the impurities will crystallize out. When required 
 quite pure for analysis, it is obtained by heating the crystallized 
 bicarbonate to a temperature below redness, but sufficient to expel 
 its water and half of the carbonic acid, dissolving the residue, and 
 filtering. 
 
 Carbonate of Silver. Ag. 0, C0 2 . This is precipitated as a 
 pale yellow insoluble powder, by adding carbonate of potassa to 
 nitrate of silver. It blackens by exposure to light. It is formed 
 and dissolves in small quantity in solution of nitrate of silver when 
 a soluble carbonate is added to it to neutralize free nitric acid, and 
 the small quantity taken up communicates to the solution a slightly 
 alkaline reaction, which renders it necessary to add a drop or two of 
 acetic acid to counteract the bad effect which the slightest alkalinity 
 produces. 
 
 Carbonate of Soda. Na. O, C0 2 . Manufactured for washing 
 purposes in immense quantities, by the decomposition of common 
 salt by sulphuric acid, subsequent fusion of the product with chalk 
 and coal in a reverberatory furnace, and purification. This mode of 
 making it adulterated, leaves it with a small quantity of chloride of 
 sodium and sulphate of soda, but it is generally extremely pure. It 
 is used in the same way as carbonate of potassa, and also in making 
 nitrite of soda, for producing nitrite of silver, in removing the size 
 from paper, in separating chloride of copper when chloride of gold 
 is obtained from the standard coin, and in moderating the action of 
 chloride of gold used as a toning bath. 
 
 Carbonates. The carbonates used in photography, when soluble, 
 are all alkaline in their properties, owing to the weakness of carbonic 
 acid, and this must be particularly remembered, not only when they 
 are used, but also when the chlorides, bromides, iodides, cyanides, 
 and hyposulphites, are employed, because they are all more or less 
 adulterated with corresponding carbonates. When hard water, con- 
 taining carbonate of lime, is used for making nitrate of silver solu- 
 tions, the alkalinity produced must be removed, and pictures must 
 not be washed in such waters immediately after going through solu- 
 tions containing carbonate of soda, or the lime will be precipitated 
 on the picture. 
 
 Carbonic Acid. C 0 2 = 22. This gas is a very feeble acid, 
 and reddens litmus paper but very little. Its effect as an acid, in 
 
72 
 
 CAR 
 
 retarding tlie action of light, or a developer, is almost inappreciable, 
 but in nature it is itself decomposed by light in the leaves of trees 
 and plants, which give off its oxygen and retain its carbon. The 
 rays which effect this are the luminous or yellow rays of the spec- 
 trum : the chemical rays are much less active. 
 
 Carbonized Plates. Copper plates have been evenly covered 
 with hydrocarbon, in the form of finely divided powder, and, bv 
 exposure to heat, have been superficially converted into carburet of 
 copper. These plates, covered with nitrate of silver and exposed to 
 light under a negative, will receive an image in pure milk-white silver, 
 but there is always too much tendency in the metallic plates them- 
 selves, to reduce the nitrate, to render the process available. 
 
 Carburet of Silver. Ag. 2 C; Ag. C; Ag. C 2 . Silver will not 
 only combine with organic matter composed of carbon, hydrogen, 
 and oxygen, but also with carbon alone, and that in three propor- 
 tions, forming a dicarburet, a carburet, and a bicarburet. The com- 
 pound containing least carbon is formed by keeping fused silver in 
 direct contact with carbon for some time ; but the others are fqrmed 
 by reduction from compounds of silver with organic acids by means 
 of heat. Now this reduction is sufficiently like the reduction of 
 organic salts of silver by light, to be worth comparison. The 
 carburet containing silver and carbon, atom for atom, is a yellow 
 substance not affected by heat, and soluble in nitric acid. If the 
 photographs which fade do so because the silver continues to undergo 
 reduction, nearer and nearer to the metallic state, is the final yellow 
 the residue of an organic salt from which sulphur, hydrogen, and 
 oxygen have departed, and of which the carbon and silver alone 
 remain ? Or do sulphur and carbon both remain P 
 
 Carrageen. Irish moss, Chondrus crispus, or Fucus crispus. 
 This is a kind of sea-weed, while Iceland moss, Cetraria Islandica, is 
 a lichen. Both have been resorted to in photography, on account of 
 the jelly which they contain. The carrageen contains 80 per cent, 
 of vegetable gelatine, 10 per cent, mucilage, and 10 per cent, of 
 chlorides, iodides, and bromides. It has been used 30 grains to the 
 pint of water as a first preparation in the wax paper process. It is 
 not precipitated by nitrate of silver, and therefore it may be sup- 
 posed not to exert so much modifying power in that process as 
 albumen does in others ; and indeed its action in waxed or other 
 papers is of little moment, so far as the image is concerned. The 
 
CAS 
 
 CAT 
 
 73 
 
 solution of Chondrus crispus must not be confounded with that of 
 chondrine, which is derived from animal sources. See “ Organic 
 Matter.” 
 
 Case ate of Sieves. Caseine is the cheesy matter of milk. It 
 combines with oxide of silver in the same way as albumen, and forms 
 an insoluble organic salt of a white colour, which becomes brick -red 
 in the light. It therefore powerfully influences reduction of the 
 silver salts in photo-chemical operations. The white salt is called 
 by some the caseate of silver. 
 
 Caseine. The principle in milk which produces cheese. It 
 greatly resembles albumen. Like it, it is held in solution in the 
 milk by the alkali with which it is combined, and it is coagulated 
 and precipitated by acids. Its action on nitrate of silver, and in 
 photographic processes, also closely resembles that of albumen. 
 Vegetable caseine, which exists in the gluten of flour, acts in the same 
 manner, and hence flour paste is a much more powerful body in 
 calotype, &c., than the starch with which Trench paper is sized, or 
 even than the animal gelatine of some English papers. It is the 
 valuable part of the serum, or whey, used in photography, and 
 therefore that liquid should be prepared so as not to be entirely free 
 from it, but to contain a definite quantity, Caseine dissolved in 
 dilute ammonia will form a solution which will coat a plate like 
 collodion or albumen, and it has already been used in this manner. 
 Its properties must be intermediate between those two fluids, since 
 it has the combining power of the albumen, and yet does not form a 
 film so hard or difficult of development as that substance ; and a 
 more ready development always implies an increase of sensitiveness. 
 
 Catalysotype. A calotype process, in which the paper is first 
 prepared with syrup of iodide of iron, instead of the ordinary iodide 
 of potassium. As the picture developes itself after exposure by merely 
 keeping the paper moist without using the usual gallic acid developer, 
 it was supposed that light set up a u catalytic” action on the silver 
 salt, which then operated on the iron salt to produce a picture. 
 The truth was, that when the paper was excited on the nitrate of 
 silver, protonitrate of iron was formed by the same decomposition 
 which produced iodide of silver, and this protosalt is a still more 
 energetic developer than gallic acid. The process is not good, 
 because the solution of nitrate of silver used for exciting is in a few 
 hours quite blackened by the iron, and the iron-syrup is itself very 
 
74 
 
 CAT 
 
 inconstant in its composition : but, with smart manipulation, and 
 fresh solutions, it is extremely quick. 
 
 Catalytic Action, ( KaroXvaiQ , dissolution.) The following 
 are a few instances of what has been called catalytic action. 
 Clean surfaces of platinum attract hydrogen and oxygen gases so 
 powerfully, that, without combining with them in any way, it draws 
 them into such close contact with itself, and with one another, that 
 they unite and form water ; platinum black will absorb in this way, 
 without chemical union, 250 times its volume of oxygen. Charcoal 
 newly burned and introduced into a mixture of oxygen and sulphu- 
 retted hydrogen, causes their combination with such force, that while 
 it remains chemically unchanged, it nevertheless becomes ignited, 
 and the gases explode. When a pound of starch is digested, and 
 kept simmering for a few days, with six or eight pints of distilled 
 water rendered slightly acid by two or three drachms of sulphuric 
 acid, the sulphuric acid, though itself remaining unchanged, causes 
 the starch to be converted into sugar. A ferment introduced into 
 solutions of sugar will cause the sugar to be decomposed into alcohol 
 and carbonic acid, without itself combining in any way with either. 
 Water is an oxide of hydrogen which is capable of taking another 
 atom of oxygen, and becoming a binoxide, still retaining the form of 
 water ; but a piece of gold put into it will suddenly decompose it 
 without becoming itself oxidized. When chlorate of potass is heated 
 to cause it to give off oxygen, it is found that the gas is much more 
 freely liberated when binoxide of manganese is also present, though 
 chemically it takes no part in the decomposition, nor is itself 
 deoxidized. In all such cases where a body effects chemical changes 
 in other bodies, itself remaining unchanged, this action of presence 
 is called catalytic action. It has been supposed by some that light, 
 when it acts on salts of silver to produce an invisible impression, 
 modifies the salt in some way to communicate to it catalytic pro- 
 perties. The most probable account of such an effect would be, 
 that the chlorine undergoes some modification similar to that which 
 oxygen in certain processes undergoes when it becomes ozone, with- 
 out losing its chemical identity. Light has been shown by Professor 
 Schonbein, in certain cases, to produce this effect on oxygen, and in 
 all cases of catalytic action it may be that the catalyser, if we may 
 use that term, renders allotropic one or more of the elements with 
 which it comes in contact. This notion of the modifying power of the 
 light is not opposed to the theory which supposes that a gradual 
 reduction of metal takes place in the camera, though in such small 
 quantity as to be inappreciable, because the modifying power may 
 
CAU 
 
 CEE 
 
 75 
 
 be the cause of the reduction. No doubt the development of the 
 image commences at a point which our eyes cannot reach, and it is 
 of no importance whether that commencement takes place in the 
 camera or the dark room. 
 
 Caustic. (Kavctc- burning). Chemical substances which destroy 
 the vitality of any part of the body, or produce effects like burning, 
 are called caustic. The name is generally applied to certain alkaline 
 oxides and acid salts, which destroy animal structure. The most 
 commonly used caustics are nitrate of silver, and potassa. The 
 fixed alkalis and earths are all caustic, as lime, baryta, potassa ; also 
 such salts as chloride of zinc and chloride of gold. 
 
 Caustic Ammonia, Potassa, Lime, &c. See “ Ammonia,” &c. 
 
 Centigrade Thermometer. This is the thermometer gene- 
 rally used on the continent, and is so called from centum a hundred, 
 because the space between freezing and boiling water is divided 
 into 100 degrees. The thermometer used in England is Fahrenheit’s, 
 where freezing water is marked 32°, and the boiling point 212°. 
 In the two scales, therefore, 32 and 0, 41 and 5, 59 and 15, 
 68 and 20, 86 and 30 are corresponding points. To convert 
 centigrade into Fahrenheit, multiply by 9, divide by 5, and add 32. 
 
 Centigramme. A weight often introduced into England with 
 French formulae. It is the hundredth of a gramme. A gramme is 
 equal to 15*432 grains troy, and a centigramme to *1543 gr. 
 
 Centilitre. The hundredth part of a litre, or of 35oz. 2 dr. 
 llmin. English fluid measure. 
 
 Centimetre. The hundredth of a metre or of 39*7 inches 
 English ; it is nearly or § of an English inch. 
 
 Ceroleine. A principle in bees’ wax, soluble to a small extent 
 in alcohol. The alcoholic solution when iodized, has been used as a 
 substitute for the wax itself in the wax paper process. Its effect 
 as organic matter is rather stronger than that of wax ; it does not, 
 when introduced with alcohol, impede the action of light and of the 
 developer so much, and is more easily manipulated ; but it requires 
 the paper to be re-waxed. Ceroleine also gives greater intensity 
 when bromide or chloride of silver is added to the iodide, than when 
 the latter is employed alone. It has been added to collodion, and 
 
76 
 
 CHA 
 
 CHE 
 
 the small quantity which dissolves in the ethereal mixture, will 
 prevent, by its adhesive character, the cracking which is caused by 
 too large a quantity of water. The proportions for preparing 
 paper are — 
 
 Solution of wax in alcohol, S. G. 817, (90 grains to the pint 
 of 20 oz.) — 30 fl. oz. 
 
 Saturated solution of iodide of potassium in alcohol, S. G. 840 — 
 1 fl. oz. 
 
 Charcoal. See “Animal Charcoal.” 
 
 Chemical Elements. Those substances are called elements, 
 which no known chemical operations will reduce to simple forms. 
 See “ Tables.” 
 
 Chemical Equivalents. Every simple substance, and every 
 compound substance also, has what, in chemistry, is called its 
 equivalent number ; thus, 1 is the equivalent number of hydrogen, 
 and 8 is the number belonging to oxygen ; nitrate of silver has the 
 equivalent number 170, and hydrosulphuret of ammonia the number 
 34. These numbers express the proportions by weight, in which 
 they combine with or are completely decomposed by each other, 
 and these proportions are always constant. One grain of hydrogen, 
 for instance, will always take 8 grains of oxygen to form water, 
 and 34 of sulphide of ammonium will always throw down the whole 
 of the silver from 170 of nitrate of silver. It is easy therefore, 
 from a table of these numbers, to see at a glance how much of any ’ 
 substance is required for any chemical operation. See “ Tables.” 
 
 Chemical Eocus and Chemical Bays, Chemical Spec- 
 trum. The rays which, in a compound pencil of white light, produce 
 chemical changes with the greatest force, are not the red, yellow, or 
 green, but the blue, violet, and even some invisible rays, and these, 
 after the pencil has been refracted by a lens, do not come to the 
 same focus as the more luminous rays. The point, therefore, of 
 their coincidence, is called the chemical focus, and the other the 
 luminous; the blue, violet, indigo,* and invisible rays, are called 
 chemical rays of light, and those rays of the sun which produce 
 chemical changes by their heat, are called chemical rays of heat. 
 See “ Light.” When light is decomposed into its coloured rays by 
 being passed through a glass prism, it is, when thrown upon a 
 white screen, received as an oblong figure called the solar spectrum, 
 and that portion of it which acts chemically, is called the chemical 
 
CHE 
 
 77 
 
 spectrum. The impression which this makes upon a piece of sensi- 
 tive paper, is also sometimes distinguished as a chemical spectrum. 
 
 Chemistry of Photography. It has been said that light 
 does not fall on any body in the universe, on which it does not leave 
 traces of its passage, and that it cannot be absorbed or reflected, 
 without in some way modifying the structure and properties of the sub- 
 stance on the surface of which it struck. All the effects which light 
 thus produces are properly the subjects into which photography 
 inquires. But it is impossible here to do more than make a general 
 inquiry as to how light acts in common and easy cases, and en- 
 deavour to apply the information thus acquired to the explanation 
 of the usual photographic processes. It will be easy for the reader 
 to adapt the same principles to the phenomena of photo -chemistry 
 in general. 
 
 The most simple case of luminous action will be mentioned first, 
 because, while it is less complex than any other, it is also a striking 
 type of the whole class of decompositions and syntheses effected by 
 the solar radiations. The element chlorine is carefully isolated 
 from all other substances, and exposed to the sun’s rays. No 
 apparent change takes place in it : its colour, volume, density remain 
 the same ; but it was observed by Dr. Draper, that in such circum- 
 stances it undergoes a remarkable modification. Eor, whereas 
 before solarization, it had no power to combine directly with 
 hydrogen, it combines with it after that event with extreme readi- 
 ness, even in the dark. No other element, when alone, has yet been 
 found to experience this kind of change in the light to the same 
 extent, and this fact, therefore, places chlorine in the highest rank 
 as a photographic re-agent : the increase of affinity of chlorine for 
 hydrogen in the light, and of bodies of the same class, as chlorine, 
 for substances resembling hydrogen in their chemical properties, 
 appears to be the thread which runs through all photographic opera- 
 tions, and the basis of the art. 
 
 The effect produced on chlorine when alone, naturally leads to 
 the expectation that a very decided action will take place, if hydrogen 
 also be present at the time of exposure. Accordingly, we find a 
 mixture of hydrogen and chlorine gases placed in the sunshine, 
 immediately combine with such force as to produce an explosion, 
 and the weakest daylight will produce a perceptible amount of 
 condensation. This mixture has been proposed as an actinometer. 
 The hydrogen and chlorine united, form hydrochloric acid, and the 
 quantity formed in a given time, is in. direct proportion to the 
 intensity of the actinic force existing at the time in the luminous 
 
78 
 
 CHE 
 
 ether. No change has yet been found to be effected in hydrogen 
 by solar action, and it may be that the action is confined entirely to 
 the chlorine. 
 
 It is not necessary that the hydrogen should be in the elementary 
 state to be capable of photosynthesis with chlorine. A great 
 number of organic and other compounds of hydrogen, are decom- 
 posed by chlorine in the light. A solution of chlorine in water, for 
 instance, when exposed to the sun, is speedily decomposed, for hydro- 
 chloric acid is formed, and oxygen given off to the atmosphere. 
 H 0 -}- C1.=H Cl. + O. The action which thus takes place in 
 hydrogenous bodies, in the presence of chlorine, is one of two. If the 
 compound body contain water, the hydrogen of this water is removed 
 by the chlorine, as hydrochloric acid, and the oxygen unites to the 
 remaining elements of the compound to form a new substance. But 
 if the compound substance contain no water, hydrogen is removed as 
 before, and forms hydrochloric acid with the chlorine ; but for every 
 atom of hydrogen thus abstracted, an atom of chlorine is substituted 
 to supply its place. Thus, in the case of alcohol, C 4 H 0 2 , what may 
 be represented as a binhydrate of quadrihydrocarbon, or C 4 H 4 , 
 2 HO, that is, one atom of quadrihydrocarbon, and two of water, 
 when chlorine reacts upon it, the first effect is to take away the 
 hydrogen of the water, and leave the two atoms of oxygen to unite 
 with the C 4 H 4 , and form acetic ether ; but when the water is thus 
 decomposed, hydrogen is no longer removed, except by the subtitu- 
 tion of chlorine to fill up the gap. One atom after another is 
 replaced in this manner by chlorine, with the production of as many 
 different compounds as there may be atoms of hydrogen exchanged, 
 until finally but one atom of hydrogen keeps its place, and the 
 resulting substance is chloral (C 4 H x C1. 2 0 3 ). Chloracetic acid is 
 another remarkable instance of this change of substance by substitu- 
 tion. This compound was discovered by Dumas. It is formed by 
 introducing a small quantity of concentrated acetic acid into bottles 
 filled with dry chlorine, and exposed to the direct sunshine. White 
 vapours are soon formed, and the chlorine disappears. In fierce sun- 
 shine the action is sometimes so intense as to cause explosion. The 
 next day crystals are found in the bottles, of chloracetic acid ; and, 
 on opening them, vapours as seen to escape of hydrochloric acid, 
 carbonic acid, and chlorocarbonic or phosgene gases. In the crystals 
 it is found that the whole of the hydrogen of the anhydrous acetic 
 acid has been replaced by chlorine : thus C 4 H 3 0 3 H 0, has become 
 C 4 C1. 3 0 3 , H 0. Chlorine also, in the presence of chloride of ethyl 
 (C 4 H Cl.), placed in the direct light of the sun, will remove two of 
 the five atoms of hydrogen by forming hydrochloric acid, and at the 
 
CHE 
 
 79 
 
 same time put two of its own atoms in tlieir place. An intermediate 
 compound is first formed, when single atoms only are exchanged, and 
 one atom of hydrocholoric acid only produced. So far is the mere 
 fact of the hydrogen being in combination, from preventing its union 
 with chlorine in the light, that in some instances the hydrochloric 
 acid is formed with great violence. Thus, when light carburetted 
 hydrogen or marsh gas (C H 2 ) is mixed with chlorine in the propor- 
 tion of one volume of the former to three of the latter, even the 
 diffused light of day is sufficient to cause a violent explosion. The 
 vessels are broken, hydrochloric acid formed, and carbon, and chloride 
 of carbon deposited. Of course the difficulty of removing the 
 hydrogen increases with the strength of the affinity which binds 
 together the compound in which it exists, but no affinity is strong 
 enough to prevent it altogether ; water, which is as stable a hydrogen 
 compound as any, is easily decomposed by chlorine and sunshine. 
 
 Nor is it necessary, it maybe further remarked, that the chlorine, 
 any more than the hydrogen, should be in the elementary state in 
 order to produce its usual reaction with hydrogen in the light. 
 Both the hydrogen and the chlorine may exist in combination in 
 compounds more or less stable, in both of which the affinities may 
 be far from weak, and yet the light may have the power of causing 
 their mutual decomposition. Thus, the sub- chloride of sulphur will 
 be gradually decomposed by water in ordinary daylight, and tetra- 
 hedral crystals of sulphur may be thus obtained. The compounds 
 of phosphorus, silenium, carbon, silicon, and perhaps sulphur with 
 chlorine, decompose water, and hydrogenous liquids in the same 
 manner. The terchloride of nitrogen is a substance very dangerously 
 explosive in the presence of hydrogenous and other combustible 
 bodies. A small globule of this under water causes a tremendous 
 explosion by mere contact with a drop of fixed oil, which disperses 
 the water with great violence, and emitting a momentary flash of 
 light breaks the containing vessel into atoms. But these compounds 
 of chlorine are not the most stable of the class. As the affinities 
 increase in strength, the difficulty of overcoming them by hydrogen 
 increases. The alkaline and earthy chlorides are not decomposable 
 in this manner, nor are the. majority of metallic chlorides. Those 
 only appear to be thus affected which are very easily reduced to the 
 metallic state, or which pass without great difficulty from a higher 
 to a lower state of chloridation, as from bichloride to chloride, or 
 from chloride to sub-chloride, as the chloride of silver. 
 
 These are the principal cases in which chlorine is combined with 
 hydrogen by the action of light. But hydrogen, though by far the 
 most important, is not the only body with which chlorine has greater 
 
80 
 
 CHE 
 
 power of combining in the light than the dark. The element ap- 
 proaching nearest to it in this property is carbon. It requires, pre- 
 vious to its combining with chlorine, to be like hydrogen in the 
 gaseous state, or else in the state in which it exists in organic com- 
 pounds. These two bodies are thus associated together universally 
 in all animal and vegetable bodies, and are also acted upon by 
 chlorine in the same manner, but in very different degree. When 
 mixtures of olefiant gas and chlorine (C 2 H 2 and Cl.), or marsh gas 
 and chlorine (C H 2 and Cl.), are exposed to light, both the hydrogen 
 and the carbon enter into union with chlorine, and in the first 
 mixture a triple compound is formed, unless great excess of chlorine 
 is present. The results in both cases are hydrochloric acid and 
 chloride of carbon. The chlorided compounds of carbon are either 
 solid or liquid, but of hydrogen gaseous. It might be supposed 
 that it was the hydrogen which in these cases determined the com- 
 bination of carbon and chlorine, but the fact that the hydrogen does 
 not take the whole of the chlorine when there is not enough to 
 saturate it, shows that this is not the case ; and instances occur 
 where no hydrogen is present, and where still the carbon and 
 chlorine are induced to unite by solar action. Equal volumes of 
 chlorine and carbonic oxide gases (Cl. and C 0) exposed to ordinary 
 daylight, unite very gradually, but in the sunshine very quickly, and 
 in the dark not at all. It was on account of its mode of formation, 
 called by Dr. Davy, phosgene (<£we, light, and yewaio, to produce). 
 Carbon also, existing in alcohol, and in many organic compounds, 
 has a great tendency to unite with chlorine, and this tendency is 
 favoured by the action of light. Phosphorus, sulphur, boron, 
 selenium, silicon, would seem, from many reactions, to be similarly 
 attracted by solarized chlorine, though in a degree not equal to 
 hydrogen and carbon ; and this property, as affecting them, is scarcely 
 resorted to in photography. 
 
 But is chlorine separated from any of its compounds by the 
 influence of light ? Its union with hydrogen and carbon, as pro- 
 duced by that agency, is sufficiently demonstrated by the preceding 
 experiments. Are its compounds ever decomposed by the same 
 agency? In the presence of hydrogen or its compounds, yes; 
 otherwise it seems probable that they are not. This is opposed to 
 the popular opinion on this subject. Cldoride of silver, bichromate 
 of potass, citrate of silver, ammonio-tartrate of iron, and other such 
 preparations, are constantly spoken of by photographers as if they 
 suffered decomposition merely as the effect of solar fight. The fight 
 is said to loosen the affinity of chlorine and silver for each other, to 
 have a deoxidizing power, to be a destructive agent. Of course, it 
 
CHE 
 
 81 
 
 would not be more strange that it should lessen affinities than that 
 it should exalt them, and the question is one entirely of experiment. 
 It does not appear, however, that any experiment has yet been 
 recorded which proves that the change produced by light is of this 
 kind ; while the experiments made by Seebeck and Scheele, with a 
 direct view to determine the point, are strongly conclusive against 
 it. Chloride of silver, covered, in a stopped phial, with sulphuric 
 acid, was found by Seebeck to remain uncoloured in the sun’s rays ; 
 taking the stopper out was sufficient to cause its discoloration, as 
 was also the addition of water to the acid. Scheele’s idea was, from 
 his experiments, that chloride of silver was not decomposed by light, 
 except in the presence of hydrogen or its compounds. Chloride of 
 silver in the dark is not discoloured, even when pure dry hydrogen 
 is present, but is instantly in the light. Compounds of chlorine, 
 much less stable than the silver salt, are not decomposed by light in 
 this manner, though they are very quickly changed when hydrogen 
 in any form is present. Such bodies appear to be very stable when 
 perfectly protected from all combustible substances which can take 
 the chlorine. Even the much stronger current of electricity, pro- 
 duced by a galvanic circle, appears to have no power of separating 
 elements in such a direct and abrupt manner. We are not without 
 indications in our ordinary photographic operations, which show that 
 the elements which are concerned in the decompositions must be 
 balanced with tolerable nicety to allow the luminous ether to inter- 
 fere with them, and such a compound as chloride of silver would be 
 too stable to allow its affinities to be overcome by so weak a force. 
 The affinity of chlorine for silver is not much weaker than the attrac- 
 tion of hydrogen for chlorine, or for oxygen, if we may judge by 
 their mutual reactions, and would therefore be as likely to resist the 
 action of light as water itself. The only example produced by 
 chemists, of decomposition taking place directly by light, so far as we 
 are aware, is that of chlorous acid. Chlorine is here (Cl. 0) as- 
 sociated with an element so nearly resembling itself in its relations, 
 that it is difficult to say which is the negative and which the 
 positive. They replace each other in many compounds : their 
 affinities for other bodies are of equal strength, and directed to the 
 same elements, and their attraction for each other in this compound 
 so slight, that the warmth of the hand is often sufficient to cause 
 their dissolution. It is said that chlorous acid is not changed by 
 several hours’ exposure to daylight, but that a few minutes’ sunshine 
 decompose it without explosion. But it is more probably the heat 
 of the sun’s rays which effect this change than the actinism, since a 
 slight elevation of temperature will decompose this gas with explo- 
 
 - G 
 
82 
 
 CHE 
 
 sion. As hydrogen in many views resembles the metals, analogy 
 would suggest that the effect of light would be to increase the 
 affinity of chlorine for silver, rather than diminish it. Eor these 
 reasons it seems necessary to regard the presence of hydrogen, or 
 some such substance, as requisite in photal reactions. The nature of 
 the hydrogenous compounds must therefore be calculated, in all 
 photographic preparations, and in all reasonings on the chemistry 
 of the processes. 
 
 Hitherto, for the sake of simplicity, no mention has been made of 
 any photo-sensitive element but chlorine; yet oxygen, bromine, 
 iodine, cyanogen, and others, might have been, at the same time, 
 adverted to. Neither of these is capable of uniting, as chlorine 
 does, with gaseous hydrogen directly when exposed to light : they 
 are, therefore, weaker reagents in those changes which are deter- 
 mined by solar radiations ; but from other indications we conclude 
 that their action on combustible bodies, which contain combined 
 hydrogen and carbon, is still very powerful when light falls upon 
 them. Thus, the drying oils, as they are called, confined in a vessel 
 with pure oxygen, at first scarcely absorb any in the dark, but, ex- 
 posed to strong light, they are speedily oxidized and converted into 
 gelatinous substances which do not stain paper. As in the case of 
 chlorine, substituted for hydrogen, which we mentioned above, the 
 oxygen here removes carbon in the form of carbonic acid, and at the 
 same time replaces it with oxygen ; no water is given off, though pro- 
 bably formed in the oil itself. A similar though weaker action takes 
 place with other fat oils. Eats and butter absorb oxygen in the light, 
 and become acid. The essential oils absorb oxygen, and this absorp- 
 tion is much more actively carried on in the light, and is the reason 
 of their being odorous. During the oxidation, some give off car- 
 bonic acid and a small quantity of hydrogen, become more and 
 more soluble in alcohol, and more solid, and are partially converted 1 
 into camphors or resins ; others give hydrogen to form water with 
 the oxygen, which water unites with the remaining elements, to form 
 solid hydrates. Bitter almond oil, in the air and sunshine, absorbs 
 two atoms of oxygen, and is converted into solid hydrated benzoic 
 acid (C 14 H 6 0 2 -h0 3 = C 14 H 5 0 3 , HO). In the similar case of 
 chlorine, bitter almond oil and light, C 14 H 6 0 2 + 2 Cl. = C 14 H 5 0 2 
 Cl. + H CL, but the hydrochloric acid (H Cl.), instead of remaining 
 in the compound as the water (H O) does, is liberated. Oil of cin- 
 namon is decomposed in precisely the same manner. Gum guaia- 
 cum, in the light, absorbs oxygen and changes colour To green. The 
 bleaching and the coloration of oils by light, are attributable to the 
 same species of reaction. In the photographic processes with 
 bitumen, this property is taken advantage of, and in the photo- 
 
 
CHE 
 
 83 
 
 lithographic processes generally. Carbon, in certain of its organic 
 forms, has also the property of uniting with oxygen in the light, 
 and in many of the gradual changes which light operates in complex 
 organic substances, both the carbon and the hydrogen which they 
 contain are oxidized. Bromine and iodine unite directly with car- 
 bon and hydrogen, existing in the gaseous state in the form of 
 olefiant gas, when exposed to the sun’s rays, though they do not 
 appear to have any such power on isolated hydrogen or carbon. 
 They are also both capable of entering like oxygen and chlorine, by 
 a process of gradual substitution, into numberless organic com- 
 pounds of hydrogen, carbon, and oxygen : and an inquiry into the 
 exact interference of light in these cases, constitutes the real field 
 of scientific photography. Whenever chemists speak of the 'pro- 
 tracted or gradual effects of these electro-negative bodies upon 
 others which are more electro-positive, the influence of light is to 
 be suspected ; and their mutual reaction, when recently prepared, 
 and when they have never seen the light at all, ought to be in such 
 cases examined. The processes of fermentation, putrefaction, spon- 
 taneous decomposition, and change of form, and spontaneous com- 
 bustion, and other changes often spoken of as if they were effects 
 without causes, should also be studied in their relation to actinic 
 phenomena. 
 
 v The other imponderable fluids, as well as light, have the power 
 of increasing the affinities of chlorine and hydrogen, and of their 
 respective congeners. Thus, hydrochloric acid may be formed by 
 passing electric sparks through the mixed gases, or by subjecting 
 them to a certain degree of heat, and both these are capable of 
 effecting combinations which light is too weak to produce. These 
 same agents are even found, where they exert different degrees of 
 force, or in different circumstances, to have both the faculty of 
 uniting elements, and of decomposing the compound which their 
 union forms. Thus, hydrochloric acid may be decomposed into 
 its elements by the electric spark as well as formed by its agency. 
 And the like effect has been observed in photography, to attend the ac- 
 tion of the actinic rays, and has been called “ reverse action of light.” 
 
 Erom a comparison of the effects produced by these fluids upon 
 the two classes of substances which have been spoken of above, in 
 different circumstances, we learn that the order of affinities, as far 
 as relates to the “ sensitiveness” of the elements, is as follows : — 
 Chlorine Eluorine Phosphorus 
 
 Oxygen 
 
 Bromine 
 
 Iodine 
 
 Cyanogen 
 
 Hydrogen 
 
 Carbon 
 
 Boron. 
 G 2 
 
 Sulphur 
 
 Selenium 
 
84 
 
 CHE 
 
 This order is the same for electricity, light, and heat alike. The 
 metals have not been classed, because if, as analogy would intimate, 
 the affinity of chlorine for them is increased by light, and not dimi- 
 nished, as some suppose, this has not hitherto been taken advantage 
 of in photography. If we wish to form a table of hydrogen and car- 
 bon compound bodies, arranged in the order of their affinity for 
 chlorine, &c. in the light, some modification of it would be necessary 
 for each element, in accordance with the nature of the decomposition 
 which would take place. 
 
 With respect to the mode in which light operates in modifying 
 the chemical relations of bodies, we are almost entirely in the dark. 
 It has been disputed in some cases, whether actual decomposition 
 is produced, or only a change in the arrangement of the constituent 
 atoms ; but it is clear that a molecular change must first be induced 
 by light, and that this must end in decomposition. The precise 
 moment when such decomposition commences is beyond our per- 
 ception, and is not, in any case, necessary to be known. What the 
 nature is of this molecular modification, is the interesting question. 
 With respect to oxygen, we know that it is capable of existing in 
 two forms — the active or allotropic form found in ozone, and the 
 usual and less active form. Allotropic oxygen is often formed from 
 ordinary oxygen by electricity, and recent experiments have shown 
 that it is also formed by light, in those cases where bitter almond 
 oil, and other such bodies are present. It is therefore fair to con- 
 clude that light acts on the bodies which resemble oxygen in a like 
 manner, and the chlorine, which has been altered in its degree of 
 activity by solarization, will be active or allotropic chlorine. And 
 so of the rest. Bodies like hydrogen, combustible hydro-carbons, 
 and phosgene gas, are so remarkable for their high refractive power 
 on light, that Newton was led to suppose that the diamond was 
 combustible merely from its index of refraction. Some essential 
 oils and hydro-carbons can only be distinguished by a peculiar 
 power of rotating a polarized ray of light to the right or to the left. 
 These, and other like phenomena, would lead us to suppose that the 
 action of light is to polarize the elementary atoms, and that their 
 different properties are the effects of their polarity. A change of 
 polarity in one element in a compound would immediately affect the 
 whole, and might lead to its decomposition in the presence of a se- 
 cond body, or to the assumption of a different arrangement of its 
 own elements if isolated. In either case the result would be a dis- 
 tinct chemical change. But light may perhaps, without producing 
 an actual change of .polarity, set up in certain cases a vibratory 
 action among the particles themselves, similar to the regular vibra- 
 
CHE 
 
 85 
 
 tions produced by sound in sonorous bodies. This kind of action 
 is sufficient to account for the phenomena observed in the formation 
 of images by the condensation of vapours, as in the experiments of 
 M. Moser. The condensed vapours then arrange themselves in the 
 lines of vibration, just as in the figures of Chladni, minute solid par- 
 ticles arrange themselves on the sonorous surface. 
 
 The facts, therefore, which are established by experiment, and 
 which must explain for us the formation of photo-chemical images, 
 are — 
 
 First, The increase of affinity in the light of chlorine, oxygen, 
 bromine, and iodine, for hydrogen and carbon. 
 
 Second, The regular decomposition which occurs in certain cases in 
 consequence of this increase, capable of being represented by 
 chemical formulae. 
 
 Third, The gradual and progressive nature of this decomposition. 
 
 It remains to apply these to the explanation of the two processes 
 to which all others used by photographers may be referred 
 
 I. SUN-PRINTING. 
 
 II. Development Printing. 
 
 I. Chemistry of Sun Printing . — Printing on paper prepared with 
 chloride of silver may be taken as an example. This paper is first 
 prepared with a soluble chloride of sodium, barium or ammonium ; 
 it is then brushed over with a solution of nitrate of silver strong 
 enough to decompose the whole of the chloride and leave an excess 
 of nitrate. Na. Cl.-pAgo. No 5 = Ag. Cl. + Nao. No 5 + Ago. No 5 . 
 The paper now contains (1) chloride of silver, (2) moisture, (3) nitrate 
 of silver, (4) vegetable fibre, (5) starch or gelatine in the form of 
 sizing, and lastly, nitrate of soda, which, however, produces no effect 
 appreciable in the results. As to their separate influence, it may be 
 said in general terms, that the chloride decomposed by light and mois- 
 ture gives a feeble violet image, that the nitrate of silver strengthens 
 it and somewhat darkens the tone, and that the vegetable fibre and 
 size, but especially the size, give a redder and warmer tone to the 
 colour, with more vigour and greater transparency. What is the 
 chemistry of these effects ? 
 
 , First, as to the chloride. Some have supposed that the chloride 
 is reduced to the metallic state by the entire removal of the chlorine ; 
 others, that the silver loses all its chlorine, but takes oxygen ; others, 
 that it loses half its chlorine and takes oxygen to supply its place, 
 becoming a mixture of sub-chloride and sub-oxide ; and others again, 
 
86 
 
 CHE 
 
 that it loses half its chlorine without taking oxygen. It seems to 
 be a sufficient objection to the idea that in the reaction the silver is 
 oxidized, that oxide of silver, in the presence of water, is reduced to 
 the metallic state by light, and that the violet colour is not at all in- 
 dicative of the presence of oxide or sub-oxide, for of these oxides one 
 is brown, and the other black. The doubt, therefore, must lie between 
 the reduction of chloride to the metallic state, or to the state of sub- 
 chloride. The chloride of silver reduced to the metallic state by 
 hydrogen in the ordinary methods, is of a dull grey colour, in no 
 degree resembling the violet compound produced by the sun’s rays 
 acting on it, but, on the other hand, the sub -chloride of silver 
 which is formed by immersing silver plate into a solution of per-chlo- 
 ride of copper or iron is of a deep violet tint, which, when another atom 
 of chlorine is given to it by an aqueous solution of that gas, becomes 
 white. The affinity of chlorine for silver is about equal to its affi nity 
 for hydrogen, if we may judge by experiments on them in a heated 
 state ; for red-hot silver will decompose hydrochloric acid, and red-hot 
 chloride of silver will give up its chlorine to hydrogen passing over 
 it. When, therefore, chloride of silver and water act on each other 
 in the light, if the silver be reduced to the metallic state, the light 
 must be powerful enough to decompose water, to prevent the oxidation 
 of the silver, and to cause the hydrogen to remove the chlorine 
 entirely from the salt, by the superior affinity for it in those circum- 
 stances. But this is scarcely in accordance with the known power 
 of light in other similar reactions. And, as the metals which have two 
 degrees of chloridation are known to part with one atom of chlorine much 
 more easily than with both, we conclude the reduction to be to the 
 state of sub-chloride only. 2 Ag. Cl. + Ho.=Ag. 3 Cl. -f- H. Cl. -f- O. 
 It is doubtful whether the atom of oxygen is immediately liberated 
 and escapes into the air, for experiments with the iodine test for 
 chlorine indicate its presence ; it would therefore seem that, as in the 
 electrolysis of water, the liberated oxygen often unites with the water 
 to form peroxide of hydrogen, so in photolysis, the atom of chlorine 
 sometimes remains attached to an atom of water as oxychloride of 
 hydrogen, until set free by other causes. 
 
 The action of nitrate of silver is next to be considered. Its own 
 decomposition by the organic matter of the paper will be examined 
 in the next paragraph, with the paper itself ; here we are concerned 
 with the darkness which it adds to decomposing chloride of silver 
 when no such matter is present. It will be found, if chloride of 
 silver is precipitated from an excess of nitrate and exposed to the 
 sun, that it darkens more than if there be no excess of nitrate in the 
 solution. Pure chloride of silver itself is so perfectly opaque, that 
 
CHE 
 
 87 
 
 after very prolonged insolation it is only superficially decomposed, 
 and immediately beneath the surface remains white : the colour is 
 therefore too feeble to produce an effective photograph. This is the 
 case when organic matter, however active, is present, and occurs with 
 all kinds of papers, albumen as well as others, when there is no free 
 nitrate. Organic matter seems to have no action on the chloride in 
 this case ; and the reason of this is doubtless to be found in the 
 solid and intractable state in which they coexist, for in processes by de- 
 velopment where the active organic matter is in solution its influence is 
 very perceptible. The reason why nitrate of silver, added to the simple 
 chloride, increases its darkening power, would appear to be that the 
 hydrochloric acid formed by the light, precipitates from the nitrate 
 new chloride continually upon the surface, which again suffers partial 
 decomposition and renewal. In this manner a larger quantity of the 
 violet salt will be formed than if no nitrate assisted in its accumulation. 
 
 But the nitrate of silver also modifies the colour and vigour by its 
 specific reaction with the fibre and size of the paper. Cotton and 
 linen fibre have both an affinity for many metallic oxides, alumina, 
 oxide of iron, oxide of tin, &c., and among others for the oxide of 
 silver ; this affinity is sufficient to cause a decomposition of nitrate 
 of silver in the light, though the light, unaided, has no power to 
 decompose that salt, either in the solid state or in solution. The 
 same solution of nitrate of silver which would of itself remain clear 
 and bright for an indefinite period in the sun, will be very quickly 
 reduced by light when in contact with a piece of Swedish filtering- 
 paper or pure cotton wool. The oxide of silver combines with the fibre, 
 and the nitric acid is set at liberty as may be perceived by its smell. 
 The warm brown oxide therefore mixes, and perhaps combines with 
 the subchloride, to intensify the deposit, and at the same time render 
 the colour less cold and slatey. The organic matter with which the 
 paper is sized is still more efficient in these respects, and so are the 
 albumen, serum, and other substances with which papers are often 
 purposely imbued. The principal classes of organic matter are, 
 the lignines, the starches, the gums, the sugars, the gelatines, the 
 proteiniferous bodies, the neutral and acid principles of fats, essential 
 oils and liquid hydro-carbons, the resins and camphors, and the 
 vegetable acids. These include an almost infinite variety of sub- 
 stances all varying in their behaviour in photographic processes. 
 In sun-printing their action does not appear to depend upon the chlorine, 
 combining with their elements in the light, but entirely on their 
 power of forming compounds with the elements of nitrate of silver. 
 All appear to possess this faculty in some measure or other. Generally 
 they form soluble compounds with the alkalis, and insoluble ones 
 
88 
 
 CHE 
 
 with lime, baryta, and silver. Those which do not combine with 
 oxide of silver to form an insoluble organic salt, do not produce so 
 marked an effect in printing as those which do. Lignine, therefore, 
 and starch, and even gelatine, are not so efficacious in giving warm, 
 vigorous images as those bodies which contain proteine, such as 
 albumen, caseine, and emulsine, but they require less after treatment 
 to produce the blacks and purples, which are in general request. 
 Eor this reason the Erench papers, prepared with starch, are by 
 many preferred to the English, sized with gelatine and resin ; and 
 for the same cause, prints on albumenized paper are very red and 
 difficult to tone. Sun-printing is thus a double reduction, where 
 a violet subchloride and a red organic subsalt of silver are formed simul- 
 taneously, the resulting picture partaking of the colour and proper- 
 ties of both. That the red salt is a salt of the suboxide, as the violet is 
 a subchloride, is shown by the fact that when citrate of silver is sub- 
 jected to the action of hydrogen at the temperature of- 212°, the pro- 
 toxide contained in that salt loses half its oxygen, and a red brown salt 
 is formed, just as when the citrate is decomposed by exposure to light. 
 In both cases the hydrogen present removes half the oxygen. Many 
 other organic salts of silver, according to Professor Graham, comport 
 themselves in the same way when heated in hydrogen. Another 
 proof is found in the fact, that the subsalt formed by hydrogen is 
 soluble in water to some extent, giving a brownish solution ; and so 
 are the impressions on our albumenized and other papers weakened 
 by the solvent action of water, especially when warm. It might at 
 first sight be supposed, that since these organic salts of silver are 
 thus easily decomposed by light, while the corresponding salts of lead 
 and other metals are not so affected, that the silver was in some 
 way the element acted upon by the light, and that the decomposition 
 was not determined by the hydrogen and oxygen attracting each 
 other. But the difference between lead and silver in these cases is 
 probably found in the different condition of their oxides, the lead 
 having no tendency to part with half its oxygen, as the silver does. 
 Other metals which have two oxides, chlorides, &c. behave many of them 
 like silver in this respect ; for example, the ammonio citrate of iron, 
 salts of the sesquioxide of uranium, the ferrosesquicyanide of potas- 
 sium, the bichloride of mercury, and others. It may be that the 
 second atom of chlorine or oxygen exists in these compounds in its 
 allotropic form, and that therefore its removal by light is possible, 
 whereas the removal of the second atom by the same means is 
 impossible. 
 
 The chloride of silver is the only silver salt employed in sun-print- 
 ing on paper. The bromide and iodide cannot be so used, on account 
 
CHE 
 
 89 
 
 of important differences in their chemical relations. If a paper, 
 prepared as the chlorided papers in all respects, except the substitu- 
 tion of iodine for chlorine, be exposed to light, it is found impossible 
 to get beyond a feeble grey or drab-coloured impression. The che- 
 mistry of this change is more obscure than that which occurs with 
 chlorided surfaces. The following is a plausible explanation. Pure 
 iodide of silver, exposed in the dry state to the action of the atmo- 
 sphere and light, remains unchanged, and is not reduced to the con- 
 dition of subiodide, as might be expected. Calotype papers, which 
 have been prepared either by what is called the double or single 
 process, may be thus exposed without change, for any period, to sun- 
 shine, but similar chlorided papers are immediately discoloured. 
 Well-washed iodized collodion plates are equally incapable of re- 
 ceiving any visible impression; and, so far as any actual decomposi- 
 tion is concerned, pure moist iodide of silver may be pronounced 
 insensitive to light. This seems to confirm very strongly what 
 has been said respecting the mode in which the removal of half the 
 chlorine of chloride of silver is brought about ; for if the light acted 
 immediately on the elements of the compound to separate them, it 
 ought a fortiori to decompose iodide of silver where the connecting- 
 affinity is weaker. But if it be allowed that an atom of water must 
 intervene, then it is plain why iodide of silver is not affected by the 
 solar rays. Iodine dissolved in water and exposed to light, has not 
 the power to overcome the more powerful affinity of hydrogen for 
 oxygen, but on the contrary, a solution of hydriodic acid exposed to 
 . the air gives up the hydrogen of the acid to the oxygen of the 
 atmosphere and sets the iodine at liberty. The great difference, 
 therefore, between chlorine and iodine lies in the fact that the for- 
 mer has a power of liberating oxygen from water in the light, of 
 which iodine is destitute. 
 
 When, therefore, papers prepared with iodide of silver and nitrate 
 of silver darken in the sunshine, an idea might be entertained that 
 the darkening proceeded from the nitrate only; but this would 
 not be correct, because papers imbued with both salts darken more 
 quickly than if the nitrate alone is present, and also because iodide 
 of silver, precipitated from an excess of the nitrate in a test tube, will 
 be found to change colour under the sun’s action, while nitrate in 
 such circumstances is not darkened except the light is assisted by 
 organic matter. Some other reaction must consequently occur in 
 this instance between the iodide and the nitrate in solution. It is 
 found on trial, that whatever change takes place there is no libera- 
 tion of iodine when iodide of silver and solution of nitrate are acted 
 on by light, nor of any of the elements, except nitric acid, which re- 
 
90 
 
 CHE 
 
 mains in the solution ; whatever change takes place is in their ar- 
 rangement only, for in the dark the yellow colour is restored, except 
 organic matter interfere to prevent : the yellow salt may be thus 
 darkened, and restored many times in succession. A similar change 
 occurs when a brown aqueous solution of iodine is exposed to 
 light : no oxygen is given off, as has been already observed, yet very 
 slowly the brown colour disappears, but is gradually reproduced 
 in the dark. When the colourless solution is examined, it is 
 found that the iodine, not being able to displace oxygen, has com- 
 bined with both elements of the water in such a way as to form 
 hydriodic acid with the hydrogen, and iodic acid with the oxygen : 
 5H 0 + 6 1=1 0 5 + 5 H I. These acids form colourless solu- 
 tions. But in the ordinary course of chemistry, hydriodic acid is 
 decomposed very quickly by iodic acid, water being formed and 
 iodine set free. This sufficiently explains the reappearance in the 
 dark of the brown colour which the light destroyed. When free 
 iodine is added to a solution of nitrate of silver, if iodide of silver 
 only were formed, then, besides nitric acid, an atom of oxygen would 
 be liberated; for Ag. O + N 0 5 + I=Ag. I + N 0 5 + 0; but here 
 also, iodine not having the power of liberating oxygen, forms with it 
 iodic acid, and so iodate of silver : 6 Ag. ON 0 5 + 6 1= Ag. 0 I 0 5 
 + 5 Ag. 1 + 6 N 0 5 . The reason, therefore, why light has not suffi- 
 cient power to decompose iodide of silver in the presence of water 
 only, and yet can so decompose it when nitrate of silver is also pre- 
 sent, is tolerably obvious. In addition to the tendency which iodide 
 of silver has in the light to give up iodine to form hydriodic acid 
 and iodic acid, another force is introduced viz., the attraction which 
 the oxide of silver in the nitrate has for those two acids ; and it is 
 easy to suppose, that though either force separately might be insuffi- 
 cient, yet the two in union might be able to complete the change. 
 The formula of the decomposition would be 6 Ag. 1 + 6 Ag. 0 N 0 5 
 = Ag. O I O s + 5 Ag. 1+6 N 0 5 + 6 Ag., if we suppose the silver 
 is reduced to the metallic state ; or if we suppose it to become re- 
 duced to subiodide only, which seems more probable, the 6 atoms of 
 metallic silver which are shown in the formula will be united to 6 
 atoms of undecomposed iodide. In the dark, if no organic matter 
 prevent, this arrangement of the elements is altered and nitrate and 
 iodide of silver reformed. When organic matter, such as paper, is 
 present, the reaction will be different if an organic salt of silver take 
 part in it instead of the nitrate. This organic matter may have 
 such an affinity for the atom of oxygen, the non-liberation of which 
 causes the above formulse to be so complex as to cause an entirely 
 different change, or it may by its affinity for the subiodide simply 
 
CHE 
 
 91 
 
 maintain the arrangement of the salts which light has induced. The 
 former is probably its action in developed prints, the latter in sun 
 prints. We cannot wonder if this be the correct view of the dark- 
 ening of iodide of silver, t^at it should be carried to a much less 
 degree than in the case of chloride, where not a fresh arrangement 
 of the salts merely but actual release of one element occurs. The 
 light is barely sufficient in the iodized paper to overcome the incli- 
 nation which the salts have to return to their first condition, and 
 this inclination increases as the iodate and the free nitric acid ac- 
 cumulate ; the reduced subiodide is of a dingy colour, with very little 
 force ; and the quantity of nitrate of silver must be small, because 
 strong nitrate of silver has the property of dissolving iodide to such 
 an extent that, if allowed by evaporation to become concentrated, the 
 whole surface of the paper would be whitened and its sensitiveness 
 destroyed ; and the organic matter has no influence directly on the 
 iodide, any more than it has on the chloride. For these reasons the 
 chloride is universally employed in sun-printing. 
 
 II. Development- Printing. — By development is meant the 
 rendering visible an invisible actinically formed image, or the 
 intensifying a visible one, by means of chemical reagents acting 
 by their ordinary chemical affinities independent of the light. 
 Both visible and invisible impressions of light may be developed. 
 A body whose presence in a certain preparation is essential to its 
 modification by light, is called a sensitizer ; if not essential to 
 the actinic influence taking effect, but only favouring the production 
 of the effect in a shorter time, it is an accelerator ; if itself decom- 
 posed in such a way by the substance which has already been 
 modified or decomposed by light as to attach itself to it, or in any 
 other way render the effect perceptible or plainer than before, it is a 
 developer. The same compound may fill all three offices, but its use 
 is different in the three cases. It is important to remember that the 
 developer is always a compound, either decomposing spontaneously, 
 or suffering decomposition by the substance produced by light in the 
 sensitive surface. In the usual processes of photographic printing 
 by development, the developer is always a mixture of solutions which 
 are gradually letting fall a precipitate by their action on each other. 
 The principle of development is therefore altogether different from 
 sun-printing, and the chemistry proceeds on altogether a different 
 principle. The power which is continually acting in sun-printing 
 is the power of light in causing certain elements to unite : the force 
 resorted to in development is simply the attraction of cohesion, by 
 which atoms which are homogeneous tend to cling together. So 
 
92 
 
 CHE 
 
 entirely is this true, that after a print has been developed, washed, 
 altered by a fixing solution, and washed again, after it has been dried 
 and knocking about for months, the process of development may be 
 continued. When the image has, therefore, once been formed in the 
 camera, or the pressure frame, all that is done further is to pour over 
 them a mixture from which silver, in some form, is being very slowly 
 precipitated, so that at the moment when the precipitate is being 
 liberated from the solution it finds itself in contact with the particle 
 to which it has a tendency to adhere, just as when chloride of silver 
 is being precipitated from the nitrate, the particles which are within 
 each other’s sphere of attraction, cohere to form a flocculent mass 
 instead of going separately to the bottom. All that is necessary pre- 
 vious to development is, that centres of attraction shall be 
 formed by the change which light has caused, having this affinity for 
 the particles which are to be precipitated upon them. What are the 
 particles which are thrown down by the usual developers P These 
 developers are nitrate of silver mixed with either sulphate of iron, 
 gallic acid, or pyrogallic acid. 
 
 When nitrate of silver and sulphate of iron are mixed in solution, the 
 iron takes the oxygen of the silver and the silver is precipitated in the 
 metallic state. When gallic or pyrogallic acid is used, the precipitated 
 silver is found to be combined with a small quantity of organic matter, 
 which has not been examined. The image differs therefore in the 
 three cases, but until a proper analysis has been made, we cannot do 
 more than point out the more truly metallic nature of the image 
 formed when a protosalt of iron is employed, than when recourse is 
 had to an organic deoxidizer. 
 
 Both chloride and iodide of silver have been extensively used for 
 printing by development. The same objection does not lie against 
 the iodide, in this mode of producing pictures, that was found in sun- 
 printing. Though giving a very feeble direct print, it is found that 
 the incipient image is more quickly formed, and the reason may be 
 easily drawn from what has been already remarked. A rearrange- 
 ment of the elements, and a production of suboxide sufficient to 
 develope upon, is very speedily effected, and that long before any 
 impression is perceptible to the eye, and when the developer is 
 applied, having a strong affinity for oxygen, the atom of oxygen which 
 prevented the formation of iodide without iodate is removed and the 
 development proceeds rapidly. But in the case of chloride, the de- 
 composition, because more complete, requires a stronger and more pro- 
 longed action of the light, and the image must be quite visible before 
 the development commences. 
 
 Two effects are observed in printing by development on iodide of 
 
CHE 
 
 93 
 
 silver which do not occur on the chloride. These are such a reversal 
 of the usual manner of impression by the light that, on development, 
 the precipitating silver is adherent on the parts which have had the 
 least intense light upon them, instead of those which have been most 
 vividly illuminated ; and the other is a reddening of the image in the 
 parts which have been struck by the highest lights. They are pro- 
 bably, both of them, to be accounted for by changes produced in the 
 sensitive surface, by light of a certain intensity and duration, in 
 which the elements return to their unimpressed state, or some element 
 is actually eliminated. The iodide of silver with the nitrate is 
 subject, as has been shown in the paragraphs on sun-printing, to hover 
 between two states, either of which it may be made to assume at 
 pleasure. When acted upon by light, iodate, iodide, and subiodide 
 of silver are formed, and nitric acid liberated. This liberation of 
 nitric acid, against its affinity for oxide of silver, must have a retard- 
 ing effect upon the gradual change operated by light. That the 
 state of the substances after insolation is somewhat forced and strained 
 when judged of by ordinary chemical affinities, is shown by the fact, 
 that darkened iodide of silver, in the absence of organic matter, will 
 return to the yellow form ; and therefore, as the nitric acid increases in 
 quantity and in power, with the intensity and duration of the illumi- 
 nation, a time is likely to arrive when it would suddenly upset the 
 constrained relations of the parts, and reproduce the original arrange- 
 ment. In this case, the most strongly lighted parts would receive 
 little or no deposit in the development. This view of the matter is 
 confirmed by the circumstance, that it is when a bath of nitrate of 
 silver, containining much nitric acid is used, and when little or no 
 organic matter is present that this reversal takes place. It has been 
 produced and remedied many times successively, and with absolute 
 uniformity, by alternately using a large comparative quantity of nitric 
 acid, and of acetic acid, nor is it observed except when pictures of a 
 leaden metallic appearance are being developed, the colour and 
 feebleness of which are indicative of nitric acid and metallic silver, 
 free from organic admixtures. It is also sometimes purposely 
 produced by exposing the plate for an instant to the light at 
 the very commencement of the development. This momentary 
 exposure perhaps acts just as the intenser light in the more exposed 
 parts of the image, to which we have adverted. If this explanation 
 be correct, a true alternation ought to take place, in the effect, since 
 when the particles are again placed in the status quo ante the 
 further impact of light would operate as at first. Some experi- 
 menters have said that this regular alternation of effects really occurs. 
 A similar phenomenon is observed when hydrochloric acid is subject- 
 
94 
 
 CHI 
 
 CHL 
 
 ed to electric sparks : the hydrogen and chlorine are separated until 
 a certain point is reached, and then another spark causes their instant 
 and explosive recombination. The effect seen in the reddening of the 
 skies of landscapes and of the strongest lights in portraits may be 
 the same thing, prevented by organic matter from fully taking effect, 
 or more probably be only the consequence of more violent action 
 upon the developer. 
 
 These observations show that if the use of iodide of silver be at- 
 tended by the great advantage of sensitiveness, the sensitiveness is 
 itself the cause of disadvantages which it requires skill to obviate. 
 On chloride of silver the sun-printing and development may be 
 mixed in any proportion, and to the experimenter’s taste without 
 entailing any of these inconveniences. 
 
 Space will not allow any detailed explanation here of the che- 
 mistry of the fancy photographic processes, or even of the number- 
 less interesting photochemical experiments on particular salts which 
 have been recorded ; they are briefly noted in their own places. 
 The principles above laid down are the main clue to the interpreta- 
 tion of all. 
 
 Chiar’ oscuro ; (Italian, “ Chiaro,” light, “ oscuro” dark). The 
 light and shade of a picture is called “ chiar’ oscuro,” a term much 
 used by artists. See “Breadth.” 
 
 China Clay. See “ Kaolin.” 
 
 Ciiloracetic Acid- An acid formed by exposing acetic acid 
 and chlorine to light. Chlorine displaces hydrogen, until C 4 H 3 0 3 , 
 H O, becomes C 4 C1. 3 0 3 , H 0. 
 
 Chlorate of Potassa. K O, Cl. 0 6 . This salt is used in 
 the production of oxygen gas, which it gives off readily when mixed 
 with a small quantity of binoxide of manganese ; its oxygen is the 
 element which causes its use in the manufacture of lucifer matches, 
 and of those compounds which are intended to burn with an 
 intensely vivid light. It has been used as a first wash for paper, 
 instead of a chloride. 
 
 Chloride. The compounds of other elements with chlorine are 
 either acids or chlorides. With oxygen and hydrogen it forms 
 acids, with metals and other elements chlorides. The chlorides of 
 the alkalis and alkaline earths are used in photography not as 
 sensitive bodies, but as convenient salts for the production of those 
 
CHL 
 
 95 
 
 which are. The chlorides which would be especially likely to be 
 affected by the light falling upon them, are the compounds of silver, 
 platinum, and metals not easily oxidized ; and also such metals as 
 iron, which have several oxides and chlorides, and combine in more 
 complex forms than those whose affinities for oxygen and chlorine 
 are much stronger, and whose salts are much simpler. Iron, for 
 instance, has four oxides and two chlorides, and the oxygen and 
 chlorine appear to have the power of replacing each other in some 
 cases. Their relations resemble those of organic compounds more 
 nearly than those of many simpler inorganic bodies, and would 
 therefore appear to promise the production of compounds suitable 
 for the production of photographic images. Cobalt, copper, iron, 
 and manganese are especially remarkable for these properties, and 
 many have been already found to be decomposed, or modified, by 
 light and organic bodies. Chlorine appears to have a greater affinity 
 for most of the metals than oxygen, so that it is removed by heat 
 and light with greater difficulty ; and though chlorine at a high 
 temperature decomposes most of the oxides, and entirely disengages 
 the oxygen, oxygen acts only on a very few chlorides, principally of 
 the class which has been spoken of as forming complex arrange- 
 ments. Iron seems to be nearly balanced between chlorine and 
 oxygen, for sesquioxide of iron dissolves in hydrochloric acid to form 
 sesquicliloride of iron and water ; but the sesquichloride, evaporated 
 and exposed to the action of the air, again forms sesquioxide and 
 hydrochloric acid. There can scarcely be a doubt that this last action 
 would be accelerated by light. Water is decomposed at ordinary 
 temperatures by the perchlorides of manganese, chromium, colum- 
 bium, and tungsten, and by the chloride of arsenic : hydrochloric 
 acid and a metallic acid are the results. Chloride of tungsten is 
 decomposed by it, with the disengagement of hydrochloric acid, and 
 the precipitate of oxide of tungsten of a violet color. Water also 
 acts upon the chlorides of antimony, bismuth, and tellurium, decom- 
 posing them into oxychlorides which are insoluble, and chlorides 
 soluble in the atoms of hydrochloric acid formed. In the reaction 
 of water upon the subchloride of tellurium some of the metal is even 
 reduced to the metallic state. The chlorides of silver, platinum, gold, 
 &e., are believed to be all decomposable by water and light, more or 
 less easily. Some that do not appear to form hydrochloric acid 
 with water, as the bichloride of mercury, do so in the light in ether, 
 or other solvents. The alkaline and earthy chlorides, whose con- 
 necting affinity is stronger, do not appear to be oxidized or 
 reduced by water, in which they are very soluble ; but chloride of 
 sodium and others will speedily give colour in the light to alcohol 
 
96 
 
 CHL 
 
 and other hydrogenous solvents. It is impossible not to see in all 
 this that the affinity which is instrumental in causing these 
 changes is that of hydrogen for chlorine, nor can the mind escape 
 the impression that this is all extremely interesting as an illustration 
 of the probable action of light in most of the instances in which its 
 chemical effects have been observed. Many of these reactions of 
 water and chlorides have been observed by chemists, without any 
 attention to the share which light might* take in the operation : but 
 this is a subject highly worthy of investigation. As the chlorides 
 are most of them used to form chloride of silver, their equivalents 
 and solubility in water, or insolubility, should be attended to. 
 
 Chloride of Ammonium. N H 4 Cl. = 54. Applied as a 
 first wash to printing papers, and in chloridizing collodion : 54 parts 
 of it for these purposes are equal to 60 of chloride of sodium, and 
 to 105 of anhydrous, or 123 of crystallized chloride of barium: it 
 dissolves sparingly in alcohol; about 1*5 per cent, in alcohol S. G. 
 834, or 6 grains to the fl. oz. The nitrate of ammonia formed is a 
 solvent of oxide of silver, s and therefore sometimes injurious : when 
 papers or plates are placed successively in the same silver bath it accu- 
 mulates. It dissolves bromide and chloride, but not iodide, of silver : 
 and gives a picture less red than chloride of sodium produces. 
 
 Chloride of Barium. Ba. Cl. = 105, or Ba. Cl. 2 Id 0 = 
 123. The crystals are much less soluble in alcohol than those of 
 the last-named salt, spirit of S. G. 834 dissolving only per 
 cent. It gives a slightly more purple colour to the reduced sub- 
 ♦ chloride than either the salt of ammonium or sodium : mixed with 
 chloride of gold, it gives a surface sensitive to light and affects the 
 tint of the reduced metal. It is used as a test for sulphuric acid, 
 and for carbonate, iodate, and sulphate of potass, existing as im- 
 purities in iodide of potassium : it is precipitated as a white salt 
 when these impurities are present. It is much more soluble than 
 nitrate of baryta. 
 
 Chloride of Bromine. The mixture of chlorine and bromine, 
 so-called, used as an accelerator in the daguerreotype, is not a definite 
 compound. 
 
 Chloride of Calcium. Ca. Cl. = 56, and, when hydrated, 
 Ca. Cl., 2 H O = 74. This salt is soluble in almost any quantity 
 in water at 220° : its attraction for water being so great, that it 
 soon deliquesces in the air, becoming what was once called oil of 
 
CHL 
 
 97 
 
 lime. On this account it is employed freshly fused to deprive gases 
 of aqueous vapour, and as a means of depriving ether and alcohol 
 of water. It is copiously soluble in alcohol *. ten parts of anhydrous 
 alcohol S. G. 794 dissolve seven of the chloride, and the solution 
 in cold weather affords crystals containing 60 per cent, of alcohol, 
 instead of water of crystallization. It may be conveniently used in 
 testing comparatively two samples of collodion for water, by dissolving 
 6 or 8 grains, while hot after fusing, in a drachm of the plain, or in 
 half a drachm of iodized collodion. As often sold it is in the 
 form of the hydrate, and, when warm, dissolves in its contained 
 water : this must be boiled away, and the dry salt then fused. When 
 employed on printing paper it gives a very red picture : the nitrate 
 of lime formed is deliquescent, and may prove injurious if the paper 
 be long kept. Newly-made anhydrous chloride of calcium is phos- 
 phorescent. It is often present in common salt, and then makes it 
 very moist. It must not be confounded with chloride of lime, 
 which contains oxygen as well as chlorine. 
 
 Chloride of Copper. There are two chlorides of copper, a 
 subchloride Cu. 2 Cl. and a chloride Cu. Cl. ; the latter is the salt 
 generally employed. When copper wire is twisted into a spiral 
 form, made hot, and plunged into a vessel of chlorine, it burns like 
 iron in oxygen, though with less intensity : thick vapours of the 
 chloride are formed, which are every now and then rifted by sparks 
 of vivid flame. It parts readily with half its chlorine, and has been 
 on this account added to hyposulphite of soda to produce toning 
 properties, which it does in the same manner as free iodine ; also to 
 assist iodide of potassium in bleaching papers which have been pre- 
 pared with the sensitive silver salts and darkened by the sun ; and 
 in giving chlorine to silver plates immersed in its solution, with a 
 view to form a coating of violet subchloride of silver that shall give 
 the colours of the spectrum. 
 
 Chloride of Gold. There are two chlorides of gold, viz., Au. 
 Cl. and Au. C1. 3 : the former is prepared by heating the latter to 
 the temperature of melting tin, till no more chlorine is evolved, and 
 is decomposed by water into the latter and metallic gold. The 
 second of these two only is soluble, and is the salt generally called 
 the chloride of gold. Gold unites directly with chlorine, and it also 
 dissolves in an aqueous solution of chlorine. Gold cannot decompose 
 hydrochloric acid, either dry or in solution, because the attraction 
 of chlorine for hydrogen is greater than for gold. It can liberate 
 the hydrogen in neither case : but if nitric acid be also present in 
 
 H 
 
98 
 
 CHL 
 
 the solution, then the hydrogen, instead of escaping, forms water 
 with the oxygen of the nitric acid and disengages N 0 2 , which is 
 held by a much more feeble force than the hydrogen of either hydro* 
 chloric acid or water. The attraction of chlorine for gold is 
 stronger than that of N 0 2 for 0 3 . But the affinity of chlorine for 
 gold is not great, and hence it is easily driven off by heat. It is also 
 reduced to the metallic state by protosulphate of iron, and by gallic, 
 acetic, citric, oxalic, and other acids which take oxygen, and then 
 forms hydrochloric acid from the elements of water. When neutral, 
 the compound and its solution are red ; but if hydrochloric acid be 
 present, yellow. It is of important use in photography : in the art 
 it is constantly made use of for toning the red newly -fixed images, 
 for making the sel d’or, or double hyposulphite of gold and soda, 
 for gilding daguerreotypes, and even in chloridizing collodion, and, 
 mixed with other chlorides, in preparing papers fbr the printing 
 frame. It has been further applied to the reduction in strength of 
 over-printed positives, and to the restoration of those which have 
 faded. Solution of chlorine, or any easily decomposed chloride, as 
 the perchloride of copper, or bichloride of mercury, will reduce a 
 print, when quite bronzed, to a reasonable condition as well as the 
 chloride of gold. To revive a print, immerse it in a bath composed 
 of 40 grains of the chloride to a pint of water rendered slightly acid 
 with hydrochloric acid : in the light of the sun a few minutes are 
 sufficient, but in the dark three or four hours are necessary to com- 
 plete the restoration, when the print must be fixed again with fresh 
 hyposulphite. The other uses are explained elsewhere. It is best 
 kept in solution. To make it, place a half-sovereign in a vessel 
 containing nitro-hydrochloric acid and digest with a gentle heat: 
 add fresh aqua regia when action has ceased if it be not entirely dis- 
 solved. The solution will contain chloride of gold and chloride of 
 copper, and free acid : add, therefore, carbonate of soda enough to 
 neutralize the acid and precipitate a green salt of copper, which may 
 be left till the next day to settle. The solution will contain about 
 85 grains of chloride of gold, and if rendered slightly acid, and 
 made up to 85 fl. drachms, will be stable, and each drachm will 
 represent a grain of the chloride. 
 
 Chloride of gold is a photo-sensitive salt : a piece of paper 
 moistened with it and exposed to light becomes purple in conse- 
 quence of its decomposition. The oxides of gold are more or less 
 purple, and the finely divided metallic gold reddish brown : the 
 chloride of gold would, therefore, appear to be reduced to an oxide, 
 Au. C1. 3 , + 3 H O = Au. O + 8 H Cl. This oxide has not the 
 instability of the teroxide in the light. Pictures toned with gold 
 
CHL 
 
 99 
 
 in the form of teroxide have been known to become redder in time 
 owing to the reduction of the gold to the state of pure metal. 
 The terchloride of gold is the basis of the chrysotype process, but 
 it is merely a developer in this case. A persalt of iron in the pores 
 of the paper is reduced, by light, to the state of protosalt in the 
 sunned portions, and upon this, by the reducing power of the iron 
 itself, gold is precipitated in the metallic state, or silver if nitrate 
 of silver be brushed over the paper instead of terchloride. If chlo- 
 ride of gold and chloride of barium are both in the paper when the 
 exposure takes place, then instead of the gold appearing in the purple 
 colour of the oxide, the first effect is to remove the slight pinkey or 
 yellowish colour of the gold salt entirely and then slowly and feebly 
 darken it. But before any decidedly visible image is formed if 
 the paper be immersed in water, the picture is rapidly developed to 
 a full purple brown. Hot steam has the same effect as water and 
 is more energetic. The following is probably the rationale of this 
 interesting change which has not hitherto been explained. Chlo- 
 ride of gold (Au. C1. 3 ), forms with chloride of barium, ammonium, 
 &c., double salts containing an atom of each, and which crystallize in 
 a regular manner with four or five atoms of water. The gold cannot 
 be precipitated by alkalis from the solutions of these salts : and in 
 this and their constitution they resemble the double salts of citrate 
 of iron and citrate of ammonia and the like, such as the potassio- 
 tartrate of iron, the ammonio-oxalate of iron and many others. 
 When these double salts of iron are exposed to light the oxygen 
 and carbon of the organic acid are so excited — See “ Chemistry of 
 Photography,” — that carbonic acid is formed and escapes, and as a 
 consequence the salt is reduced to the state of protoxide. Similarly 
 when the double chloride of gold and barium is solarized, hydro- 
 chloric acid escapes and the gold is reduced to the state of proto- 
 chloride. But protochloride of gold, though permanent when dry, 
 is quickly resolved by water, and especially by hot water into 
 metallic gold and perchloride. The first whitening effect observed 
 is the formation of the protocloride and the subsequent darkening 
 is the commencement of the resolution of the double compound by 
 the action of the atmosphere. Similar effects occur with oxalate 
 of ammonia, acetate of lead, bichromate of potash and iodide of 
 of potassium with this chloride. 
 
 Chloride of gold can be entirely separated in the neutral state 
 from the acid solution by means of ether, which dissolves it more 
 readily than water. The hydrochloric acid remains with the water. 
 This is a neat way of neutralizing it for the preparation of toning 
 baths. The salt is extremely deliquescent, and, therefore, cannot be 
 
 H 2 
 
100 
 
 CHL 
 
 conveniently kept dry : its solution in pure water, slightly acid, will 
 not decompose, but an alcoholic or ethereal solution might decom- 
 pose in the light, if kept long. 
 
 Chloride of Iron. What is usually called the chloride of 
 iron is the sesquichloride or perchloride (Fe. 2 C1. 3 ), it is formed 
 by dissolving the peroxide (Fe. 2 0 3 ), in hydrochloric acid and 
 evaporating to dryness. It is soluble in alcohol and ether, and 
 deliquescent. The alcoholic solution is the tincture of the shops ; 
 boiling alcohol dissolves its own weight. Added to hyposulphite 
 of soda it produces sulphur compounds which impart good tones 
 to photographs, but make them liable to fade : it acts on the soda 
 compounds as free chlorine would, and loses chlorine. Mixed with 
 iodide of potassium and spread over darkened chloride paper, it 
 bleaches out the dark impression in the light, the potassium giving 
 iodine, and the iron chlorine, which combine with the reduced silver. 
 
 Chloride of Lead. A white fusible compound crystallizing 
 in needles from hot solutions, and in some respects resembling 
 chloride of silver. It is insoluble in alcohol, but very soluble in 
 hyposulphite of soda, in combination with which it makes a good 
 toning bath for positives. Chloride of lead is obtained by digesting 
 litharge in hot hydrochloric acid. 
 
 Chloride of Lime. Bleaching powder, soluble in ten parts of 
 water and then called bleaching liquor. Formed by exposing hydrate 
 of lime to the vapours of chlorine. Similar compounds are pro- 
 duced by substituting bromine or iodine for chlorine, and they con- 
 stitute the best form of accelerator for imparting bromine to the 
 iodized daguerreotype plate. They contain calcium, oxygen, and 
 chlorine or bromine, and are now generally supposed to be hypo- 
 chlorites, &c. Chloride of lime is thus represented as Ca. Cl. 
 + Ca. O. Cl. 0. The greatest attention must be paid in forming 
 these salts to keep down the temperature, or entirely different and 
 useless compounds will be produced ; these even form at common 
 temperatures in the course of time. The carbonic acid of the air is 
 quite strong enough to decompose them and evolve the hypochlorous 
 acid, &c., which has powerful bleaching properties. Chloride of 
 calcium contains no oxygen : the lime salt is sometimes not entirely 
 removed from paper after bleaching, 
 
 Chloride of Magnesium. Mg. Cl. This salt is of great use 
 in the collodion process, when it is thought desirable to add a 
 
CHL 
 
 101 
 
 chloride as well as an iodide to the iodizing solution, for it is 
 soluble in twice its weight of alcohol. Its effect is to increase the 
 density of negatives, and prevent solarization of the skies ; it is also 
 useful in the dry process, for increasing the sensitiveness of the 
 plates by rendering the perfect removal of the free nitrate of silver 
 unnecessary. In printing transparent collodion positives upon glass, 
 it increases the density and vigour of the blacks, and improves the 
 tone of the picture. The nitrate of magnesia formed in the N.S. 
 bath is an acid salt, with an effect resembling that of free nitric 
 acid. 
 
 Chloride of magnesium is extremely deliquescent. It is obtained 
 by passing hydrochloric acid over red-hot magnesia. Sea- water 
 contains about one-fifth part as much of it as of chloride of sodium. 
 
 Chloride of Mercury. Hg. Cl. Calomel. When 100 is taken 
 as the atomic weight of mercury, this salt is a subchloride or 
 dichloride, since it takes two equivalents of 100 each to form with 
 one equivalent of chlorine (36), one equivalent of the chloride 236 ; 
 but when 200 is taken as the atomic number of mercury, then this 
 salt is called the chloride. The opinion of chemists in general is, 
 that 100 should be taken as the equivalent number of mercury, and, 
 if so, calomel will be a subchloride, though in commerce it is still 
 called the chloride, and corrosive sublimate the bichloride. Calomel 
 is insoluble ; like the subchloride of silver it is resolved by solvents 
 (hydrochloric acid) into metal aud a higher chloride : it phosphoresces 
 when scratched. It is decomposed by alkali, and ammonia, and black 
 suboxide is one of the results. This black suboxide is what is 
 formed when photographs are whitened by bichloride of mercury, 
 and the produced chloride of silver and chloride of mercury are 
 acted upon by ammonia. The bichloride of mercury is soluble in 
 18 parts of water at 60°, and in 3 of hot water, in 2 parts of 
 alcohol and in 3 of ether, also in its own weight of hydrochloric 
 acid. By parts , are intended parts by weight. See “ Bichloride 
 of Mercury.” 
 
 Chloride of Nickel. Formed by dissolving nickel or its 
 oxide in hydrochloric acid, and heating the salt to redness. Soluble 
 in hot water. 
 
 Chloride of Platinum. The chloride of platinum usually 
 spoken of is a bichloride (Pt. C1. 2 ) easily soluble in water, ether, 
 or alcohol. The protochloride (Pt. Cl.) is insoluble like the cor- 
 responding salt of gold. When the bichloride of platinum is mixed 
 
102 
 
 CHL 
 
 with lime water, or solution of carbonate of soda, and exposed to 
 light, a white precipitate is caused in the lime water, and a red 
 precipitate in the soda solution. The white powder has been deter- 
 mined by Weiss and Dobereiner to be a hydrated compound of 
 chloride of calcium with lime and peroxide of platinum, having the 
 formula Ca. Cl. + Ca. 0, 2 Pt. 0 2 + 7 H 0 ; and the red 
 crystalline precipitate to be Na. O, 3 Pt. 0 2 , 6 H 0. These two 
 compounds are sufficiently complex and might seem to indicate a 
 very intricate reaction when the solar rays fall upon the mixture. It 
 will be found, however, on examination that the increase of attrac- 
 tion of chlorine for hydrogen in the light will account for the change 
 in a very simple manner, thus : — 
 
 2 Ca. 0 + 2 Pt. ni . 8 + 10 H 0 = Ca. Cl., Ca. 0, 
 
 2 Pt. 0 2 , + 7 H O + 3 H Cl. 
 and 
 
 Na. 0 C 0 2 + 3 Pt. C1. 2 + 12 H O = Na. 0, 3 Pt. 0 2 , 
 
 + 6 H 0 + 6 H Cl. + C 0 2 , 
 
 from which formulae it will be seen that in both cases hydrochloric 
 acid is formed, and that the platinum becomes oxide instead of 
 chloride. The light does not appear to have the power of causing 
 water alone to decompose chloride of platinum as it does chloride 
 of silver, but when other substances are present which have an 
 affinity for oxide of platinum, and, therefore, may be supposed to 
 exert some force in its formation, then the decomposition by light 
 will take place. Paper has the power of slightly decomposing 
 chloride of platinum in the light, for with some days’ exposure, the 
 effect is visible. In one of Sir John Herschel’s experiments neutral 
 bichloride of platinum was exposed to the sun and brushed over 
 afterwards in the dark with nitrate of silver. The picture was 
 weeks before it began to develope, but still it developed in all its 
 details. It has been used as a toning agent both before fixing and 
 also in the toning bath, just as chloride of gold, but it is not so 
 energetic, and the tone is less pleasing. 
 
 Chloride of Potassium, K Cl. = 76. This dissolves in 3 
 parts of water at 60°, but is insoluble in alcohol. This salt is a 
 residue in several chemical processes and is often present as an im- 
 purity in the iodides and bromides of the metals, and in nitre as 
 occurring in commerce. In preparing papers it is not much used, 
 though the nitrate of potassa formed in sensitizing would not be so 
 likely to damp the paper and spoil it when kept, as nitrate of soda. 
 
 Chloride of Silver, Ag. Cl. = 144. Formed by precipitation 
 
CHL 
 
 103 
 
 when a soluble chloride is added to nitrate of silver or other soluble 
 salt of silver, and of essential service in photography. It is per- 
 fectly insoluble in water, ether, and alcohol ; but soluble in alkaline 
 chlorides, hyposulphites, cyanides, bromides, iodides, &c., and in 
 ammonia. When dissolved in a chloride or other such salt, double 
 salts are formed soluble in a small quantity of water, but not in a 
 larger ; hence adding water to a clear solution decomposes them, 
 throwing down the silver salt. Hyposulphite of silver is often thus 
 seen in the interior of papers, where dilute solutions of hyposul- 
 phite of soda have been used in fixing photographs. And in con- 
 sequence of this law of solubility the more concentrated the alkaline 
 solution, the more silver salt will it dissolve. When chloride of 
 silver is dissolved in this manner, it is no longer chloride, but hypo- 
 sulphite, cyanide, or as the case may be, the chlorine going to the 
 alkali. It is also soluble to a minute extent in nitrate of silver, 
 and thrown down in a similar manner by water in excess. It differs 
 from iodide of silver in being more soluble in haloid salts, but less 
 so in nitrate of silver; iodide of silver is not soluble at all in 
 ammonia. At 500° of temperature it fuses and concretes into a 
 substance grey and semi-transparent resembling horn. It is re- 
 duced to the metallic state by fusion at a bright red heat, with 
 twice its weight of potassa, or soda, or carbonate of either ; also 
 by boiling with potassa in solution, especially when sugar is also 
 present ; and by zinc, tin, lead, iron, copper, mercury, and some 
 other metals when moist, and more quickly when hydrochloric or 
 sulphuric acid aids the decomposition by its conducting power, 
 for the force in these cases is that of electrolysis. Zinc filings 
 triturated with moist chloride of silver decompose it with such 
 heat, that the zinc and metallic silver fuse together. 
 
 Chloride of silver is said by some to be decomposed by light 
 without the intervention of other elements : but this view is not at 
 present supported by sufficient evidence. When decomposed by 
 sunlight in the presence of water, chlorine has been spoken of as 
 existing free in the water, and if this be correct then it is a 
 proof that the water was not the cause of the decomposition, but if 
 it exists in the water in the form of hydrochloric acid and not 
 chlorine, then it is a proof that water took part in the change. 
 That hydrochloric acid is formed and oxygen liberated cannot be 
 denied, but it is said some chlorine is also found in the free state, 
 but no satisfactory proof of this has been produced. Chloride of 
 silver has been decomposed by light in an atmosphere of chlorine, 
 but in that case the experiment was tried on the salt as formed in 
 the pores of paper which contains hydrogen. Niepce found it to be 
 
104 
 
 CHL 
 
 discoloured in vacuo when bitumen and other organic matters 
 which in the air are decomposed by light remained intact, but a per- 
 fect vacuum is not attainable, and chloride of silver is much more 
 sensitive than bitumen. When presented to light under sulphuric 
 acid in a stopped bottle, quite full, no change takes place ; and that 
 the acid, as such, does not prevent its discolouration is proved by 
 the fact, that the change goes on if the stopper of the bottle be re- 
 moved so that air can reach the surface of the acid. If air, or mois- 
 ture rather, can act on the chloride through the sulphuric acid we may 
 well suppose it can in an imperfect vacuum. Chloride of silver will 
 thus be reduced by light even under strong nitric acid. It has been 
 given as a proof that it is directly decomposed, that it blackens when 
 carefully freed from moisture and exposed under benzole in a tube 
 from which the air is expelled by boiling the benzole. But benzole 
 contains hydrogen, and is very much inclined to unite with chlorine : 
 this only shows that if the benzole were pure the chloride can be decom- 
 posed without being oxidized : and that it is not oxidized in these 
 experiments is also shown by the accumulation of oxygen, above 
 the water, in the bottles in which they are conducted. Pure isolated 
 chloride of silver appears therefore to be not decomposable by light : 
 and the presence of hydrogen, or a body having affinity for chlorine, 
 seems essential to its decomposition. The part of the spectrum 
 which acts on chloride of silver includes the rays from the green to- 
 wards the violet, to a space quite beyond the visible spectrum : the 
 portion from the green to the extremity of the red, collected to a 
 focus so bright that the eye can scarcely endure it will not discolour 
 chloride of silver exposed to it for hours. The point of maximun 
 effect is even beyond the violet end. Prom the power which it 
 possesses of displacing and setting free oxygen from hydrogen it 
 suffers a loss of chlorine in the light, but there is no corresponding 
 liberation of iodine and bromine from their silver salts in conse- 
 quence of their having weaker affinities than oxygen : hence the 
 chloride gives the stronger image in the light. But the actual liber- 
 ration of the oxygen requires a stronger and more prolonged action 
 of light on the part of the chlorine than is necessary to change the 
 iodide and bromide, for these change colour and darken slightly 
 though they lose no iodine or chlorine, and set free no oxygen. 
 They enter into new combinations, merely, and hence are more 
 quickly altered than the chloride, and give a picture, by development, 
 with less insolation. 
 
 Though a picture is produced by light on chloride of silver, no 
 chloride remains in the fixed image, and therefore the same result 
 can be obtained by using other silver salts : it is only useful as ac- 
 
CHL 
 
 105 
 
 celerating the action. An image formed, directly, on pure chloride 
 of silver with excess of nitrate on a glass is, after fixing, metallic 
 silver : if developed with a protosalt of iron, it is the same : if de- 
 veloped by pyro-gallic or gallic acid it contains elements of the de- 
 composed organic acid. On paper the image is always combined 
 with organic matter : on collodion it is often nearly pure metallic 
 silver. The chloride will not give strong pictures without excess of 
 nitrate, and when, in following the usual method of proceeding, the 
 picture comes out mottled with white or feeble patches, the silver 
 bath has become too weak, or not enough was brushed on, or the 
 paper did not float on it long enough, or the cold weather required 
 the paper to lie on the silver longer than usual. 
 
 Chlorine is a much more powerful element in its affinities for or- 
 ganic compounds of hydrogen, carbon and oxygen, than iodine : it 
 decomposes them by displacing hydrogen and intruding itself into 
 the gap. Iodine is much less intrusive in this way. Hence in sun - 
 prints, and prints by development, the presence of organic matter has 
 a much more powerful influence on the resulting picture, its appear- 
 rance, and its composition, when chlorides are present than when 
 iodides alone are used. The action of the chlorine in displacing 
 hydrogen is attended with the formation of bodies of acid properties 
 which unite with the reduced metal. Pictures produced on iodide 
 of silver, are, therefore, more metallic in their composition than those 
 on chloride, and bluer and colder in their appearance. The finished 
 pictures vary also, as might be expected, in their susceptibility of 
 change, by oxidizing, or sulphuretting, or other compounds : the 
 purer metal is generally more permanent, and developed prints more 
 stable, than sun-prints. A russet apple keeps better that a rosy- 
 cheeked one. 
 
 Chloride of Soda. A compound like chloride of lime. If this 
 be used instead of chloride of sodium in preparing papers, chlorate 
 of silver will be formed as well as chloride, and the sensitiveness 
 decreased. 
 
 Chloride of Sodium. Na. Cl. = 60. Common salt. A. thou- 
 sand parts of common salt contain from 930 to 990 parts of pure 
 chloride of sodium : chloride and sulphate of magnesia are the im- 
 purities. If wanted pure for analysis or for testing silver baths, it 
 must be made by exactly saturating pure hydrochloric acid with car- 
 bonate of soda, so as to make a perfectly neutral solution : but rock 
 salt is quite pure enough for printing, and other such purposes : if 
 it contains much magnesia it deliquesces. There is a peculiarity in 
 
106 
 
 CHL 
 
 the solubility of salt, viz. that it is as soluble in cold water as in 
 hot : it requires three parts of water to one of salt. Half an ounce 
 only is taken up by 100 ounces by weight, of alcohol, of S. G. 834. 
 so that a very small quantity only could remain in solution in collo- 
 dion, since it is still less soluble in ether and that part would be liable 
 to precipitation in the setting of the him. 
 
 Chloride of Strontium. Sr. Cl. = 80 . Obtained by dis- 
 solving carbonate of strontia in hydrochloric acid diluted, evapo- 
 rating to dryness, and fusing the residue. If dissolved and crystal- 
 lized, the crystals contain six atoms of water, and the equivalent 
 will be 134. This salt is readily soluble in alcohol and in twice its 
 weight of water : the crystals deliquesce. 
 
 Chloride of Tin. The protochloride of tin, Sn. Cl. is sold in 
 commerce under the name of salt of tin : it strongly attracts oxygen, 
 and, therefore, reduces many metallic solutions : on this account it 
 has been recommended as an accelerator. It is partly decomposed 
 by a large quantity of water, with the deposition of an oxychloride 
 and the formation of free hydrochloric acid: unless this acid is 
 present it quickly becomes turbid : the acid solution is a powerful 
 deoxidizer. Chloride of tin is extensively used as a mordant in 
 dyeing. 
 
 Chloride of Zinc. Zn. Cl. = 68, or, as produced by evapo- 
 rating its solution, Zn. Cl. H O = 77. It may be formed by 
 evaporating a solution of zinc in hydrochloric acid to dryness, and 
 heating the residue red-hot in a glass tube with a small aperture. 
 The solution of this salt is always acid, and cannot be neutralized 
 till all the zinc is precipitated. Zinc has a very powerful affinity for 
 chlorine. Its acidity is not in favour of its use photographically : 
 it is soluble in its own weight of alcohol. 
 
 Chlorine (^Xwpoe, green). Chlorine is so called from its colour : 
 it is a greenish yellow gas. It is the most powerful electro-negative 
 element, generally displacing oxygen. There are some who still think 
 it to be an oxygen compound, and, if so, bromine and iodine also. 
 These bodies all seem to be powerfully modified by solar light, and 
 resemble each other in their chemical properties, but they are not 
 all generally considered to contain oxygen. To obtain chlorine, a 
 mixture of one part by weight of coarsely powdered black oxide of 
 manganese, and two parts of common hydrochloric acid may be 
 heated over a lamp in a glass retort : the evolved chlorine may be 
 
CHL 
 
 107 
 
 collected over warm water, as cold water absorbs it. The qualities 
 which distinguish chlorine in chemistry are its strong affinity for 
 hydrogen and the metals, and its bleaching power : in photography, 
 the increase of its usual affinities by the action of light. Its bleach- 
 ing power is a consequence of its affinity for hydrogen, since, on 
 coming in contact with colouring matter in the presence of moisture, 
 it decomposes the water, especially in the light, and the nascent 
 oxygen unites with the colouring matter to form a new and colour- 
 less compound. In the same way it destroys organic substances in 
 the atmosphere, carrying with them infectious disorders, and is, 
 therefore, used as a disinfectant : hydrogenous gases and miasmata 
 are thus destroyed, as well as vegetable and animal dyes containing 
 hydrogen, and even the colour of ink, which is formed by gallic acid, 
 of which hydrogen is a component. Engravings which are produced 
 with printers’ ink, a mixture of charcoal, vegetable dyes, and organic 
 oils and resins, are not proof against the action of chlorine : even 
 the carbon in such combinations may, by the joint action of light 
 and this gas, be earned away as chloro-carbonic acid. Photographs 
 are particularly liable to be thus attacked. Many substances are 
 decomposed by chlorine with such violence as to cause combustion. 
 Many organic substances appear to be capable of forming with 
 chlorine as many different compounds as they contain atoms of 
 hydrogen, the hydrogen being displaced by chlorine by single atoms 
 at a time, which go off as hydrochloric acid. 
 
 As has been said, the affinity of chlorine for hydrogen and carbon 
 is much exalted by the sun’s rays, and organic bodies, unaffected by 
 chlorine in the dark, are often rapidly altered by it in the light, and 
 sometimes the alteration takes place with explosive violence. The 
 nature of the effect produced on chlorine by light has not yet been 
 determined : some suppose that it is rendered permanently allotropic ; 
 others, that an effect is produced upon it similar to the induction of 
 electricity on insulated conductors in the proximity of an electrified 
 body, which state or effect immediately ceases on the withdrawal of 
 the exciting force. But the investigations of more than one experi- 
 menter seem to prove, that an effect is produced upon chlorine in the 
 light which does not terminate with the exposure to light, but con- 
 tinues in the dark, and the darkening which occurs in iodide of 
 silver, exposed with nitrate to the sun, is only gradually removed 
 when it is afterwards taken into a dark place. It has been said by 
 some that the light combines with the element to form a definite 
 compound, but this is not consistent with what is now generally 
 admitted as to the nature of the phenomena of light, and is not 
 accordant with the undulatory theory which is now considered by 
 
108 
 
 CHL 
 
 most philosophers to be established. The action on the elementary 
 body is probably an induced polarity of some kind, which continues 
 for a longer or shorter period, even after the disappearance of the 
 exciting force. 
 
 Chlorine is the only body capable of dissolving gold and platinum, 
 and is a more powerful body even than oxygen. It constitutes 
 with oxygen, bromine, iodine, and fluorine, a natural group of very 
 similar bodies, of which it is the most energetic. In testing for 
 chlorine in the free state, it is necessary to distinguish it from 
 ozonized oxygen, which, being more active than ordinary oxygen, 
 might, in some of its reactions, be mistaken for it. 
 
 At the temperature of 60° water dissolves twice its volume of 
 chlorine, and more as the water is cooled : in this respect it resembles 
 the binoxide of hydrogen, which gives off at high temperatures the 
 oxygen which it holds in solution. Chlorine water acts like chlorine 
 itself, but is decomposed by sunshine, with the formation of hydro- 
 chloric acid and liberation of oxygen. 
 
 Chloro-Bromides, &c. These and similar compounds are 
 often mere mixtures of chlorine and bromine, or iodine, in combina- 
 tion with water or lime, and used as accelerators in the preparation 
 of daguerreotype plates. 
 
 Chloroform. A compound of chlorine and formyle. Tormyle 
 is a hypothetical hydrocarbon, C 2 H ; and chloroform is the ter- 
 chloride of it, C 2 H Cl 3 . It is a heavy limpid fluid of S. G . 1*48, 
 insoluble in water, but soluble in ether and alcohol, from which it is 
 thrown down by water. It dissolves camphor, wax, resins, gutta 
 percha, and caoutchouc. Solution of gutta percha in it has been 
 used as a film on glass to receive collodion, intended to be removed 
 after the picture is taken, and also as a means of removing the film 
 after the operations are all finished. It is successfully applied in 
 both these ways to support the collodion instead of glass when used 
 to print from, and has also been recommended as a varnish to pro- 
 tect photographic images in general. Added to collodion, it has 
 been found sometimes to render it more fluid, tough, and contractile, 
 and to- remedy in part the want of half-tone often produced by old 
 or impure ether ; but it must be pure when used for these purposes, 
 or it will be worse than nothing. It increases the density of the 
 image slightly, but is not an accelerator. It is made by distilling a 
 mixture of one pound of chloride of lime, three of water, and three 
 ounces of alcohol. About three ounces of chloroform pass over. 
 
CHL 
 
 CHE 
 
 109 
 
 Chlorophyl. (x^wpoe green, and (pvWov a leaf.) The green 
 colouring matter of leaves, which is not produced in the absence of 
 light, and is produced by decompositions in which oxygen is libe- 
 rated. When the green matter is oxidized by the action of chlorine 
 or other bodies it is deprived of its colour. This action on living 
 plants is the opposite of the effect of light on decaying matter, which 
 is oxidized and gives off carbonic acid. 
 
 Chromate of Copper. Cu. 0 Cr. 0 3 . This salt is the buff 
 precipitate which falls when solutions of chromate of potassa and 
 sulphate of copper are mixed : it is sparingly soluble in water, and 
 neutral. When bichromate of potassa is used, no precipitate occurs, 
 the bichromate of copper formed being soluble. When a strong 
 mineral acid is added to the sparingly soluble chromate in water, 
 half the copper is removed, and forms a neutral salt with part of 
 the acid, and the residue, as bichromate of copper, dissolves in the 
 remaining acid water. This mineral acid may be neutralized ; but, 
 if alkali be added more than is necessary for this, then the bichromate 
 is decomposed, and neutral chromate is again thrown down. The 
 solution of bichromate, prepared in either way, and spread upon paper, 
 is decomposed by light. As the neutral chromate is not found to 
 be affected in the same way, or, at least, in the same degree, it is 
 possible that the second atom of chromic acid is alone changed : see 
 “ Chromic acid.” If the neutral chromate itself were decomposed, 
 then some chromic acid would be deoxidized, and a dichromate 
 of copper formed, and perhaps a chromate of chromium. The 
 bichromate of copper has been tried as a means of procuring photo- 
 graphs on glass, which, when burnt in a glass furnace, should give 
 a permanent image in coloured glass : the colour to be given by the 
 oxides of copper and chromium. 
 
 Chromate of Iron. Chromate of the protoxide of iron cannot 
 exist, since the iron abstracts oxygen from the chromic acid, and both 
 become sesquioxides. The chromate of the sesquioxide of iron is 
 soluble, like the corresponding chromate of sesquioxide of chromium : 
 the formula is Ee. 2 0 3 , Cr. 0 3 . This compound has also been proved 
 to be decomposed by light and organic matter, and two atoms of 
 the chromic acid must be resolved into Cr. 2 0 3 and 0 3 , and a sub- 
 chromate produced. It is not impossible that the oxide of iron is 
 at the same time reduced to protoxide, which will unite with the 
 Cr 2 0 8 , as a base with an acid. The sesquioxides of both iron and 
 chromium are very feeble bases, and with strong bases even act as 
 acids : the latter is the more stable and more acid compound. 
 
110 
 
 CHE 
 
 Chromate of Mercury. 4 Hg. O, 3 Cr. 0 3 . This is a basic 
 chromate, and becomes still more basic when exposed to the sun’s 
 rays : metallic mercury is at length formed. 
 
 Chromate of Nickel. A reddish-brown powder and neutral 
 is the chromate of protoxide of nickel : the acid chromate is soluble. 
 It resembles the chromate of copper. 
 
 Chromate of Potass a. K 0 Cr. 0 3 . This is the salt from 
 which the other chromates are all prepared. It is soluble in twice 
 its weight of water, but not in alcohol. See “ Bichromate of Potass.” 
 
 Chromate of Silver. Ag. 0, Cr. 0 3 , is precipitated of a 
 crimson colour, when chromate of potassa is added to nitrate of 
 silver. It is also formed more gradually by the slow decomposition 
 of bichromate of silver : the decomposition of one-half of the acid 
 is effected by boiling, or by exposure to air and light : the neutral 
 chromate forms then in dark green crystals. The sesquioxide formed 
 remain in solution with the bichromates. Chromate of silver is 
 decomposed by light and organic matter, with the production of 
 metallic silver and oxide of chromium, and the organic matter is 
 oxidized. 
 
 Chromatic Aberration. See “ Aberration.” 
 
 Chromic Acid and Chromates. Chromic acid is a teroxide 
 of chromium Cr. 0 3 : and resembles ferric acid Pe. 0 3 . The com- 
 pounds of iron and chromium are, indeed, generally so similar in 
 their reactions, that we might almost expect them to act, photo- 
 graphically, in the same manner. Chromic acid is more stable than 
 ferric, and peroxide of chromium than peroxide of iron : chromium, 
 therefore, appears to have the stronger affinity for oxygen. Ferric 
 acid is much more easily decomposed than chromic, though it is 
 very difficult to keep either in the free state : the former, indeed, can- 
 not be isolated, for it splits up into Fe. 2 0 3 and 0. It is only in 
 combination with very strong bases, such as potassa, that it can 
 exist. Chromic acid, on the other hand, will form salts with a great 
 number of bases, and, even, with such a weak base as Cr. 2 0 3 : but 
 these salts are more easily decomposed in proportion to the weakness 
 of the base : the acid then, like the ferric, splits up into sesquioxide 
 and oxygen. The strong bases generally will combine with either 
 one or two equivalents of chromic acid, but the second equivalent is 
 extremely liable to undergo the same change as free acid. Chromic 
 
CHR 
 
 111 
 
 acid is decomposed instantly by the contact of organic matter, which 
 takes the oxygen, and it may be, therefore, employed as a bleaching 
 agent, and, as this change is favoured by light, it may be used in a 
 great variety of ways photographically : the principle of the opera- 
 tions being the exaltation by light of the affinity of oxygen for car- 
 bon. Oxygen and carbon are thus affected in a degree only second 
 to chlorine and hydrogen. Alcohol is sufficiently active to decom- 
 pose chromic acid : it is itself oxydized into aldhehyde and formic 
 acid, which forms formic ether. This unstable body will not even 
 allow itself to be filtered through paper without reduction. 
 
 There is a strong analogy between the photographic processes by 
 means of the salts of chromium, iron, uranium, and aluminium, and 
 the operations of dyeing. These salts are in both cases decomposed 
 by the organic matter to be printed on, and the reduced oxide, 
 which combines with the oxidized organic fabric, becomes a basis 
 on which other oxides will subsequently adhere. The dyer, even, 
 becomes at times altogether a photographer, when he makes use of 
 the sun to impart greater brilliancy to his colours than their unaided 
 affinities would produce. And the photographer becomes a dyer, 
 when, having reduced his oxides on his paper by the light, he pro- 
 ceeds to deposit on them, as a mordant, other oxides by means of 
 nitrate of silver, chloride of gold, ferrocyanide of potassium, and 
 other substances used as developers. The colour of the dye is also 
 often developed in a way precisely analogous. The mordant oxide 
 appears to be such a one as can play the part of both base and 
 acid, and hold out a hand both to the organic matter on the one 
 side and the metallic oxide on the other. 
 
 It is often difficult to point out the exact formula which takes 
 place when the salts of chromium and iron are reduced photo- 
 graphically and afterwards developed. When bichromate of copper, 
 for instance, is decomposed, the reaction would be easy to under- 
 stand if the sesquioxide formed were immediately precipitated in 
 the solid state. But this oxide appears to form with the elements 
 of the undecomposed compound complex double salts. The sesqui- 
 oxides of iron, chromium, aluminum, manganese, are isomorphic, 
 and form aluminoid salts with sulphuric acid and potassa of a com- 
 plex kind : and such aluminoid double salts, with some slight 
 modifications, appear to be formed in the decomposition of the 
 bichromates. Chromic acid salts are isomorphous with the sul- 
 phates, and double chromates and chromosulphates are easily formed. 
 When a neutral chromate is deoxidized, the decomposition is, pro- 
 bably, first into subchromate, sesquioxide of chromium, and oxygen, 
 and, perhaps, afterwards, entirely into sesquioxide and protoxide ; 
 
112 
 
 CHR 
 
 but, when a bichromatic solution is used, these double aluminoid 
 compounds are probably first formed. The first effect of light upon 
 them is to deepen the colour, because of the formation of sesqui- 
 oxide ; but further action often bleaches this colour, and this, most 
 likely, indicates the formation of a double salt, or of a salt composed 
 of sesquioxide as the acid in the place of chromic acid. The action 
 upon a developer is in both cases the same in kind, and depends 
 upon the developer used : with nitrate of silver, chromate of silver 
 appears to be formed in the parts not changed by light, and contain- 
 ing the original chromate, and with ferrocyanide of potassium com- 
 pounds resembling Prussian blue, but containing chromium, in place 
 of iron, as the base. Chromic acid will destroy a photograph more 
 quickly than nitric. 
 
 Chromium. A metal found in nature in the form of sesquioxide, 
 united, as the acid, to the protoxide of iron, as the base : (xpw/ia, 
 colour). It is remarkable for forming coloured compounds, and 
 hence its name. 
 
 Chromatype. A name given to that class of photogenic de- 
 compositions in which chromic acid is deoxidized : the sesquioxide 
 of chromium does not appear to undergo reduction to the state of 
 protoxide, and in this it differs from the persalts of iron in ferrotype. 
 The following are the modes adopted for getting photographs by the 
 chromium salts : — 
 
 (a) Soak the paper in a saturated solution of bichromate of pot- 
 ash, and dry it by rapid agitation in front of a brisk fire out of the 
 light. It is now of a bright yellow colour, but exposure to the sun 
 under a negative, will produce a positive, by darkening the exposed 
 parts to a deep orange colour. Washing well in water removes the 
 unchanged yellow salt from the lights, but the reduced sesquioxide 
 in the shadows remains combined with the paper. The paper should 
 be well sized, or the bichromate will be only feebly decomposed. 
 
 ( b ) Brush a sizing of starch very uniformly over the paper, and 
 then steep it in a weak alcoholic solution of iodine, and if the coat- 
 ing of blue iodide of starch be not uniform repeat the operation. 
 Steep it in bichromate and dry it as before. The print will be ne- 
 gative, even, from a negative, and positive from a positive, if after 
 exposure and washing it is again steeped in the solution of iodine 
 which renders the unexposed parts of a dark violet colour. 
 
 (c) To a saturated solution of bichromate of potassa, add a satu- 
 rated solution of sulphate of nickel, in quantity more than sufficient 
 to decompose the whole of the bichromate, or 1 drachm of the latter 
 
CHR 
 
 113 
 
 (the potassa) in crystals to 2 drachms of the former, also in crystals. 
 Apply this to the surface of the paper and expose under a negative. 
 The exposed parts become brown and the rest remains yellow, and if 
 now washed in water the result is a positive picture. But if the 
 exposure be continued under the negative beyond the dark stage the 
 browning disappears and the exposed parts are white, a colourless 
 double salt being formed there. When nitrate of silver is applied as 
 a developer the chromate of silver is deposited in the unsunned por- 
 tions, where the original solution remains unchanged, but not on 
 the whitened parts. Pure water will remove nearly all except the 
 precipitated chromate, which, if thought desirable, may be converted 
 into chloride, and exposed again, and developed as in the usual silver 
 processes. Sometimes the colourless double salt formed will de- 
 « compose the nitrate of silver, owing to its imperfect formation. The 
 chromate of silver, by which the shadows are represented, is itself 
 decomposed by light, and, therefore, must be changed in some way, 
 before permanence is attained. 
 
 (d) Dissolve neutral chromate of copper in ammonia. The solu- 
 tion, whicji is now a mixture of chromate of ammonia and of animo- 
 niacal solution of oxide of copper, is of a green colour, formed by the 
 mixed yellow and blue of the separate solutions. Papers prepared 
 with this behave like those soaked in the chromate of copper. 
 
 Chrysotyfe Process. (Gr. xP V(T °£> gold.) This is one of 
 Sir John Herschel’s ingenious processes. The operations are as 
 follow : — 
 
 First. Immerse a sheet of paper in a moderately strong solu- 
 tion of ammonio-citrate of iron, and dry it in the dark. The 
 strength of the solution should be such as to dry into a good yellow 
 colour, not at all brown. 
 
 Second. Expose the paper to light, either in the camera or pres- 
 sure frame, under a negative, until a very faint impression is ob- 
 tained. 
 
 Third. Brush over the paper a neutral solution of chloride of 
 gold, of such strength as to have about the colour of sherry wine- 
 The picture immediately appears and is rapidly developed to a purple 
 
 tint. 
 
 Fourth. Wash the developed print in several changes of water, 
 and fix it with’, a weak solution of iodide of potassium : then wash 
 again thoroughly, and dry. 
 
 The picture is now finished. 
 
 The rationale of the process appears to be as follows : — 
 
 The per salt of iron is reduced by light to a salt of the protoxide. 
 
 I 
 
114 
 
 cie 
 
 CIT 
 
 When a solution of chloride of gold is added, water is decomposed, 
 and metallic gold precipitated. The chlorine takes the hydrogen of 
 the water and forms muriatic acid, and the protosalt of iron takes 
 the oxygen and becomes oxidized to a persalt. The precipitated 
 gold forms the purple shadows of the photograph. The iodide of 
 potassium fixes the picture by converting any chloride of gold which 
 may remain in the paper into a soluble double iodide of gold and 
 potassium. 
 
 Circle of Least Confusion. See “ Local lines.” 
 
 Citrate of Mercury. 3 Hg. O, Ci. This salt is decomposed 
 by light in the presence of excess of nitrate of mercury, like the 
 corresponding silver salt, but more slowly ; and pictures may be 
 developed upon it by the protosalts of iron. Also when preci- 
 pitated with iron from the protonitrate by ammonio -tartrate of iron 
 or other salts of the like composition, it darkens speedily and in- 
 tensely. The citric acid has a feeble reducing power, and is pro- 
 bably slowly oxidized into carbonic acid and water. 
 
 Citrate of Silver. 3 Ag. 0, Ci. + H0. This falls as a shin- 
 ing white powder on mixing citrate of soda with nitrate of silver, 
 and has been used as the basis of a printing process, for the par- 
 ticulars of which the reader is referred to the article on printing. 
 Citrate of silver is insoluble in water, but soluble in ammonia, no 
 doubt forming citrate of ammonia and ammoniacal oxide of silver. 
 When heated to 212° in the presence of hydrogen, part of the 
 oxygen of the oxide of silver in the citrate, is converted into water 
 by uniting with the hydrogen, and citrate of the suboxide of silver 
 instead of citrate of the oxide remains. This has been represented 
 as the change which light also produces on this salt, but it is 
 scarcely a satisfactory explanation. It appears more probable that 
 the citric acid is also at the same time decomposed. If half of the 
 oxygen be removed by light, it must either combine in its nascent 
 state with some hydrogen or carbon in the atmosphere, or.it must 
 be entirely set free, or it must oxidize the citric acid or the paper. 
 It is easy to account for its being removed by hydrogen and heat 
 to form water, but when it is separated from the silver by light, 
 it is combined, there can be little doubt, with the organic matter 
 present, and the reduction is due to the joint action of the citric 
 acid and light. Citric acid and the other vegetable acids, all par- 
 take in a greater or less degree of the qualities of gallic and pyro- 
 gallic, and are capable of being entirely oxidized into carbonic acid 
 
CIT 
 
 115 
 
 and water. Heat increases this tendency to oxidation by increase 
 ing the affinity of carbon and hydrogen for oxygen or chlorine* 
 and light does the same. Citric acid will reduce the chloride of 
 gold to the metallic state, and combine with the chlorine, and when 
 subjected to heat it undergoes changes analogous to the conversion 
 of gallic into pyrogallic. The decomposition takes places gra- 
 dually, and by stages, for citric acid in its oxidation is capable of 
 forming a variety of other organic compounds, and the silver is 
 reduced more and more nearly to the metallic state. The tendency 
 of citrate of silver to be decomposed in this manner is shown by 
 the action of heat upon it. The affinity of the carbon and hydrogen 
 for the oxygen of the silver is increased, by heat, so much that when 
 the salt comes into contact with an incandescent body a kind of 
 explosion takes place, and carbon and metallic silver alone remain. 
 Many other organic salts of silver comport themselves after the 
 same fashion. It seems natural to conclude that what takes place 
 here, almost instantaneously, is effected by light, according to its 
 usual mode of acting on organic substances, viz., by a series of 
 gradual and progressive substitutions. The gradual approach to 
 the metallic state, in prints taken on citrate of silver, is proved by 
 the difference observed in the reaction of nitric acid, ammonia, and 
 hyposulphite of soda upon it in the different stages, by the growing 
 indifference to oxidizing compounds, the greater permanence of the 
 more reduced parts, the greater facility of amalgamating with 
 mercury, and the bronzing or metallic appearance of the strongly 
 sunned portions. The colour also changes from the red which is 
 indicative of organic matter to blue, which is characteristic of 
 metallic silver. No such alteration of properties appears to attend 
 the reduction of pure chloride ; it never bronzes, and appears to be 
 resolved at all stages of the exposure to light into chloride of silver 
 and pure metal. The less citrate of silver prints have been exposed 
 to light the more organic matter will continue in combination with 
 the silver, and the more liable will they be to suffer deterioration by 
 the action of elements for which the organic matter has affinity : 
 the colour is a measure of this liability with the same preparations. 
 When ammonia-nitrate is used with the citrate, the image is bluer 
 and more metallic than with plain nitrate, because alkalis increase 
 the affinity of vegetable acids for oxygen, as is seen in the mixture 
 of ammonia and pyrogallic acid. The decomposed citric salt is 
 more or less acid to the organic compounds of silver which are 
 precipitated by the usual developers, and, therefore, attracts them ; 
 and as the silver which falls from the more powerful developers is 
 
 i 2 
 
116 
 
 CIT 
 
 more completely reduced than that which citric acid deoxidizes, the 
 developed prints are more permanent than the sun prints. 
 
 Citrate of Silver Printing Processes. These processes 
 only differ from the other printing processes in the composition of 
 the salting bath, in which citrate of soda is substituted for a portion 
 of the salt. The exact proportions will depend upon the kind of 
 effect which it is desired to produce. The effect of adding citrate of 
 soda to the salting bath is as follows : — 
 
 When a paper containing citrate of soda is floated upon a bath of 
 nitrate of silver, a double decomposition takes place, nitrate of soda 
 and citrate of silver, together with excess of nitrate of silver being 
 produced. Now citrate of silver is sensitive to light, and is dark- 
 ened to a red citrate of the suboxide of silver by exposure to light. 
 This organic subsalt of silver is very energetic in producing the 
 various photographic effects due to organic matter in combination 
 with silver. It reddens the tint of the proof, and renders it more 
 vigorous on the surface, and less liable to assume a cold inky tint 
 when toned with sel d’or. But at the same time it renders the 
 paper less sensitive, a fault which is of no consequence when the 
 light is good. On the whole, therefore, the use of citrate of silver 
 in the printing processes may be considered an improvement, when 
 it is required to produce a certain class of effects. 
 
 Citric Acid. This acid is gradually formed in lemons and other 
 fruits, by the oxygen of the air acting on the mucilages which are 
 found in the fruits in their less matured condition ; just as tannin 
 is converted into gallic acid by the same means. It is precipitated 
 by chalk as citrate of lime, and liberated afterwards by a stronger 
 acid. A gallon of lemon juice gives 8 ounces of citric acid, and at 
 times more. A drachm of lemon juice is, therefore, equivalent to 
 about 3 grains of the acid, and, when lemon juice is substituted for 
 the vegetable acids, this proportion should be remembered. As it 
 contains more oxygen in proportion to its carbon and hydrogen 
 than acetic acid it is a much stronger acid, but weaker than tartaric 
 acid. Anhydrous citric acid, such as it exists in citrate of silver 
 is C 12 , H 5 O n = Ci. =165, but the ordinary crystals contain 4 
 atoms of water, and are = 201. Other crystals with more and 
 less water may be formed. The usual crystals are soluble in less 
 than their weight of water, and are soluble in alcohol, but not in 
 ether. The dilute aqueous solution, like gallic and other acids, 
 soon becomes mouldy. It unites with three atoms of base to form 
 citrates : sometimes the place of one or two of these atoms of base 
 
CLE 
 
 117 
 
 is supplied by water. It cannot exist without an atom of water, 
 and when this water is taken from it, it is resolved into aconitic 
 acid and hydrated citric or other compounds, or even into oxalic 
 and acetic acids. It is used as acetic acid in photography, and 
 also as a test of iodate of potassa in iodide of potassium, and to 
 form citrate of soda with the bicarbonate of that alkali for the 
 first preparation of printing papers. Papers soaked in a solution 
 of one part citric acid to 10 water will have the particles of metal 
 which are often so annoying, removed as soluble ammonio-citrates 
 if they are afterwards put into dilute ammonia of 20 parts water to 
 one part liquid ammonia. This acid is a weak deoxidizer : boiled 
 with a solution of chloride of gold it removes the chlorine without 
 giving off gas; clilorocarbonic acid is probably formed, for citric 
 acid may be regarded as a compound of oxalic and acetic acids, 
 and water ; and oxalic acid C 2 0 3 , as a compound of carbonic acid 
 C 0 2 , and carbonic oxide C O ; and carbonic oxide in the light 
 unites with chlorine to form chlorocarbonic acid. 
 
 Cleansing. The cleansing of bottles, papers, stirrers, and ap- 
 paratus in general, is a matter of essential importance in many 
 photographical operations; and the great rule must be to clean 
 immediately after use, when purity is obtained with much more cer- 
 tainty and much less labour. All dirty glasses, &c., should be im- 
 mediately put in a particular place and attended to at the first leisure 
 moment. The following directions will be found useful : — 
 
 (g) To clean Albumen from Glass Plates. Use a solution of caustic 
 potash, or ammonia, for albumen is soluble in alkaline solutions ; 
 then wash with water ; and lastly, with dilute nitric acid and plenty 
 of water. 
 
 (b) Collodion Bottles. Leave the stopper out until the ether and 
 alcohol have evaporated, and the film is hard and horny, when it 
 will be easily removed without any adhering to the glass by means 
 of cold water and a bottle brush. Drain and rinse out with a little 
 alcohol. 
 
 (c) Developing Measures and Trays. Wash well with tow or a 
 rag and common water, then with a little strong nitric acid if the 
 black precipitate has dried in the vessel. The acid should remain 
 some time in contact with the blackened parts, even after they 
 appear clean : lastly, plenty of water and a clean dry cloth. 
 
 ( d ) Gallic Acid and Gallo- Nitrate. Bottles in which gallic acid 
 has been allowed to become discoloured or mouldy require the use 
 of nitric acid, which decomposes most organic matters, before they 
 are fit to receive solutions of other compounds. 
 
118 
 
 CLE 
 
 (i e ) Glass Plates and Vessels. These should be washed and 
 rinsed, if possible, as soon as they are done with, and before they 
 have got dry : even common water if allowed to dry on them will 
 leave matters which often require considerable force to remove. If 
 plain water will not clean them a little tow and the ashes from the 
 fire will generally remove every thing. Sand or gravel should not 
 be used with white glass vessels, since it always scratches the soft 
 flint glass of which English vessels are made. They must after 
 washing be rinsed, drained, and wiped with a clean coarse cloth. 
 New window glass or crown glass requires particular care in clean- 
 ing, and especially, in hot weather. Glass plates which have re- 
 ceived pictures before are, contrary to the general opinion, better 
 than new, if carefully cleaned after each experiment. Nitric acid 
 with or without tripoli is the best detergent for new glass : better 
 without tripoli, if possible. If a plate be cold and then take the 
 the breath in an even film, it is clean. If a collodion picture has 
 been developed with pyrogallic acid, it requires only water, but, if, 
 with an iron salt, nitric acid will be necessary. 
 
 (/) Grease. Greasy glasses should not be washed but, in the 
 first place, wiped with tow to remove as much as possible of the 
 grease, and then a dry cloth should be used until the surface appears 
 clean. It should afterwards be washed with nitric acid or caustic 
 potassa and tow which removes the thin film of grease remaining, 
 and it may then be rinsed, drained, and wiped as before. A special 
 duster should be appropriated to remove grease. 
 
 (g) Lenses. Do not use silk, for it is apt to scratch : a soft wash 
 leather, free from the powders used in cleaning it, is best ; if the lens 
 be greasy, soft tissue paper will clean it quickly, especially if moistened 
 with a little alkali. Eub off the alkali with fresh paper, and finish 
 with the wash leather kept for this purpose. 
 
 ( h ) Nitrate of Silver Stains. Nothing is so good as nitric acid 
 for porcelain, or glass/ For linen, or the hands, mix together 
 alcohol 10 ounces, iodine | ounce, nitric and hydrochloric acid each 
 l of an ounce. Apply a little to the stain, and when it has become 
 yellow dissolve it out with cyanide of potassium, and wash well. 
 
 (i) Papers containing Metallic Spots. Make two solutions, one 
 of one ounce of tartaric acid in 10 of water, and the other of one fl. 
 ounce of liquor ammoniae in 20 ounces of water. Let the papers soak 
 in the first for a quarter of an hour one over the other, then place 
 them for a few minutes in the second. Einse in plain water and 
 hang up to dry. The metal is removed as ammonio -tartrate. 
 
 (j) Resinous , Bituminous , and Tarry Varnishes. As much as pos- 
 sible should be scraped off with a knife ; then use tow with a little 
 
CLE 
 
 119 
 
 strong caustic potash or sulphuric acid; rub the glass well, and 
 after a few minutes the resin &c., will wash olf with water, and the 
 glass may be cleaned in the ordinary way. Wood spirit is a clean 
 solvent of these substances. 
 
 (Jc) Spots of over-development in Photographs. The solution given 
 above for nitrate of silver stains has been found to answer, but it 
 requires care in using. 
 
 (7) Sulphate of Iron Stains, andiron Moulds , and Ink Stains. To 
 remove these from linen make use of a solution of oxalic acid, and 
 from glass vessels hydrochloric. 
 
 (m) Sulphuret of Silver Films in Hypo-bottles. Kemove all that can 
 be rubbed off with tow and water and then use nitric acid which will 
 form nitrate of silver : finish with plenty of water. 
 
 (n) Turpentine. Strong alkali and tow will soon soften it so that 
 it will be moveable by water, or perhaps better, sulphuric acid, which 
 will decompose it. 
 
 (6) Varnishes and Varnished Collodion Pictures. These are all 
 easily removed by wood spirit even if asphalt or brunswick black has 
 been applied : or, if time be not pressing, let them soak for some days 
 in water, when the varnish will peel off. 
 
 (p) Yellow Lights in Photographs. A very weak solution of bi- 
 chloride of mercury will speedily remove the yellow appearance after 
 the hyposulphite of soda is entirely gone, but the prints must be 
 removed immediately on the effect taking place or the picture will 
 be attacked. 
 
 Clearness. This quality in a photograph implies the absence 
 of any action except what is induced by the impact of light, the 
 exact amount of exposure, both as to the intensity of the light and the 
 duration of its action, the exclusion of all light except that reflected 
 by the object which is photographed, perfect optical arrangement so 
 that the image is sharp and evenly illuminated, a uniformly sensitive 
 surface to receive the impression, the arrangement of light so that 
 the illuminated object to be taken appears perfectly modelled, and 
 clean, uniform and smart manipulation. The things to avoid are 
 dirty plates, impure chemicals, too weak or too strong solutions, or 
 those of unknown or inconstant strength, too much heat or cold, bad 
 water, dirty hands or frames, and unclean vessels which cause action 
 independent of the light ; lenses with foci not coincident or with no 
 sharp focus, and of too short focus, and cameras shaky or not light 
 tight, or not adjusted to the focussing glass, or open in front to rays 
 from any objects beside the one to be photographed, or deficient in 
 means of absorbing the light scattered by the lenses or the mounting, 
 
120 
 
 CLT 
 
 COB 
 
 which cause an imperfect image; illuminated smoke or vapours 
 between the object and lens, bright back-ground or sky to the picture, 
 and dew or grease on the lens, which introduce into the camera 
 diffused light that veils the picture ; feeble light, or too intense, or 
 not falling up on the object so as to throw out the relief, light coming 
 too obliquely from parts of the object, and exposure too long or too 
 short, which injure the purity of the modelling ; and careless, or 
 unequal, or dilatory manipulation which makes one part of the plate 
 more sensitive or develope more quickly than another. 
 
 Cliche. The Trench word for negative, or mould. 
 
 Clips. Little clasps made of wood for hanging papers to dry. 
 They are best when they are tipped with shellac or other varnish 
 that the solutions cannot penetrate, and a separate set should be 
 kept for each operation : they should be provided with S hooks of 
 wire so that they may be hung over a line. Such hooks made with 
 black pins or silver wire are very useful substitutes for the clips 
 themselves. 
 
 Coal Oil. Coal tar naphtha. 
 
 Coal Tab Naphtha. The term naphtha is now generally 
 applied, par excellence, to wood spirit, or pyroxylic spirit, or wood 
 alcohol, by all which names it is called, but besides this wood 
 naphtha there are many varieties of bituminous liquids which are 
 also called naphthas, and to distinguish them from wood spirit, 
 mineral naphtha. Such mineral naphthas are found in the natural 
 state impregnating different kinds of soil, and often containing 
 petroleum or other bituminous substances in solution. Coal tar 
 naphtha is a product of the distillation of pit coal, and resembles the 
 bituminous or mineral naphthas in its composition : these contain 
 only carbon and hydrogen, but pyroxylic alcohol contains oxygen 
 also. The purest mineral naphthas have the formula C 6 H 5 . They 
 are used as solvent of bitumen &c., for varnishes, and will not mix 
 with naphtha usually so called. 
 
 Cobalt. A metal resembling, in its photographic applications, 
 iron, nickel, and chromium. It is the basis of smalt, and the blue 
 colour of writing papers. Its compounds with sulphur and arsenic 
 have been found to be affected by light. 
 
 Cobalto-Cyanogen. This compound is analogous to ferro- 
 
COI 
 
 COL 
 
 121 
 
 cyanogen, and forms compounds, which like the ferrocyanides are 
 decomposed by light. 
 
 Coincidence of Foci. See “ Lens.” 
 
 Collodion. This viscid solution is one of the most generally 
 used articles in photogenic operations, and is remarkable as the basis 
 of processes of extreme sensitiveness, and giving pictures surpassing 
 all others for generally good qualities. It is a solution of pyroxyline 
 in ether and alcohol, holding also in solution a certain quantity of 
 some soluble iodide, bromide, or chloride, or a mixture of them. 
 The principal points to be attended to in its manufacture have now 
 been determined by long experience and many trials. There are 
 two chemical compounds known by the name of pyroxyline, one ' in 
 which the vegetable fibre operated upon increases in weight about 
 75 per cent., and a second where the increase is only about 50 per 
 cent. The first is insoluble in ether and alcohol, and not used by 
 photographers. The second is the soluble compound, and upon its 
 preparation very much depends. Though chemically the same it 
 varies in its properties, and modifies the physical and chemical 
 character of the collodion according to the mode in which it has 
 been made. The physical characters which vary are the fluidity, 
 smoothness, contractility, adhesiveness, softness, porosity, cohesion, 
 strength, and transparency of the film formed when the collodion is' 
 poured on a glass : the chemical characteristics of the collodion, 
 which are liable to variation with the pyroxyline, are sensitiveness to 
 light, and the density and colour of the image. As to the film, 
 fluidity, smoothness, softness, porosity, and adhesiveness go together ; 
 and likewise glutinosity, coarse cellular structure, horniness, imper- 
 meability to fluids, contractility, and want of adhesiveness go 
 together : and as to the chemical qualities, greater sensitiveness to 
 feeble lights characterizes one kind of pyroxyline and the power of 
 giving dense images another. Two circumstances are favourable to 
 the production of good physical qualities, viz., a certain dilution of 
 the acids employed and a comparatively high temperature, and the 
 chemical qualities depend upon the temperature only. When the 
 most concentrated acids are used insoluble pyroxyline is produced : 
 when the least degree of dilution is formed which, with 120° of 
 temperature, gives the soluble pyroxyline, the collodion has the bad 
 qualities of glutinosity, coarse structure, &c., but by gradually in- 
 creasing the amount of water, added to dilute the acids, the good 
 qualities grow in degree until a limit is reached where the film in 
 setting becomes short, powdery, and milk white, and partially dis- 
 
122 
 
 COL 
 
 solves in the acids : by stopping a little short of this, and increasing 
 the temperature from 120° towards 170°, the physical qualities 
 undergo still further improvement until a limit is also reached in 
 this direction, when the fibre employed again dissolves in the acid. 
 As to the chemical qualities, when the best physical qualities are 
 obtained that acids at 120° will give, it is found that the sensitized 
 film is very quick, but the chemical action is confined to the silver 
 salts, so that the image is weak blue and metallic ; the collodion 
 itself is then a kind of organic matter extremely inert and does not 
 combine in any marked degree with the reduced metal, or take part 
 in the reduction. But if the temperature be raised towards 170° 
 the case is different ; the collodion exerts a decided chemical in- 
 fluence in the reduction of the silver salts, and behaves in* the same 
 manner as albumen, citric acid &c., in sun-printing. The image is at 
 first red, and is capable of becoming very intense and opaque ; it is 
 more soluble than before in cyanide of potassium, and in every re- 
 action shows that the silver is now combined with organic matter. 
 If, therefore, collodion be required for “ positives,” we select that kind 
 of pyroxyline which gives a collodion with the best physical qualities, 
 but organically inert, for by that means we get a more metallic and 
 brighter tone, and the image being less dense is better modelled and 
 rounded. If negatives are desired we use the pyroxyline made with 
 a higher temperature. 
 
 Why does the high temperature render the collodion more apt to 
 take part in the reduction of the silver, and to combine with it ? 
 Cotton fibre is nearly pure lignin, and the tendency of acids is to 
 cause lignin to pass successively into starch, gum, sugar, glucose, 
 saccharic, and other acids, which have each a greater reducing power 
 than the preceding member of the series, though they resemble each 
 other extremely in their ultimate composition. Heat favours this 
 action of acids upon these organic substances, and the probability 
 is, that when vegetable fibre is submitted to a comparatively high 
 temperature in the making of pyroxyline, some change of this kind 
 may take place, in a greater or less degree at the same time that the 
 lignin is changed by the acid as before. The acid we may suppose 
 first changes the fibre into an isomeric body, more analogous to gum, 
 for lignin and gum are both represented by C 24 H 20 O 20 , and then 
 the further substitution of N0 4 , for a certain number of atoms of 
 hydrogen, completes the change into pyroxyline. This view is 
 confirmed by an experiment, in which a remarkable collodion was 
 produced very fluid, with 30 grains of pyroxyline to the ounce of 
 mixed ether and alcohol. Swedish filtering paper was allowed to lie 
 half a minute in a mixture of equal volumes of sulphuric acid and 
 
COL 
 
 123 
 
 water, until parchmentized, and then used, as had been previously 
 done, by Mr. Hardwich, for conversion into pyroxyline : the weakest 
 acids, and the highest temperature were resorted to, that it was pos- 
 sible to try. The parchment dissolved for the greater part, but 
 after 2 minutes immersion, the acid, the vessel, and all were plunged 
 into water. Scraps of yellow looking parchment were collected, 
 washed and dried ; collodion made with this showed negative qualities 
 in the very highest degree. The image was exceedingly red at first, 
 and could be intensified to almost any extent. • Like all organic 
 matter, it was also found to interfere decidedly with the sensitiveness 
 of the film to light, and this more and more the longer the col- 
 lodion was kept. This really photographic action of collodion 
 cannot be disputed, though it has generally been said that it is inert, 
 and that its good qualities are entirely mechanical. It may be 
 considered as demonstrated, that where intensity of image is de- 
 manded, organic matter must take part in it. It might be supposed 
 that this pyroxyline dissolving 30 grains to the ounce, was a different 
 compound from the ordinary kinds : it might have been, but dif- 
 ferent kinds of pyroxyline have been made dissolving from 1 to 15 
 grains, which were made without loss, and all of which increased in 
 weight about 50 per cent, in the making, so that they were pro- 
 bably all of the same composition. 
 
 With respect to the solvents of the pyroxyline, the ether and 
 alcohol, a similar regard must be had to the kind of collodion re- 
 quired. Tor collodion positives it is necessary that they should 
 both be free from organic matter, or they will form a collodion not 
 sensible to feeble lights, which will show itself in pictures wanting 
 half tint in the shadows, and overburdened with material in the 
 lights, in fact, too negative. Careful distillation from caustic 
 potash is necessary in this case, and also a prudent choice of alcohol 
 in the first instance, It is extremely difficult to decompose the grain 
 oil, and other matters occurring in some alcohols made from roots 
 and seeds, and they should be rejected for photographic purposes. 
 Tor negatives it is not essential that the spirits should be perfectly 
 pure so long as they are neutral, and good rectified ether and alcohol 
 will answer well : to make negative pyroxylines, proceed thus : — 
 
 Take one ounce of old linen or calico that has been many times 
 washed. Boil it in weak caustic potassa until it ceases to give 
 colour to the solution, and then let it remain as long as convenient 
 in the solution. This modifies it in the same way as linen and 
 cotton fabrics are jnodified by alkali, when they are intended to be 
 bleached ; that is to say, they are rendered more susceptible of oxi- 
 dation. Then to 10 fl. ounces of sulphuric acid, S. Gr. 1-84, in a 
 
124 
 
 COL 
 
 warm basin, add 12 fl. drachms of water, and next 10 fl. ounces of 
 nitric acid, S. G. 1*45. The temperature will probably rise to 140°, 
 or thereabout. If it does not, place the basin in a deep dish con- 
 taining boiling water until it does, and if it goes beyond that tem- 
 perature, wait until it cools to that degree. Take the basin if neces- 
 sary from the hot water, and immerse in the mixed acids the ounce 
 of linen previously washed from the alkali, dried and torn into strips 
 of 6 or 7 inches long, and 2 or 3 wide. When the linen is immersed, 
 the temperature will probably rise to 150°, and if it does not, replace 
 the basin in the hot water. Keep the whole at a temperature of 
 150° for twenty minutes moving the linen so as to secure uniform 
 reaction. Then pour off the acid, plunge the basin and its contents 
 into a very large vessel of water, and stir it up vigorously. Allow 
 the pyroxylme to settle, and immediately pour off the water ; squeeze 
 the cotton and place it in another vessel of water ; continue this 
 washing until no acidity is shown by litmus paper, and then allow 
 the pyroxyline to soak some hours in water. Lastly, squeeze well 
 in a clean dry cloth, and spread the pyroxyline in a warm room to 
 dry. It will be found necessary with almost every fresh sample of 
 nitric acid, to try by experiment on small quantities, whether the 
 above proportion is the weakest possible mixture of the acids that 
 will act without dissolving much of the linen. If none at all is dis- 
 solved, it is almost certain that more water may be added. The 
 pyroxyline ought to dissolve 6 or 8 grains to the ounce, and give 
 perfect fluidity and a structureless film. 
 
 To make positive pyroxyline. Use cotton wool instead of linen. 
 Keep the temperature about 130°, and allow the acids to act for 5 
 or 6 minutes : it should dissolve 4 grains to the ounce. 
 
 Collodion Negative Process. In this process a negative 
 photograph is taken on a glass plate coated with collodion. The 
 various operations are as follow : — 
 
 To prepare iodized Collodion. This is made by adding to plain 
 collodion a certain quantity of an alcoholic solution of iodide of 
 potassium. 
 
 To make the plain collodion ; dissolve to the ounce of pure ether 
 S. G. 765, (made by adding alcohol S. G. 845 to ether S. G. 720,) 
 about 3 J grains of pyroxyline, or more, according to the nature of 
 the pyroxyline and the kind of film desired. 
 
 To make the iodizing solution ; dissolve 14 grains of pure iodide 
 of potassium to the ounce of alcohol, S. G. 810. The iodide of 
 potassium is not freely soluble in the alcohol. It must therefore 
 be pounded in a mortar, and the bottle containing the mixture well 
 
COL 
 
 125 
 
 shaken occasionally, and immersed in a basin of tepid water. All 
 this is very troublesome The neatest and simplest way of making 
 the iodizing solution is to procure absolute alcohol S. Gr. 794, which 
 will require about the one-twentieth part of its bulk of water to rais? 
 its S. G-. to 810. The iodide of potassium may therefore be dis- 
 solved in the water, and then added to the alcohol. 
 
 The S. Gr. of the plain collodion is about 769, and of the iodizing 
 solution about 839. These specific gravities correspond with those 
 of the excellent compounds made by Mr. Thomas, and called by him 
 “ Xylo-iodide of silver,” (which contains apparently neither xyloi- 
 din, nor iodine, nor silver,) and solution X ; their chemical com- 
 position and properties being also apparently identical with Mr. 
 Thomas’s preparations. 
 
 To iodize the plain collodion ; add one part of the iodizing solu- 
 tion to three parts of the plain collodion ; shake well together, and 
 use the next day. If used immediately the sensitiveness is at its 
 highest pitch, but sometimes the negative may be a little wanting in 
 density, and perhaps disfigured by minute floating particles which 
 adhere to the film in consequence of the mixture not having been 
 allowed to settle, as well as with minute white spots occasioned by 
 undissolved crystals of iodide of potassium which have been pre- 
 cipitated by mixing the iodizer with the ether, and have not had 
 time to be redissolved. 
 
 Iodized collodion deteriorates by keeping, and at the end of a 
 few months is generally unfit for use, in consequence of the pyro- 
 xyline undergoing a decomposition which renders the film rotten. It 
 is at first of a lemon yellow colour, which changes in the course of 
 time to a deep red. In this state it is perfectly useless, and should 
 be thrown away, as the ether obtained by redistillation from it is too 
 impure to be used again for a similar purpose. Only small quan- 
 tities of iodized collodion should therefore be made at a time, and it 
 should be kept in a dark place as exposure to light greatly accele- 
 rates its decomposition. The bottle which contains it should be 
 made of orange coloured glass. 
 
 The change of colour produced in iodized collodion by keeping is 
 due to the liberation of iodine. This is occasioned, in all probability, 
 by the oxidation of the potassium, which forms potass, and unites 
 with an organic acid produced by the decomposition of the ether, 
 alcohol, and pyroxyline. The photographic effects of old, as com- 
 pared with new iodized collodion, may be imitated very closely, by 
 adding a little acetate of potass and free iodine to the iodizing solu- 
 tion. The negatives then require a much longer exposure, and 
 exhibit greater density in the blacks, and less gradation of tone, 
 
126 
 
 COL 
 
 which are the effects produced by old iodized collodion, with the 
 exception, however, of the increased porosity or rottenness of the 
 film in the latter case. When old iodized collodion is used, the free 
 iodine liberates free nitric acid from the nitrate of silver in the nitrate 
 bath, and this lessens the sensitiveness of the film, while increased 
 density is produced by the formation of organic salts of silver, 
 which in all the photographic processes have the property of inten- 
 sifying the image, as well as in some degree of impairing the sen- 
 sitiveness of the excited plate or paper. 
 
 When iodide of cadmium is used instead of iodide of potas- 
 sium, the iodized collodion does not become gradually deteriorated 
 in the same way, but the nitrate of cadmium which is formed and 
 accumulates in the nitrate bath has an acid reaction, which like nitric 
 acid, impairs the sensitiveness of the film, and the density of the 
 negative. Iodide of cadmium also injures the fluidity of collodion. 
 
 Iodide of ammonium is a very unstable salt, and liable to be de- 
 composed into free iodine and ammonia. Collodion iodized with 
 this iodide becomes rapidly discoloured (particularly in the light), 
 although when first iodized it yields remarkably good negatives. 
 The free iodine introduced into the nitrate bath, and the instability 
 of the nitrate of ammonia which is formed by the double decom- 
 position of nitrate of silver with iodide of ammonium, both tend to 
 overcharge the nitrate bath with free nitric acid. Iodide of ammo- 
 nium is therefore a bad iodizer to employ for negative collodion, 
 although a very good one for positive collodion, to which the same 
 objections do not apply. 
 
 The alcohol used for making negative collodion need not be so 
 absolutely free from the common impurities of alcohol as that which 
 is required for positives, for these impurities act rather beneficially 
 than otherwise in increasing the density of a negative. In positives, 
 however, where density is not required, but a pure white metallic 
 precipitate, they act injuriously on the tone of the picture. Posi- 
 tive collodion should be made with chemicals of the utmost purity, 
 and yet it is generally sold at a cheaper rate than negative 
 collodion. 
 
 Photographic pyroxyline is perfectly soluble to a mixture con* 
 taming equal parts of absolute ether (S. Gr. 720), and absolute 
 alcohol (S. G. 794); and certain iodides, such as the iodides of 
 calcium, ammonium, or cadmium, which may be used as iodizers, 
 are soluble in anhydrous alcohol. It is possible, therefore, to make 
 iodized collodion which shall be absolutely free from the presence of 
 water. But the film thus obtained, is found to be so extremely 
 repellant of water, that it is impossible either to excite it properly 
 
COL 
 
 127 
 
 in the nitrate bath, or to get the developer to flow evenly upon it. 
 Water is, therefore, a necessary ingredient in photographic collodion. 
 On the other hand, when collodion contains too much water, the 
 film cracks all over in drying, and the picture is destroyed. Great 
 nicety is therefore required in so regulating the specific gravities of 
 the ether and alcohol as that the collodion shall contain the proper 
 amount of water, and no more. 
 
 It is sometimes advisable (in hot weather for instance) to use 
 more alcohol in the collodion, in order to keep the film moist longer, 
 as well as to increase its sensitiveness by modifying its mechanical pro- 
 perties. When absolute alcohol (S. G. 794) is used, considerable 
 latitude is allowable in the quantity which collodion may contain. 
 Lor instance, if to an ounce of iodized collodion, an ounce of absolute 
 alcohol, and the corresponding quantity of pyroxyline and an iodizer 
 be added, the collodion so made will yield excellent negatives 
 scarcely distinguishable in any respect from the original collodion 
 containing only half the quantity of ether. As alcohol is much less 
 volatile than ether, this seems to indicate tlist for certain purposes 
 the usual proportions of ether and alcohol may be advantageously 
 modified. In making this experiment the reader must, however, 
 remember that by “absolute alcohol” is meant alcohol S. G. 794. 
 The alcohol frequently sold as “ absolute,” and even called so by 
 chemical writers who ought to know better, contains as much as 
 5 per cent, of water, and would, of course, ruin the collodion, if used 
 in the quantity indicated above. 
 
 Plain collodion may be kept for a very long time without deterio- 
 ration, in a cool, dark, dry place. It should be kept in a tall, well- 
 stoppered glass bottle, which should be shaken as little as possible 
 when decanting the collodion, so as not to disturb the particles of 
 undissolved cotton, &c., which settle to the bottom. 
 
 When too much iodide is added to collodion the film cannot con- 
 tain the whole of the iodide of silver that is formed in it by the nitrate 
 bath. It therefore lies loosely upon the surface, or is partially washed 
 off. On the other hand, if the film contains too little iodide, it ap- 
 pears thin and blue when looked at in the light, and does not give 
 quite so intense an image, the sensitiveness being also somewhat 
 diminished, though not much. It is better to have too little iodide 
 in the film than too much 
 
 When too much iodizing solution is added to plain collodion, the 
 film becomes thin and insensitive, because there is not enough 
 pyroxyline and too much iodide and water in it, so that the 
 sensitive iodide is washed off and transferred to the bath. When 
 a film contains too little pyroxyline, and too much iodizing solution, 
 
128 
 
 COL 
 
 the effect is first indicated by the formation of broad parallel lines 
 or bands, like the edges of retreating waves upon a flat beach, 
 all round the edges of the film ; and when this fault exists to a 
 great extent in collodion it becomes hardly possible to obtain a trace 
 of a picture, even after a very long exposure. The remedy is simply 
 to add plain collodion until good results are obtained. This addi- 
 tion of plain collodion to iodized collodion which works badly will 
 frequently remedy the evil. Another cause of the insensitiveness of 
 collodion, and poverty of the negative, occurs when from any mis- 
 management or inaccuracy in mixing the ingredients the iodide of 
 potassium is allowed partially to crystallize or solidify before the film 
 is put into the nitrate bath. This produces a compact and insensi- 
 tive coating of iodide of silver, which is blue and not sufficiently 
 transparent by transmitted light. The negative on such a film as 
 this is weak and insensitive. It appears, therefore, that even when 
 the materials are good, a variety of mishaps may occur through mis- 
 management in combining them. 
 
 Having now described the mode of preparing iodized collodion, 
 ready for use, we proceed to the various operations of taking the 
 negative. 
 
 To coat the plate with iodized Collodion. The plate must first 
 be cleaned in the manner described in the article, “ Cleaning,” and. 
 then well polished with a cambric rag, or leather buff, immediately 
 before pouring on the collodion, for unless the plate is wiped thoroughly 
 dry and well polished before use, it will be covered with streaks or 
 marks where the damp rag last touched it. The breath condenses 
 upon a clean, dry, polished plate in an even sheet, without exhibiting 
 marks or irregularities. 
 
 Hold the plate horizontally, by one corner, between the finger and 
 thumb of the left hand, if a small plate, or place it on a plate holder 
 (see Plate Holder) if too large to be conveniently held in this way, and 
 pour upon the middle of it rather more collodion than is sufficient to 
 cover it with a good thick layer. Then, tilt the plate so as to let 
 the collodion flow towards the thumb, but without touching it, and 
 afterwards to the other corners in succession, and pour off the 
 surplus into the bottle from the corner opposite to that by which 
 you hold it. This done, keeping the corner of the plate still resting 
 upon the neck of the bottle, and holding it vertically, rock it three or 
 four times quickly through a wide angle, in order, to prevent the 
 formation of lines in the collodion ; then place it upon the dipper ready 
 to be plunged into the nitrate bath. 
 
 Be careful to wipe the neck of the collodion bottle occasionally, as 
 bits of dry collodion which are formed there are liable to become 
 
COL 
 
 129 
 
 detached and deposited on the plate ; also, avoid dust in the dark 
 room, and blow off any floating particles which settle upon the plate, 
 before coating it. 
 
 To excite the Film . The ether evaporates very quickly from a 
 coated plate, and the fluid collodion speedily gelatinizes. This is 
 called “ setting.” When the collodion has sufficiently set, that is, 
 as soon as it ceases dropping from the comer of the plate, and is 
 safely placed upon the dipper, immerse it, without pausing, in 
 the nitrate bath ; a pause during the immersion producing a line 
 across the plate. Then, move it from side to side in the bath, for 
 a few seconds, in order to prevent the formation of streaks in 
 the direction of the dipper, and leave it immersed for a couple of 
 minutes or so. Then raise it out of, and lower it into the bath half 
 a dozen times; in order to wash off the ether and get rid of the 
 greasy, streaky, appearance of the film ; let it drain for a few seconds 
 over the bath ; wipe the back of the plate with blotting paper, and 
 place it in the slide, taking care never to invert the slide while the 
 plate is in it, and thus allow the free nitrate which drains towards 
 the bottom to flow back over the partially dry upper surface of the 
 plate. 
 
 The strength and composition of the nitrate bath is a matter of 
 the greatest importance. The formula for making it is simple 
 enough, but the bath is liable to many kinds of irregularities in its 
 mode of action, the principal of which will be described, and the 
 remedies pointed out. 
 
 The Nitrate Bath is made by dissolving 30 grains of nitrate of 
 silver to the ounce of distilled water, and leaving an iodized collo- 
 dion plate in it until the iodide of silver is dissolved out. This may 
 appear simple enough, but a solution of nitrate of silver may be 
 either acid, neutral, or alkaline, and its condition in these respects 
 materially affects the character of the negative. 
 
 Crystallized nitrate of silver is generally contaminated with free 
 nitric acid, and frequently adulterated with nitrate of potass. The 
 latter impurity has no other bad effect than that of weakening the 
 solution in which it is substituted for nitrate of silver, but free 
 nitric acid in the bath has a very marked and injurious effect when 
 it exists in too great excess. 
 
 Free nitric acid in the nitrate bath impairs the sensitiveness of the 
 film, and produces in a greater or less degree, according to its excess, 
 grey, metallic, and feeble negatives, which it is difficult to intensify 
 sufficiently. A bath containing an exceedingly faint trace of free 
 nitric acid is very good for all purposes where the light is strong, 
 such as views, skies, &c., because it keeps the lights of the negative 
 
 K 
 
130 
 
 COL 
 
 very clean, at the same time that it prevents the blacks from acquir- 
 ing an undesirable degree of opaoity, but for in-door work it is better 
 to neutralize completely the whole of the nitric acid, and acidify the 
 bath slightly with an organic acid, such as the acetic. 
 
 There are three ways of preventing the presence of free nitric acid 
 in the nitrate bath. The first is, to fuse the nitrate of silver ; the 
 second to neutralize the nitric acid with carbonate of soda; the 
 third to add acetate or citrate of soda to the nitrate bath. 
 
 By fusing nitrate of silver the whole of the free nitric acid is 
 driven off. The salt is then in general rendered slightly alkaline, a 
 fault which may easily be remedied by the addition of acetic acid to 
 the solution ; and sometimes, which is much worse, nitrite of silver is 
 formed, in consequence of the presence of nitrate of potass in the 
 silver salt. Heat drives off oxygen from nitrate of potass, and 
 forms nitrite of potass, which in its turn is oxidized at the ex- 
 pense of the nitrate of silver, and nitrite of silver produced. The 
 heat required for fusing nitrate of silver is not sufficient to decom- 
 pose it and form nitrite. Nitrite of silver is a most objectionable 
 substance to introduce into the nitrate bath. A very few crystals 
 of it added to a large bath will completely put it out of order, and 
 cause it to produce foggy negatives, looking as if they had been ex- 
 posed to diffused light. Acetic acid will not remedy this evil. The 
 only remedy is to add nitric acid, and so convert the nitrite into 
 nitrate. It appears therefore, that fused nitrate of silver is liable to 
 contain nitrite, and is consequently a dangerous substance to intro- 
 duce into a bath, unless the purity of the nitrate of silver has been 
 fully established, before fusing it. 
 
 The second plan of getting rid of free nitric acid from the bath, 
 is to neutralize it with pure carbonate of soda, a solution of which 
 should be added drop by drop, until the bath becomes decidedly al- 
 kaline, after which it should be acidified with acetic acid. This is 
 an excellent mode of dealing with an acid bath. 
 
 The third plan is, to add acetate or citrate of soda to the bath. 
 The soda having a stronger affinity for the nitric than for the organic 
 acid, leaves the latter and combines with the former, thus produc- 
 ing nitrate of soda, which is inert, and acetate or citrate of silver, 
 both of which latter salts are slightly soluble in the nitrate bath, and 
 have a marked effect in increasing the redness and density of the ne- 
 gative, and at the same time rendering the film less sensitive. 
 
 A bath made with fused nitrate of silver acts very well when the 
 nitrate is perfectly pure, and the alkalinity of the solution neutralized 
 by acetic acid. 
 
 The effect of free acetic acid in the bath is to keep the lights clean, 
 and slightly to impair the sensitiveness of the plate. 
 
COL 
 
 131 
 
 When a nitrate bath is accurately neutral it gives the most sen- 
 sitive film, but in this state it is very liable to get out of order, and 
 either become alkaline from the introduction of carbonate of potass 
 with the collodion, or acid from the fermentation of organic matter 
 which it may contain. Acidity is frequently produced in this way 
 when travelling, in consequence of the bath being kept for a long 
 time in a state of agitation. 
 
 It is impossible to obtain a good picture with an alkaline bath. 
 The developer is immediately decomposed and a precipitate thrown 
 down all over the plate. The film is not in this case more sensitive 
 than when the bath is neutral. 
 
 The acidity of a bath shoidd never be neutralized with ammonia, 
 because ammonia has a strong tendency to escape from its compounds. 
 Carbonate of soda is the proper alkali to employ. When added 
 to the bath yellow insoluble carbonate of silver is formed which is 
 immediately decomposed, and carbonic acid liberated with efferver- 
 scence. This goes on until the whole of the nitric acid is neutra- 
 lized, after which the insoluble carbonate of silver is precipitated. 
 This is again decomposed by the acetic acid afterwards added, and 
 acetate of silver formed. 
 
 On the whole it will be seen that the best formula for the nitrate 
 bath is that in which pure recrystallized nitrate of silver is used, con- 
 taining only a very small excess of nitric acicL The negatives are 
 not then so excessively dense as to produce chalky positives, and 
 the process is generally manageable as regards exposure for out-of- 
 door subjects, where the range of light and shade is the greatest that 
 occurs. When there are moving figures in a view which cannot be 
 taken instantaneously, and when violent contrasts of light and shade 
 occur, an exposure of two or three minutes is far preferable to one 
 of twenty or thirty seconds, — and then this kind of bath will be 
 found the best to employ, for the plate may then be exposed until 
 the picture is covered with detail without any part being overloaded 
 with a dense impenetrable precipitate. Excessive density is the 
 great evil to avoid in negative photography, and beautiful modula- 
 tion of tone with perfect rendering of all the details the great desid- 
 eratum ; provided always that a proper amount of contrasts between 
 the highest lights and deepest shades is at the same time introduced. 
 
 A nitrate bath sometimes works badly in consequence of being too 
 weak. Its strength should be tested occasionally with a silver- 
 metre. See “Nitrate Bath.” 
 
 The Bath may be kept for years in a glass, or gutta percha vessel, 
 when the gutta percha is pure and unadulterated. Porcelain is 
 objectionable, as nitrate of silver acts on the glaze. There is a 
 
 K 2 
 
132 
 
 COL 
 
 substance used in America for baths and dishes, called “ Photogra- 
 phic Ware,” which resembles wedgewood ware, and has no glaze ; 
 this is said to answer very well. The nitrate bath should be filtered 
 occasionally. 
 
 To develope the image — Pour over the plate a solution containing: — 
 
 Distilled water .... 1 ounce 
 
 Pyrogallic acid . . . .1 grain 
 
 Glacial'acetic acid . . . 20 minims, 
 
 or 
 
 Distilled water ... 2 ounces 
 
 Pyrogallic acid . . .2 grains 
 
 Citric acid . . .. 3 grains. 
 
 Tt is convenient to make a small quantity of developer of three 
 times the above strength, and to dilute it when wanted. You have, 
 then, at hand a little strong solution to use, should occasion require. 
 The developer will not keep longer than three or four weeks. A 
 little alcohol added to it makes it flow more freely over the plate. It 
 should be filtered if any floating particles appear in it. Citric acid 
 is much cheaper to employ than acetic acid, and it answers very 
 well, giving negatives of a more inky tint than acetic acid ; but it 
 must be carefully weighed, as a grain or two more or less may make 
 a considerable difference, citric being a very powerful acid in pho- 
 tography. 
 
 Although it is customary to call the acidified solution of pyrogallic 
 acid the “ developer,” yet it must be borne in mind that this is not 
 a developer unless it also contains nitrate of silver. The small quan- 
 tity of free nitrate of silver which remains on the wet plate is, in 
 general, sufficient to complete the development, but sometimes it 
 is necessary to add a few drops to the developer, towards the end 
 of the process in order to render the negative sufficiently intense ; 
 and this addition is always necessary when the bath contains a little 
 free nitric acid. The picture should be developed as soon as possi- 
 ble after exposure, because, as the plate dries, the free nitrate 
 becomes concentrated, and dissolves the iodide of silver, thereby 
 destroying the impression produced by light. 
 
 It frequently happens, in hot weather, that the plate gets so dry 
 during the exposure, that it is difficult to make the developer flow 
 over it at once, and this produces stains, which spoil the picture. In 
 such a case the following is a good plan to adopt : — 
 
 After removing the plate from the slide, immerse it in a bath 
 of distilled water. Then wash the measure which contains the 
 developer thoroughly in water, afterwards with cyanide of potassium, 
 
COL 
 
 133 
 
 and then with water again. This renders it chemically clean. Now 
 pour into it the necessary quantity of developer, and add a few 
 drops of fresh nitrate of silver solution. Bemove the plate from the 
 distilled water, and develop immediately with the above mixture, 
 which will now flow freely, and the picture will come out as quickly 
 and as dense as before, and free from stains. The developing pyro- 
 gallo-nitrate of silver should not become discoloured in the measure. 
 If it does, it is a proof that the measure is not thoroughly clean, 
 and the result will probably be a bad proof, dirty, and stained. 
 
 When the details are fully out, and the blacks of the proper inten- 
 sity, wash the proof with water poured over it gently from a jug, 
 and fix it in the manner to be described. 
 
 When a picture has not received sufficient exposure, the blacks 
 become very intense while the lights remain clear and devoid of 
 detail. Under these circumstances it is impossible by any modifi- 
 cation of the treatment to produce a good negative. When the 
 picture has been over exposed the effects are different according to 
 the state of the bath and collodion. When the bath is acid with 
 nitric acid, the effect of great over-exposure is to reverse the action 
 of light in the black parts of the negative, and render them more 
 or less transparent. W r hen the bath is acid with acetic acid, and 
 the picture exhibits the effects due to organic matter, the over-exposed 
 parts do not become dark so rapidly as those which have received 
 the proper exposure, but remain red and transparent, having a 
 burnt appearance, the edges of the red parts being, however, fringed 
 with black, and the “ solarization,” as it is called, being more marked 
 in the centre of the over-exposed part. When the bath is neutral, 
 and the iodizer contains chloride of magnesium, and the developer 
 citric acid, the over-exposed parts are sometimes those which acquire 
 the greatest density and opacity, and what is called “ solarization” 
 is not then produced. 
 
 Over-exposed negatives give flat and feeble positives, which are 
 deficient in contrast and cannot be properly printed to a good 
 tone. 
 
 There are limits therefore to the time of exposure which cannot 
 be passed with impunity. When diffused light finds its way into the 
 camera, fog is an evil which is greatly exaggerated by over-exposure ; 
 and when the chemistry of the process is a little at fault, the effects 
 of over-exposure are sometimes very remarkable and absurd. 
 
 Negatives may be developed with proto-sulphate of iron acidified 
 with acetic or citric acid ; and some persons think this developer a 
 much better one than either of those before described. The 
 
134 
 
 COL 
 
 proportions are variable, and depend on circumstances. In our 
 hands the following formula succeeds very well : — 
 
 Distilled water . . . .1 ounce 
 
 Proto-sulphate of Iron . . . 5 grains 
 
 Acetic acid . . . . .5 minims 
 
 The acid should be added to the proto-sulphate solution imme- 
 diately before use, and the proto-sulphate solution should be kept 
 as much as possible in the light. There should be no free nitric 
 acid in the nitrate bath, and the collodion should be such as to 
 assist the production of an organic compound of silver. When this 
 developer is used too strong, a white transparent stain is produced 
 upon that part of the plate on which it is first poured ; but so long 
 as this evil does not occur the developer may be strengthened with 
 advantage. The exposure need not much exceed one-half that 
 which is required when pyrogallic acid is used. The chief merit of 
 the iron developer is that it yields negatives abounding in half tone, 
 and with exquisite detail. Any amount of density may be obtained 
 with proper treatment. The negatives are brown , and not blue. 
 
 Some operators commence the development with iron, and finish 
 with pyrogallic acid, the iron being, of course, carefully washed off 
 before applying the pyrogallic. This is an excellent plan. 
 
 When the precautions for avoiding “fog 55 are strictly observed, 
 negatives developed with iron are as perfectly transparent in the 
 lights, as those developed with pyrogallic acid. 
 
 On the whole the iron developer appears to be superior to the 
 other, in the hands of a skilful operator; and it is particularly 
 serviceable in cold weather. 
 
 As a universal rule, a negative should be red and not grey in the 
 first stage of the development. 
 
 To fix the picture — Pour over it either a saturated solution of 
 Hyposulphite of soda, or a solution of cyanide of potassium from 
 5 to 10 grains to the ounce. As soon as all the yellow iodide of 
 silver is dissolved, wash the plate well with water, and set it up to 
 dry. 
 
 Hyposulphite of soda does not reduce the density of a red organic 
 image so much as cyanide of potassium, but the plate requires to be 
 washed much more thoroughly and carefully after it, for if this be not 
 attended to, the hypo left in the film is apt to crystallize and destroy 
 the picture. 
 
 Collodion negatives are not permanent unless they are varnished. 
 The pyroxyline film is gradually decomposed by contact with air 
 and moisture, and an oxide of nitrogen produced, which destroys 
 
COL 
 
 135 
 
 the photograph. When properly varnished, however, they appear 
 to be extremely permanent. Good spirit varnish is the best, and 
 amber varnish one of the worst to employ. (See “ Varnish.”) When 
 spirit varnish is used the plate should be warmed before the fire, 
 both before and after the application, but not made too hot, or the 
 negative would be dissolved off the plate. The varnish is poured 
 on in the same way as the collodion. It dries very quickly. Other 
 varnishes are applied in the same way to a cold plate, which should, 
 however, be thoroughly dry. 
 
 The negative is now finished. 
 
 It may be well to add a few words on the chemistry of this im- 
 portant process, although the general chemistry of photographic 
 processes has been discussed at some length in a separate article on 
 the Chemistry of Photography, q. v. 
 
 The collodion film is nearly inert, producing in a very slight degree 
 the effects due to organic matter ; it must be considered as little 
 more than the vehicle for supporting the sensitive iodide of silver, 
 which is more sensitive to light when precipitated in a test tube, 
 than when collodion is added to it. Collodion is not an accelerating 
 substance; on the contrary, it diminishes the sensitiveness of 
 the mixed iodide and nitrate of silver. The reason why the 
 collodion process is more sensitive than any other, is partly because 
 the film is moist and transparent, and the sensitive chemicals are 
 distributed through it in a finely divided state, and partly because 
 the nitrate bath is nearly neutral, and the developer highly energetic. 
 
 The iodide of potassium in the film produces with the nitrate of 
 silver in the nitrate bath a double decomposition, iodide of silver 
 and nitrate of potass being formed. The latter salt appears to be 
 inert in the various operations. 
 
 On exposure to light, the iodide of silver and free nitrate in 
 the film are reduced, and an image formed, which, in the collodion 
 process, is too faint to be visible. 
 
 The developer, when mixed with the free nitrate on the plate, forms 
 an unstable compound, extremely liable to be decomposed, and only 
 preserved for a few minutes from spontaneous decomposition by the 
 organic acid which is added to it. When this is poured over the 
 reduced image, the reduced particles act as centres of attraction, 
 which first decompose the pyro-gallo-nitrate in contact with them, 
 and then cause it to accumulate atom by atom upon them, in this 
 way causing the image produced by light to acquire fresh material, 
 and become visible and intense. 
 
 The fixing solution dissolves the iodide of silver, forming, in the 
 
136 
 
 COL 
 
 case of cyanide of potassium, a double cyanide of silver and potassium 
 together with iodide of potassium, and, in the case of hyposulphite of 
 soda, a double hyposulphite of silver and soda, together with iodide of 
 sodium, 
 
 The exact chemical composition of the blacks of the image is not 
 known. The red substance formed in the early stage of the develop- 
 ment is probably an organic subsalt of silver, and the black substance 
 precipitated at the end of the development either metallic silver, 
 or a compound of metallic silver with carbon. If an unvarnished 
 negative be dried before the fire, and then rubbed lightly with a piece 
 of leather, the material of the image exhibits exactly the appearance 
 of polished metallic silver. This is no proof, however, that a small 
 quantity of carbon may not be present, because in the case of steel, 
 (which is a carbide of iron,) the metallic appearance and lustre is 
 perfectly preserved. It is not unlikely that, in the decomposition of 
 gallo-nitrate, or pyro-gallo-nitrate of silver, a compound of meta-gallic 
 acid with suboxide of silver is first produced, and afterwards reduced 
 either to metallic silver, or carbide of silver. The matter is, however, 
 at present very obscure. 
 
 Positive Collodion Process, By this process, a direct positive 
 is produced or a collodionized tablet, by developing the image 
 that is obtained in the camera. 
 
 The collodion positive and negative processes differ in the following 
 important particular, viz : — 
 
 In the negative process the object is to obtain a picture, in which 
 the material of the image shall be more or less opaque when looked 
 through ; — in the positive process, to obtain a picture in which the ma- 
 terial of the image shall be^a dead white when looked at. In a negative, 
 density of various gradations is what is required, without any reference 
 to the appearance of the surface deposit. In a positive, whiteness of 
 the surface deposit is what is required, without any reference to density. 
 In the negative process, organic matter is introduced, first as acetic 
 acid or acetate of soda in the nitrate bath, and secondly, as gallic or 
 pyrogallic acid, acidified with acetic or citric acid for the developer ; 
 the object being to obtain density in the image by combining organic 
 matter with the reduced silver, so as to obtain sufficient opacity, and 
 and not a grey, feeble, metallic image. In the positive process, on 
 the contrary, the object 7 is to avoid , as much as possible, the intro- 
 duction of organic matter in any part of the formula, first by acidi- 
 fying the nitrate bath with nitric, instead of acetic acid, and secondly, 
 by developing with pi inorganic developer, such as a mixture of 
 
COL 
 
 137 
 
 the proto-salts of iron with nitric acid, and in this way endeavouring 
 to produce not an opaque organic image, brown on the surface, but 
 a thin, white, metallic deposit. 
 
 This principle being borne in mind, the rationale of the positive 
 process will easily be understood, and as the manipulation very much 
 resembles that for negatives it may be more briefly described, and 
 the points of difference only between the formulae indicated. The 
 plain collodion is the same both for positives and negatives, but the 
 iodizing solution for positives should contain iodide of ammonium, 
 instead of iodide of potassium, in about the same quantity, i. e. from 
 1 2 to 14 grains to the ounce of alcohol. The alcohol of the iodizing 
 solution should be chemically pure, and free from the fusel and grain 
 oil contained in spirits distilled from grain or roots. These organic 
 matters introduced into collodion, although comparatively harmless 
 in negative collodion, affect injuriously the tone of a positive by 
 producing an admixture of organic silver salt with the white metal. 
 
 The nitrate bath should be rather stronger in silver than that for 
 negatives, and should be acidified with nitric acid. The formula 
 is as follows : — 
 
 Distilled water . „ . .1 ounce 
 
 Nitrate of silver . . . . 40 grains 
 
 Nitric Acid ♦ . . . .1 minim 
 
 The time of exposure is about half that required for negatives. 
 
 The Developer is made thus : — 
 
 Dissolve one ounce of powdered nitrate of baryta in 16 ounces 
 of distilled water, and add 2 drachms of nitric acid, S.Gr. 1*4. Next 
 add 1^ ounces of powdered proto-sulphate of iron. Shake well until 
 the iron salt is dissolved. The mixture becomes white and turbid, in 
 consequence of the formation of sulphate of baryta. Let it stand a 
 few hours until this has settled to the bottom of the vessel ; then 
 decant and filter the solution, which, if right, will be of an apple green 
 colour. Add two ounces of alcohol to enable it to flow freely over 
 the collodion film. It is now ready for use, but gradually deteriorates 
 by keeping. It may be kept about a month in a cool place. The 
 nitric acid slowly oxidizes the proto-salts of iron and the solution 
 turns yellow. * In this state it is much slower in its action, and a little 
 fresh proto -sulphate should be added to it. An ounce of proto-sul- 
 phate of iron decomposes about an ounce of nitrate of baryta, forming 
 insoluble sulphate of baryta, and soluble proto-nitrate of iron. The 
 remaining half ounce of undecomposed proto-sulphate of iron forms 
 the energetic part of the developer. 
 
 The nitric acid should be added as stated, and not after the iron 
 
138 
 
 COL 
 
 Salt as some peroxidation of the iron might then occur, which would 
 occasion a browning of the solution, and be injurious. 
 
 The mode of developing the picture is quite different from that 
 in the negative process. It must be done almost at a blow, and is 
 completed as soon as there is a visible indication of the details in the 
 shadows when the plate is laid upon a black ground. These details 
 come out very quickly, and the development generally occupies only 
 a few seconds. If it be carried too far the boldness and vigour of 
 the contrasts is destroyed. Over-exposure produces a blue solarized 
 appearance, as in the Daguerreotype process. 
 
 The picture is to be washed, and fixed with cyanide of potassium, 
 then washed again, dried and varnished in the same way as a nega- 
 tive ; after which, the back of the plate is to be coated with black 
 varnish. Some operators varnish the picture itself with black varnish 
 because then it is non-reversed on looking at it through the glass ; but 
 this is a bad plan, because the varnish injures the tone of the 
 whites, and is very liable to crack and destroy the picture. When 
 applied to the back of the plate, it can easily be rubbed off and re- 
 newed if it cracks, and no harm is done to the picture. See “ Var- 
 nish.” 
 
 Positives should be taken in a non-reversing slide whenever the 
 reversion of the image is objectionable. 
 
 Some operators take positives on purple glass, which forms an ex- 
 cellent background to the picture, and renders the use of black var- 
 nish unnecessary. This is an excellent plan. 
 
 Positives may be taken upon a variety of substances, such as black- 
 varnished paper and card, black-enamelled iron tablets, black patent 
 leather, black glazed canvas &c., &c., Some of these processes are 
 very ingenious and useful, and deserve a particular description 
 
 intake Positives upon Paper , or Card. The paper or card must 
 first be gelatinized by floating the side on which the picture is to be 
 taken on a warm strong solution of gelatine, and drying it. It is 
 then cut a little smaller every way than a glass plate, to which it is 
 to be attached in the following manner : — Lay the back of the paper 
 upon the glass, heat the blade of a penknife in a spirit lamp, rub it 
 upon a lump of wax, and then pass it quickly all round the edge of the 
 paper so as to fasten it to the glass plate by an edging of wax, with- 
 out allowing any liquid to get behind it. Next coat the paper with 
 black varnish, and let it dry. Then take the positive in the ordinary 
 way, and when finished cut off the edges of the paper and detach it 
 from the glass. Positives taken on paper or card in this way may 
 be safely transmitted by post. The object of the gelatine is to pre- 
 vent the black varnish from soaking into the paper. 
 
COL 
 
 139 
 
 To take Positives on Patent Leather , Glazed Canvas , Enamelled 
 Iron Tablets &c., The leather or canvass is to be attached to the 
 glass by an edging of wax in the manner described for paper, and 
 the positive taken on the glazed surface at once, without black-var- 
 nishing it. Enamelled iron tablets may be treated in precisely the 
 same wav as a plate of purple glass. The back of the tablet should 
 be varnished so as to prevent its injuring the nitrate bath. 
 
 Collodion positives may be transferred to a great variety of tablets 
 by methods which are described in the article on Transferring. 
 See “Transferring.” 
 
 Collodion ized-Paper Process. In this process a negative 
 is taken upon a sheet of paper coated with collodion. The process has 
 not yet been extensively practised, but it appears to offer some im- 
 portant advantages, in one or two respects, over every other process 
 now employed by the tourist. It will be better to describe the 
 manipulation first, and point out these advantages afterwards. 
 
 The operations in the collodionized*paper process consist in 
 attaching a sheet of paper to a glass plate, coating the paper with 
 collodion, and proceeding in the ordinary way to the completion of 
 the picture ; then removing the paper from the glass by soaking it 
 in water, and treating it as an ordinary paper negative. 
 
 The only operations we have to describe are those of attaching 
 the paper to the plate, and removing it when the picture is finished ; 
 the others being identical with those on collodionized glass. 
 
 To attach the Paper to the Glass Plate . Immerse it in a warm 
 solution of gelatine, made by dissolving six grains of gelatine to the 
 ounce of water. Let it remain for a minute or two in the solution, 
 then remove it, and immediately apply the back of it to a clean 
 glass plate, pressing it into close contact with the glass by means of 
 a bent glass rod. Let it dry thoroughly. When dry it will stick 
 firmly to the glass. 
 
 Coat the paper with collodion, excite it by immersion in the 
 nitrate bath, and develope and fix the picture precisely as in the 
 collodion process. Then soak the plate in water, and remove the 
 paper negative as soon as it will leave the glass without risk of being 
 torn. It should then be well washed in several changes of water, 
 dried, and waxed in the usual way. 
 
 The advantages of the process are, that the excited paper is highly 
 sensitive, — more so than an excited collodionized plate, — so that 
 skies and instantaneous pictures may be taken upon it ; and, also, that 
 it gives far better definition and half-tone than the ordinary methods 
 
140 
 
 COL 
 
 upon paper. The photographic tourist may, therefore, employ paper 
 for the same class of subjects as glass, without subjecting himself to 
 the expense, trouble, and risk of travelling with boxes of glass plates, 
 and valuable glass negatives. On the other hand, when all has been 
 done to render a paper negative evenly transparent, it can hardly be 
 rendered quite equal to glass for supporting a negative picture. 
 Such are the merits and drawbacks of an excellent process, which 
 deserves to be more extensively practised. 
 
 Colour. — If the coloured spectrum produced by passing a ray 
 of sunshine through a prism, and thereby decomposing it into 
 coloured rays of different refrangibilities, be thrown upon a sheet of 
 sensitive photographic paper, the paper is most darkened by the 
 violet rays, and least by the yellow and red. This shews that the 
 chemical power of light resides chiefly in the violet rays. Now the 
 colour of an object when illuminated by white light is supposed to 
 depend upon its absorbing all the coloured rays which fall upon it, 
 and, when combined, produce white light, except the rays of the 
 particular colour which it emits ; so that, on this hypothesis, it 
 might be supposed that the images of yellow and orange coloured 
 objects would scarcely produce an impression upon a photographic 
 tablet, while those of objects of a blue or deep violet colour would 
 produce a comparatively marked effect. This is found by experi- 
 ments to be true to some extent in the case of coloured objects 
 copied in the ordinary way, and to a greater extent in the case of 
 light transmitted through coloured media. Tor instance, if a paint- 
 ing in fresco were exposed to full sunshine, and copied in the ordi- 
 nary way, the photograph would not be so perfectly truthful in its 
 mode of rendering shade for colour, as an engraving from the same 
 picture, because the yellows and reds would be darker, and the blues 
 and violets lighter in the photograph than in the engraving, in which 
 light colours would be properly represented by lights, and dark colours 
 by shades. But, if the same fresco were faithfully copied by a trans- 
 parent painting upon glass, and this copy were photographed by 
 superposition upon a sensitive tablet, and a positive printed from a 
 negative so obtained, the untruthfulness of tliis positive, as regards 
 light and shade, would be much more marked than in the former 
 case. It appears, therefore, that a coloured object emits from its 
 surface certain actinic rays besides the rays of its peculiar local 
 colour, and that the photographic copy of a coloured object obtained 
 in the ordinary way is not quite so inaccurate in its mode of ren- 
 dering colours by shades as might at first be supposed. 
 
COL 
 
 141 
 
 As a rule, the photographer should remember that blues and 
 violets come out lighter , and yellows and reds darker , in a direct, or 
 printed positive, than they ought to be. 
 
 It seems probable that different coloured artificial lights, or sun- 
 shine transmitted through coloured media, might be successfully 
 employed in illuminating coloured objects to be copied by photo- 
 graphy. A variety of highly interesting experiments might be 
 made in this direction, with the view of removing, if possible, from 
 photography the reproach under which it now somewhat justly 
 labours. As yet very little has been done in the way of careful 
 scientific experiments in the photography of coloured objects, and 
 our information is consequently scanty on this head. The difficulty 
 found in copying old oil paintings arises in a great measure from their 
 exceedingly dingy and low tone of colour, which will be perceived 
 by holding a white card against the highest lights ; and also from 
 the varnished and uneven surface, texture of the canvas, and other 
 defects. 
 
 Yellow paper, or calico, or glass, used for cutting off actinic 
 light, should have a tinge of orange, rather than green, as the red 
 are much less actinic than the blue rays. 
 
 Colouring Matters. Organic colouring matters are generally 
 destroyed by the action of light and oxygen, or chlorine. The 
 affinity of the carbon and hydrogen which they contain for oxygen 
 and chlorine is increased by light, and the oxygen which they take 
 up, and which may escape as carbonic acid, alters their composition, 
 and destroys their colour. They have been examined photogra- 
 phically by Sir John Herschel : he found the juice of the red poppy 
 and the double purple groundsel to be especially sensitive to the 
 bleaching power of light, and that the fading of flowers was due to 
 the same cause, which increased in force as the vitality of the plant 
 was diminished. The juices formed in spring, in the more active 
 stage of the plant’s growth, are also more easily destroyed when they 
 are expressed, than those which are elaborated at a later season. 
 The colouring matters of leaves go by the names of chlorophyl, 
 xanthophyl, and erythophyl, or green leaf, yellow leaf, and red leaf. 
 A solution of the first in ether passes, by the action of light, into 
 the yellow colour of autumn, that is, into the second variety : the 
 red matter of leaves has already been oxidized, and is much more 
 stable in the air, but long exposure changes it to brown. 
 
 These fugitive animal and vegetable dyes have slightly acid pro- 
 perties, like gum and sugar : hence they may be precipitated with 
 the oxides of iron, lead, and aluminium, and, in this state, are, 
 
142 
 
 COM 
 
 CON 
 
 much more permanent; and these oxides having an affinity for 
 cotton and other fibres, fabrics may be dyed with these compounds, 
 though they have no attraction for the colour itself, and many of 
 these compounds are purer in colour when formed in the light than 
 in the dark. Many photographic processes are analogous to these. 
 
 These weak organic acids are, like pyrogallic acid, more easily 
 oxidized when mixed with an alkali, and, therefore, fabrics intended 
 to be bleached are rendered alkaline before they are exposed, by being 
 bowled, or boiled, with a weak solution of caustic potassa ; and like 
 pyrogallic acid, though in a less degree, they have the power of 
 decomposing solutions of nitrate of silver, and carrying the oxide 
 down. 
 
 Combining Proportions. See “ Equivalents. 55 
 
 Concentrated Solutions. These are solutions in which the 
 fluid contains as much of the solid matter as it can hold at the actual 
 temperature. 
 
 Condenser. The condenser is a thick convex lens, of moderate 
 focus, uncorrected either for spherical or chromatic aberration, and 
 used for the purpose of directing light through any transparent 
 object intended to be copied, either in the copying or microscopic 
 camera, in the manner described in the article on copying. Its 
 use and mode of action will be understood from the following 
 diagram. (Pig. 12.) 
 
 A B is a transparent object to be copied, l m the diaphragm of a 
 lens, or the front lens of a combination, 0 an artificial light (the 
 oxy-calcium light, for instance), D E the condenser. 
 
 The condenser is larger than the object to be copied ; its principal 
 focal length should be a little shorter than its distance from the incan- 
 descent lime ball 0 ; and the image of O should be formed very 
 indistinctly, and with great spherical aberration in the neighbour- 
 
CON 
 
 143 
 
 hood of l m. All the rays included within the angle DOE will 
 then be transmitted through A B and the copying lens, and the 
 images of the transparent parts of A B will be formed by diverging 
 pencils, as shown at the points A, C, B, and in the manner described 
 in the article on copying. {See “ Copying.”) The condenser should 
 be of such a form, and be so placed, as to give the greatest possible 
 spherical aberration. Plano-convex is a very good form, with the 
 plane side next to the light. Some operators put the convex side 
 next to the light, but that is incorrect, because it is required to 
 increase , and not to diminish the spherical aberration of the image 
 of 0. It is evident that if the lens D E had no spherical aberra- 
 tion, the pencils l A m, l C m, l B would become straight lines, and 
 the condenser would be useless. 
 
 When sunshine is to be used instead of artificial light, the form of 
 the condenser must be modified, so as to suit the parallel rays inci- 
 dent upon it. The construction is shewn in Eig. 13. 
 
 Gr H E D is a cylindrical tube, having at each end a plano-convex 
 lens. H K is a reflector, by means of which the sun’s rays are 
 directed into the tube. The first convex lens gives a confused 
 image of the sun at its principal focus O, and rays then diverge 
 from O, and pass through the second lens in the same way as from 
 the lime ball in Eig. 12. 
 
 A solar camera may be constructed by continuing the ordinary 
 copying camera until the front of it includes the condenser G E, 
 and adding a reflector K H, capable of receiving the same adjust- 
 ments as the reflector of a solar microscope. 
 
 It will be seen in Eigs. 12 and 13, that, without the condenser 
 D E, very little of the light that is radiated from O, would pass 
 through the negative and enter the camera through the stop in 
 front of the lens. The condenser collects diverging rays, and causes 
 them to converge to the points desired. 
 
 The condenser commonly made by opticians consists of a plano- 
 convex lens, with its convex surface in contact with the most convex 
 surface of a “ crossed lens.” See “ Crossed Lens.” This arrangement 
 
144 
 
 CON 
 
 is intended to diminish spherical aberration, and is therefore wrong 
 in principle. In ordinary lenses, spherical and chromatic aberration 
 are defects , but in a condenser they are advantages . This should be 
 distinctly understood. 
 
 Conjugate Foci. Tt is an admitted principle in geometrical 
 optics, that when a ray of light has passed through any optical in- 
 strument composed of lenses and reflectors, it will, on having its 
 direction reversed, return by the same path as that which it pre- 
 viously followed. If, therefore, a pencil of rays diverging from a 
 luminous point Q be refracted through a convex lens, or convex 
 system of lenses, and brought to a focus q, it follows that on re- 
 versing the direction of every ray, and considering q as the origin 
 of a pencil proceeding the contrary way to the first, every ray will 
 return by its former course to Q, the origin of the original pencil, 
 which may therefore be considered as the focus corresponding to the 
 origin q. From this circumstance the points Q, and q are called 
 “ Conjugate Foci.” 
 
 Conversion of Positives into Negatives, and vice versa. 
 This conversion is often spoken of as possible, but few instances of 
 true conversion appear to have been recorded. Collodion positives, and 
 similar pictures, are real negatives, and it is only by reflection that 
 they appear positive. As negatives they are feeble, and the inten- 
 sifying of them so that they may be printed from is not conversion, 
 but merely further development. In development the last part of 
 the attached silver is more metallic than the first, and a negative 
 has been known to appear very positive by reflection, when much 
 metallic silver has been precipitated upon it ; but neither is this a 
 true conversion, it is merely an alteration in the superficial cha- 
 racter, and the negative prints as before. When a collodion pic- 
 ture first commences its development, if it be exposed momentarily 
 to daylight, a real positive image by transmitted light is often pro- 
 duced, instead of a negative. But this does not constitute con- 
 version, since no such change occurs when a negative image is once 
 distinctly formed. There are several processes in which, by slight 
 modifications, either the sunned or the unsunned portions may be 
 developed, the darkest at pleasure, so that either a negative or posi- 
 tive viewed by transmission is produced ; and in some of Sir John 
 Herschel’s processes, the dark parts of a developed negative may 
 be bleached, and the other parts darkened, when a case of real con- 
 version is constituted. Such processes are called Ampliitype, (from 
 a/jKpi both ways). 
 
COP 
 
 145 
 
 Copal. A resinous substance much used in varnishes. There 
 are three kinds, the East Indian, West Indian, and African. The 
 copal of the East Indies and coast of Guinea is the best. That of 
 India exudes from the Elceocarpus copalifer , and is obtained in 
 hard yellow lumps ; that of Guinea is found in the sands of 
 rivers. • 
 
 Copal is soluble in ether, and oil of rosemary, but not very soluble 
 in alcohol, until it has been pulverized and exposed for some months 
 to the action of air and light, by which it is oxidized. Eused 
 copal is soluble in oil of turpentine. Alcohol containing camphor 
 dissolves copal much more readily than pure alcohol. The methods 
 of dissolving copal, and making copal varnish, are described in the 
 article on Varnish; q. v. 
 
 Copper. Salts of copper have not been much used in pho- 
 tography. The chloride might be expected to behave something 
 like chloride of silver, since it parts with half its chlorine with tole- 
 rable readiness, and is besides prone to oxidation. It has been 
 observed, that the brown chloride becomes blue by exposure to air, 
 and that the whole subchloride absorbs oxygen, and becomes green 
 (Brunswick green). Light would most probably favour these 
 changes. The cupro-cyanides might be expected to act like the 
 cobaltocyanide. 
 
 Copying. This term is used in photography, when manuscripts, 
 maps, paintings, engravings, and works of art of that class are to be 
 reproduced. 
 
 In copying black marks upon a white ground, by the collodion pro- 
 cess, it is generally advisable to have recourse to organic matter in 
 the bath and collodion, in order to obtain density at the expense of 
 sensitiveness and half tone. An old and rather discoloured collo- 
 dion should therefore be used, because it contains an organic salt of 
 potass, and the nitrate bath should be more strongly acidified with 
 acetic acid, and a little acetate of soda added to it. The work to 
 be copied should then be placed either in sunshine or a strong light, 
 and a small stop used to the lens. Care must be taken not to give 
 so much exposure as to redden and enfeeble the blacks of the 
 negative. 
 
 A rather strong pyrogallic developer may be used, and a little 
 silver added towards the end of the development. A small stop is 
 necessary in order to give sharpness to the edges of the picture, and 
 a strong light to give density to the blacks of the negative. 
 
 L 
 
146 
 
 COP 
 
 The copying of oil paintings is a matter of great difficulty, 
 because reflected light from the varnish is likely to enter the camera 
 and blacken some part of the negative. A lens of long focus 
 should in all cases be employed, for reasons which will be under- 
 stood by reference to the following diagram, which is exaggerated in 
 its proportions, in order to render it more intelligible. 
 
 Let AB be a varnished 
 oil painting, L the lens, and 
 a b the image ; the lens being 
 one of short focus and placed 
 near to the painting. Sup- 
 pose the painting illuminated 
 by light falling obliquely upon 
 it as shown by the arrows. 
 
 Then some of the rays in- 
 cident about E, will after re- 
 flexion at the varnished surface pass 
 through the lens and fall upon the sensi- , 
 tive plate at e , producing a dark patch b 
 in the negative. 
 
 Next, suppose a lens of longer focus 
 used, and the picture placed further from it, the image being the 
 same size as before, as shown by the dotted lines ; then, no re- 
 flected rays will fall upon the sensitive plate, but they will all pass to 
 the left of the lens. 
 
 In copying oil paintings, it is important, therefore, to use a lens 
 of long focus, so as to avoid introducing very oblique pencils. 
 The following is another point of equal importance. If light objects 
 are immediately behind the camera, the light from them will strike 
 the painting nearly at right angles, and then be reflected into the 
 camera. This will be avoided by the arrangement shown in Eig. 10, 
 which explains itself. 
 
 Oil paintings should be placed in the sunshine and a stop used 
 to the lens. There is no advantage in using a weak light in pho- 
 tography. A strong light gives a quality similar to that which is 
 called “ penetration” in microscopy ; that is to say, it brings out 
 the feebler details of objects. 
 
 Daguerreotypes should be copied in full sunshine, the original 
 being placed at the end of the projecting front of an appropriate 
 camera, and the light thrown obliquely upon it, (as in Fig. 10,) 
 through an open lid in the top of the box, the inside of the box 
 being blackened, or lined with black velvet. 
 
 A E B 
 
COP 
 
 147 
 
 It now remains to add a few words on the mode of copying 
 transparent negatives, by means of a copying camera. 
 
 A good deal has been said on this head in the article on the 
 copying camera, ( see “ Copying Camera,”) and it only remains for us 
 to discuss the various methods of obtaining a luminous background 
 to the transparent negative. As this is a point which should be 
 thoroughly understood, we shall endeavour to make it as clear as 
 possible with the aid of a diagram. See Pig. 11. 
 
 The great principle to be borne in mind in the optics of photo- 
 graphy is, that the object to be copied is an assemblage of bright 
 points, each of which is the origin of a pencil of rays, and that the 
 image formed by the lens is an assemblage of foci of these pencils. 
 Prom every point of the object a diverging pencil of rays proceeds, 
 which after refraction through the lens is brought to a focus on^ 
 4he focussing screen. Now, in copying a transparent negative, the 
 case is evidently different from that of copying an engraving or print 
 placed in the same position with respect to the lens. Por instance, 
 in Pig. 11 ; if AB is an engraving and C any white point of it, 
 
 L 2 
 
148 
 
 COH 
 
 CEI 
 
 and brought to a focus at c ; — but if A B is a transparent negative 
 and C a piece of transparent glass, it is evident that C cannot be 
 the origin of a diverging pencil of light in the same way as before, 
 unless rays transmitted through C from a luminous background 
 behind, can be made to intersect at C, and then pass on as if C had 
 been an origin of light. 
 
 This being understood, let us suppose that the dotted line DE 
 behind AB is either the sky or an illuminated white screen. Then 
 if C be considered a minute transparent hole, the sky or screen 
 could be seen through it by an eye placed any where between l 
 and m , and, therefore, rays MC^, LC l, would intersect at C, 
 and produce the effect of a pencil diverging from C. And similarly 
 of every other transparent point of the negative. It appears, there- 
 fore, that when a transparent negative is copied with the sky or an 
 illuminated screen as a background, the same effect is produced as 
 if the transparent parts of it were opaque but luminous, and emitted 
 diverging pencils of light. 
 
 In all cases, the diameter of the stop, or lens, is small compared 
 with the distance of the lens from the negative ; therefore the 
 angles l Am, l‘Cm, l~Bm, are small ; and if, instead of the sky or an 
 illuminated screen, DE were a convex lens transmitting a pencil 
 of light DAEB, which converges with great spherical aberration 
 towards a focus in the neighbourhood of l m , the same effect would 
 be produced as before. A lens of this kind is called a “ Condenser,” 
 {See “ Condenser,”) and transparent negatives may be copied by 
 means of it, with either an artificial light placed behind it, or reflected 
 sunshine transmitted through it. 
 
 When any object is to be copied by superposition in the pressure 
 frame, the ordinary printing processes may be employed which are 
 described in the article on Printing. See “ Printing.” 
 
 Corrosive Sublimate. Bichloride of mercury ; q. v. 
 
 Cosmorama Stereoscope. See “ Stereoscope.” 
 
 Cribriform. (Latin ; cribs, a gridiron). When a paper nega- 
 tive is viewed by transmitted light, it sometimes appears covered 
 with intersecting diagonal lines, like network. This appearance is 
 called “ cribriform.” It is produced by the wire frame in which 
 the paper is made, and is generally remedied by waxing the paper 
 properly ; but sometimes an insoluble precipitate is formed by the 
 developer in the pores of the paper, and then the cribriform appear- 
 ance is more marked. Immersion in a bath of dilute muriatic acid 
 
CEO 
 
 CYA 
 
 149 
 
 will sometimes remove this precipitate, and then by a long immer- 
 sion in melted wax the paper may be rendered evenly transparent. 
 
 Crossed Lens. This is the form of single convex lens which 
 has the least spherical aberration. The refractive index of the glass 
 should be 1*5, and the radius of the posterior surface six times that 
 of the anterior surface, both surfaces being convex. 
 
 Crystal Varnish. See “Varnish. 5 ’ 
 
 Curvature of the Image. This is a term used in optics to 
 denote the fact of an image not lying upon a plane, but upon a 
 spherical or other curved surface. 
 
 The “real image, 55 formed by a convex lens, is an assemblage of 
 the foci of pencils which proceed from the various points of the object. 
 The situation of these foci, whether on a plane or curved surface, is 
 independent of the amount of spherical aberration which may exist 
 in each or any of the refracted pencils. “ Curvature of the image 55 
 is therefore a totally different thing from “ spherical aberration, 55 
 and the terms must not be confounded. 
 
 There may be considerable curvature of the image when there is 
 no spherical aberration, and vice versa. 
 
 As photographs are generally taken on a flat and not a curved 
 surface, curvature of the image is a very serious evil in a photographic 
 lens. The mode of remedying it by increasing the radius of the field 
 of these instruments, at the expense of other good qualities, is fully 
 described in the article “ Lens. 55 See “ Lens. 55 
 
 Cyanide of Hydrogen. An acid resembling the hydrochloric, 
 hydriodic and other acids which are called hydracids, because they 
 contain hydrogen and not oxygen. The hydrogen, however, is not 
 the acidifying principle, but the other element : they ought therefore 
 to be called chlorhydric, iodhydric acid, See. 
 
 Cyanide of Mercury. Hg. Cy. This is the usual source from 
 which cyanogen is procured. The attraction of mercury for cyano- 
 gen is so strong that peroxide of mercury decomposes nearly all the 
 metallic cyanides, not excepting that of potassium. 
 
 Cyanide of Potassium. K Cy. The principal employment 
 of this salt in photography is in forming a small proportion of 
 cyanide of silver with the iodide, and in fixing. The former of these 
 uses is not to be commended. As met with in commerce, it often 
 
150 
 
 CYA 
 
 contains half its weight of carbonate of potassa, which is detected by 
 the effervescence produced when an acid is added to its solution : 
 from the usual mode of its preparation, it often also contains cyanate 
 of potass, but not in very large quantity, and this is not injurious. 
 It is deliquescent, and especially so when contaminated with carbonate, 
 but it is not soluble in cold alcohol. By dissolving it in boiling 
 alcohol the impurities are removed, and on cooling, pure cyanide of 
 potassium is deposited. When dissolved in water it may be crys- 
 tallized in cubes. It is as poisonous as prussic acid (hydrocyanic 
 acid) itself : the best antidote, and one very convenient to the pho- 
 tographer, is protosulphate of iron. It is used from 2 to 20 grains 
 to the ounce of water in fixing photographs, according to the purity 
 of the article. Its properties for this purpose differ from those of 
 hyposulphite of soda, and it is more energetic. Like “ hypo ” it 
 forms double salts with the chloride and other compounds of silver, 
 which contain one equivalent of cyanide of silver and one of cyanide 
 of potassium : they are not decomposed by water as the double salts 
 produced by other fixing agents are, so that no apprehension need be 
 entertained of precipitating cyanide of silver in the washing : and it 
 dissolves much more silver on this account than weak hypo will do. 
 It also acts more quickly ; being weak, it is soon saturated, and is 
 more quickly and completely removed by washing, so that very little 
 comparative washing is required. It is never found to crystallize in 
 the film, and but very few instances have occurred in which the 
 washing has been so careless as to leave enough in the film to react 
 on the picture. Cyanide of potassium has also an affinity for 
 oxygen, and has therefore considerable reducing powers, so that the 
 oxides of copper and other metals thrown into it in the fused state, 
 are presently brought to the state of pure metal, and the cyanide 
 becomes cyanate. This reducing power is also seen in fixing collo- 
 dion pictures, which have a more metallic lustre when fixed with it 
 than when the “hypo” salt is used. All these are important advan- 
 tages, but it is not adapted to fixing pictures on albumen, paper, and 
 negative collodion, from the power it has of dissolving the organic 
 basis, and reduced silver in combination with it, as well as the un- 
 altered chlorides and other salts. Wffien it is used to fix pictures 
 on the very active negative collodion, which has been mentioned in 
 the article on collodion, it becomes quite red with the organic matter 
 and silver which it removes from the plate, and the picture becomes 
 faint and yellow. The same thing, precisely, occurs when it is used 
 to fix paper impressions, and therefore it is not adapted to these 
 organic pictures. When a collodion negative has density, and to 
 spare, or when the image is blue and metallic, negatives may be fixed 
 
CYA 
 
 151 
 
 with this salt without danger. The solution of cyanide of potassium 
 deteriorates very slowly when exposed to air, by the gradual produc- 
 tion of carbonate of potassa. 
 
 Cyanide of Silver, Ag. Cy. A white salt produced by 
 precipitating nitrate of silver with cyanide of potassium. It be- 
 haves like the chloride* but is less sensitive to light, and its use 
 in photography has not been found practically advantageous. 
 
 Cyanogen. C 2 N, This is not an elementary substance, but a 
 compound of two atoms of carbon and one of nitrogen. It behaves, 
 however, precisely like an element, and resembles the group of which 
 chlorine is the most characteristic specimen. It thus unites with 
 hydrogen to form an acid, and with the metals to form cyanides. Its 
 affinity for the less oxidable metals is much greater than that of 
 chlorine. 
 
 Cyanogen Soap. Soap containing cyanide of potassium. 
 
 Cyanotype Processes. These are certain processes, discovered 
 by Sir John Herschel, in which cyanogen is employed. 
 
 1$£ Process. A piece of paper is first washed with a solution of 
 ferrid-cyanide of potassium, (red prussiate of potass,) and dried. In 
 this state it is of a yellowish colour and highly sensitive to light. By 
 sufficient exposure, either in the camera, or pressure frame under a 
 negative, the salt becomes converted into prussian blue in those parts 
 where light has acted. To fix the picture, the paper must first be 
 soaked in water, then in a weak solution of carbonate of soda, and dried. 
 
 Before washing, the shadows of the photograph are of a lavender 
 tint on a yellow ground, but after washing, of a deep blue tint on a 
 white ground. Prussian blue is not a permanent substance ; perhaps 
 immersing the papers in tannic acid might blacken the colour and 
 increase the permanence of the precipitate. 
 
 The action of light in this process is probably to convert the 
 ferrid-cyanide of potassium into the ferro-cyanide of potassium 
 (yellow prussiate of potass) and form prussian blue. 
 
 %nd Process. A sheet of paper is first washed with ammonio- 
 citrate of iron, and dried. It is then of a yellowish colour, and sensi- 
 tive to light. After sufficient exposure in the camera, or pressure frame, 
 i. e., until an image is faintly perceptible, the paper is brushed over 
 with a solution of ferro-cyanide of potassium (yellow prussiate of 
 potass). This brings out the picture of a deep blue tint, which 
 becomes almost black if the paper is allowed to get dry. 
 
152 
 
 DAG 
 
 The photograph, in this state, is not permanent. It darkens all 
 over in the dark by exposure to air. If washed with ammonia, or 
 carbonate of ammonia, it is for a few moments obliterated, but soon 
 reappears with reversed lights and shades. In this state it is fixed, 
 and after being thoroughly washed in water, the shadows are pure 
 Prussian blue, which deepens by keeping. By this process negatives 
 may be multiplied ; but it is so uncertain in its results, as to be of 
 little or no practical value, at present. 
 
 The rationale of the process appears to be that the ammonio-citrate 
 of iron, which is a persalt of iron, is reduced by fight to a salt of the 
 protoxide. This reacts on the ferro-cyanic salt, and produces the 
 blue precipitate. 
 
 3 rd P?'ocess. Brush over a sheet of paper a mixed solution of 
 ammonio-citrate of iron and ferrid-cyanide of potassium, contain- 
 ing about equal proportions of these salts. Dry it in the dark. 
 Tt is sensitive to fight. After exposure, immerse it in water and dry 
 it, when a blue picture will be produced. If this be immersed in a 
 weak solution of proto-nitrate of mercury, it is quickly bleached and 
 destroyed. The mercury salt being then thoroughly washed out of 
 the paper, the picture may be reproduced by ironing it with a very 
 hot iron. The colour is then brown, which fades gradually in the 
 dark, but may be restored at any time by ironing the paper as 
 before. 
 
 These cyanotype processes may be greatly varied by substituting 
 for the ferro and ferrid-cyanide of potassium the corresponding 
 cyanogen compounds of many other metals. A wide field is open for 
 experiments in this direction. 
 
 Daguerreotype Process. By this process a direct positive 
 is obtained upon a silvered metal plate. It was first published by 
 M. Daguerre, in Paris, in January, 1839. 
 
 The picture is produced by means of the impalpable fumes of 
 iodine, bromine, and mercury, acting on a silvered plate which has 
 been polished to the utmost. The photo-chemical reactions con- 
 sequently occur in an inappreciably small quantity of material, 
 which is spread on the photographic tablet with a uniformity not 
 to be obtained by any other means. Hence it follows that a fine 
 daguerreotype exhibits microscopic perfection of detail, and perfect 
 gradation of shades. Other processes, when compared with this, 
 are coarse and imperfect in the results, and of all known methods of 
 representing a natural object, the Daguerreotype process is the most 
 perfect and scientific. 
 
 In a fine daguerreotype, the utmost delicacy of fine is combined 
 
DAG 
 
 153 
 
 with perfect gradation of shade ; the details of the deepest shadows 
 are as legible as those in full light ; and the most elaborate finish is 
 blended with exquisite softness of effect. In tone, nothing can be 
 more beautiful than the varied hues of gold and purple which the 
 image presents when viewed in different positions in a strong light. 
 
 Unfortunately, however, the very circumstance which gives such 
 perfection to the result, viz., the polish of the plate, brings with it 
 certain objections in an artistic point of view ; and these objections 
 are considered by many to be so serious that they are willing to 
 rest content with an inferior photograph upon a different tablet. 
 Hence the present popularity of the glass and paper processes. But 
 the accomplished photographer, who sees in every process its own 
 peculiar merits, will hardly rest satisfied with a sketchy production, 
 when called on to exhibit the utmost perfection of his art. We 
 would therefore earnestly call the attention of our readers to this 
 elegant and beautiful process, at present so undeservedly cast into 
 the shade by others of less merit. 
 
 The manipulation of the daguerreotype process is extremely easy 
 and certain. It involves the following five operations, viz. : 
 
 1st. — To clean and polish the plate. 
 
 2nd. — To render it sensitive. 
 
 3rd. — To expose it in the camera. 
 
 4th. — To develop the image. 
 
 5th. — To fix the image. 
 
 The finished picture is placed in a frame or passe-partout, behind 
 a glass. The image is permanent, and cannot be rubbed out by the 
 finger ; nothing but a chemical solvent will obliterate it. 
 
 The silvered plates generally employed in this process are manu- 
 factured in Trance, England, and America. The English plates 
 are the best — but they are dear. The amateur had better make his 
 first experiments on the American plates of Scovill and Co., or on 
 the cheaper kinds of Trench plates, of which there are many 
 varieties. He must choose them as free from scratches and specks 
 as possible, as these defects cannot be remedied. The plates are 
 made by laying a thin sheet of silver foil upon a copper plate, and 
 then passing it between rollers, by which means the two metals are 
 made to adhere perfectly. The silver should contain no alloy. 
 When the same plate has been used several times, the silver wears 
 off, and the plate should then be re-silvered by the electrotype 
 process ; but all scratches and specks are faithfully copied, so that 
 this process does not afford a remedy for, these defects. A scratched 
 plate is useless, except for experiments. The plates may be cut to 
 any shape by a strong pair of scissors. 
 
154 
 
 DAG 
 
 In what follows, the plate is assumed to be the quarter size. 
 
 1st Operation. — To Clean and Polish the Plate . — If necessary, 
 cut a little bit off each corner, and bend the edges in a plate 
 bender. 
 
 To clean the plate, procure some Canton flannel, and a box of 
 prepared rotten stone with a muslin strainer. In a pint bottle mix 
 one part nitric acid and twenty parts distilled water. 
 
 If the plate has been used before, and has been submitted to the 
 gilding solution, the image must be removed by a liquid sold under 
 the name of “ Gilding Dissolvent the directions for using which 
 will be found on the label of the bottle. 
 
 If the plate has by accident been in contact with mercury, it must 
 be heated over a spirit lamp until the whole of the mercury has 
 been volatilized and expelled. 
 
 Then proceed to clean it in the following manner : — 
 
 Cut the Canton flannel into pieces about two inches square. Lay 
 one of them on the plate, and pour on the back of it a few drops of 
 dilute nitric acid ; rub the damped flannel all over the plate, going 
 well up to the corners. Then powder a little rotten stone on the 
 plate, and rub it all over with the bit of flannel, with a circular 
 motion, until it is clean . Beware of scratching the plate. By 
 scratches are not meant the fine circular marks left by the rotten 
 stone, and which are removed on the butf, but the deep cuts made 
 by a crystal of sand or dust on the flannel. When the plate is 
 clean, it presents the grey appearance of unpolished silver, and is 
 free from stains or marks. When you have cleaned the face of the 
 plate, clean the back with the same piece of flannel, and then with a 
 fresh piece of flannel rub the face again until it is dry. The clean 
 plate must not be touched by the fingers. 
 
 To polish the plate, procure some prepared charcoal and a couple 
 of buffs. Tie up about an ounce of the charcoal in a piece of 
 cambric, and keep it in a box with a lid, in order that no dust may 
 stick to it. The buffs are made in the following manner. Procure 
 two pieces of wood 16 inches long, 6 inches wide, and 1 inch 
 thick. Screw a strong handle- on the back, at the end of each. 
 Cover one with fine velveteen, and the other with cotton velvet. 
 The cord of the velveteen is to be put across the buff. When not 
 in use keep them face to faGe. 
 
 Powder some charcoal all over each buff with the dabber, and 
 rub them together. Then, taking the velveteen buff in the left hand, 
 and resting the end of it on a table, lay the clean plate face down- 
 wards on it, and with the fingers of the right hand on the back of 
 the plate, rub it up and down the buff vigorously, in every direction, 
 
DAG- 
 
 155 
 
 sideways, longways, diagonally, until you have polished it, and re- 
 moved the circular marks of the rotten stone. Change continually 
 the position of your fingers on the back of the plate, and in order 
 to prevent them from slipping off on to the buff, breathe on them 
 occasionally, or wear a leather glove. If they slip on to the buff, 
 immediately powder some charcoal upon the spot. 
 
 When the plate is thoroughly polished it looks black, and not 
 grey. Polish well up to all the corners and edges, and then put it 
 away into the plate box. Clean and polish enough plates for the 
 day’s use, before proceeding to manipulate. A plate may be cleaned 
 and polished in two or three minutes in the manner described, but 
 it is hard work, and may be handed over to an intelligent assis- 
 tant. The remaining processes are extremely neat and elegant ; 
 much more so than the collodion or paper manipulation. 
 
 2 nd Operation. — To render the Plate Sensitive . — Powder some 
 charcoal upon the velvet buff, and rub the polished plate lightly up 
 and down for half a minute. The fine lines left by the buff across 
 the plate, must be horizontal, when you are looking at the picture ; 
 remember this when giving the last rub to the plate. 
 
 The plate is now ready to receive the sensitive coating. 
 
 Procure a double coating box, containing two compartments, one 
 for iodine, the other for bromide of lime, and furnished with re- 
 flectors. See that the sliders fit air-tight over the glass pans. Put 
 an ounce of iodine into one pan, and four ounces of bromide of lime 
 into the other. Spread them evenly over the bottom, and lay some 
 cotton wool thinly divided over the iodine, in order that the fumes 
 may be filtered, as it were, and rise more equally. 
 
 Place the coating box between yourself and a small window, 
 shaded by a white blind, and open the back shutters of the coating- 
 box so that a feeble white light may enter, and be reflected from the 
 mirrors. 
 
 Place the plate face downwards over the iodine, draw out the 
 glass slider, and observe by means of the mirrors the changes of 
 colour which the plate assumes. These occur in the following 
 order, viz., pale yellow, deep yellow, rose colour, steel colour, the 
 original colour of the plate ; after which the colours recur in the 
 same order as before. 
 
 Leave the plate over the iodine until it assumes the deep yellow 
 colour bordering on rose. This may occupy from one to three 
 minutes, according to circumstances. Then pass it on to the 
 bromine. Leave it over the bromine until it assumes a blue steel 
 colour. This may occupy from twenty seconds to tw^o minutes, 
 according to circumstances. Then shut the back shutters of the 
 
156 
 
 DAG 
 
 coating box, and draw a yellow blind, formed of three layers of 
 yellow calico, before the window. Return the plate to the iodine, 
 and let it remain from ten to twenty-five seconds, according to cir- 
 cumstances. 
 
 The white light which falls on the plate while you are observing 
 the colour, produces an effect, for the plate is certainly sensitive at 
 that time, but the second exposure to iodine obliterates that effect, 
 and renders it harmless. If therefore any accident occurs in taking 
 a picture, even should it be exposed to broad daylight in its sensi- 
 tive state, the effect of the light may be obliterated by exposing it 
 once more for a few seconds to iodine. The plate need not be 
 cleaned, polished, and sensitized again. 
 
 The plate is now ready for the camera, and the sooner the picture 
 is taken the better. It may, however, be kept in a sensitive state 
 for some hours, when proper precautions are taken. 
 
 3 rd Operation. — To Expose in the Camera . — The exposure in the 
 camera is effected in the usual way. The time of exposure is a 
 matter which can only be determined by trial. The details in the 
 deepest shadows may always be obtained by sufficient exposure ; it 
 is a mere question of time ; but the lights become blue and solarized 
 by over exposure. It is difficult, therefore, to include successfully 
 very violent contrasts of light and shade, and this should not be 
 attempted as a rule, for the great charm of this process consists in 
 the beauty of the half tones. In landscapes, however, the blue 
 solarization of the sky is an improvement to the picture, and the 
 blue tint generally softens down to a warmer tone on the horizon, 
 which gives a very charming effect. 
 
 4 th Operation. — To develop the Image . — -Employ a mercury box 
 of any shape, provided with a thermometer.* Filter about a table- 
 spoonful of mercury into the pan, through a filter with a very small 
 bore, but do not let the filtered mercury fall from a height into the 
 bath, as it oxidizes in passing through the air, and the globules on 
 reuniting become covered with a grey film of oxide, which floats on 
 the top, and prevents the vapour from rising so quickly as if the 
 surface were clean. 
 
 No picture is visible when you remove the plate from the camera. 
 Place it over the mercury, and heat the mercury with a spirit lamp 
 to a temperature of about 140° Fahrenheit. In a few minutes the 
 picture will be developed. The time will depend entirely on the 
 size of the box, a large box requiring more time than a small one. 
 From three minutes to a quarter of an hour are about the extreme 
 limits of the time required. Remove the plate and examine it now 
 and then by a yellow light. Push the development as far as pos- 
 
DAG- 
 
 157 
 
 sible, in order to get out all the details. If you over-develop the 
 picture, or much overheat the mercury, the shadows will be covered 
 with minute specks or pellicles. If the plate has been exposed to 
 too much diffused light, a white film of mercury will be deposited 
 upon it, which will obscure the details, or injure the vigour and inten- 
 sity of the blacks. If you under-develop the picture, the lights 
 will want solidity, and the details of the shadows will be defective. 
 
 When the image has been developed in the mercury box, the 
 plate may be exposed to moderate daylight’; and the fixing process 
 may be deferred till a convenient opportunity. 
 
 5 th Operation —To Fix the Picture . — The fixing process includes 
 two operations. The first consists in removing the coating of 
 iodine and bromine, by means of hyposulphite of soda. The second 
 consists in gilding the image by means of a hot solution of sel d’or, 
 which has the effect not only of improving the tone, but also of 
 cementing the image firmly to the plate, so that it cannot be rubbed 
 off with the finger. 
 
 Make a solution of pure hyposulphite of soda in distilled water ; 
 the strength is not material, say one ounce hypo to ten ounces 
 water. Filter it carefully into a small upright glass bath, like that 
 used for nitrate of silver. Hold this obliquely in the left hand, and 
 let the plate slide in, with its back on the under side of the bath. 
 We recommend this mode of immersing the plate in the hypo, because 
 if the plate is immersed in a horizontal bath, it sometimes happens 
 that two waves of fluid meet in covering it, and this forms a line 
 across the picture, which cannot afterwards be removed. If the 
 back of the plate is dirty, it contaminates the hypo, and occasions 
 stains on the face ; hence the importance of cleaning the back of 
 the plate. The hypo v«ry quickly removes the steel colour of the 
 iodine, and leaves the mercury, which forms the lights of the 
 picture, adhering to the silvered plate, the black polished silver 
 forming the shadows of the picture. When the steel colour has 
 disappeared, pour the fluid into the funnel, let the plate slide out 
 of the bath into your hand (taking care that the face of the picture 
 does not touch the glass), then holding it horizontally, pour upon it 
 filtered rain-water, from a bottle having a cork into which a small 
 glass tube is inserted. This washing removes the last traces of 
 hyposulphite of soda. 
 
 In this state, the picture may be easily wiped off the plate. 
 
 The next operation consists in gilding and fixing the .picture. 
 
 Place the plate (still wet) on a levelling stand furnished with 
 screws. Then pour upon it the following solution of sel d’or : — 
 
 20 ozs. distilled water, 
 
 1 gramme (15 grains) sel d’or. 
 
158 
 
 DAG 
 
 Pour the solution into a small funnel having a tuft of cotton 
 wool in the neck, and let it filter drop by drop upon the plate. Cover 
 the plate with as much fluid as it will hold. 
 
 Light a spirit lamp with a large wick, and holding it underneath 
 the plate, heat the fluid to the boiling point. Do not let the flame 
 remain for two seconds on the same spot, but pass it rapidly from 
 place to place, and from corner to corner. Watch the changes of 
 tone which the picture assumes. After a few seconds it darkens 
 slightly, and then begins to clear up. The lights become whiter and 
 the shadows blacker. After a time bubbles* show themselves in the 
 boiling fluid. The toning and fixing has now about reached its maxi- 
 mum. Do not let any bubbles adhere to the plate ; a spot would 
 occur wherever this happened. To prevent this, strike the table 
 every now and then with your left hand, to communicate vibration to 
 the plate, and detach the bubbles. 
 
 When the picture has been sufficiently toned, take it by one cor- 
 ner in a pair of nippers and pour off the fluid. Pour on it distilled 
 or filtered rain water out of a bottle as before, and then dry it by 
 applying a spirit lamp to the back, in the following manner : — 
 
 Hold it in the nippers, inclined at an angle of about 45° to the 
 floor ; begin with drying the upper corner, and proceed gradually 
 downwards. As the fluid dries off, a line of wet gradually recedes 
 downwards before the lamp ; follow this up by blowing on the plate, 
 and remove the last drop from the lower corner by a piece of blot- 
 ting paper. 
 
 If the amateur finds a difficulty in drying the plate, he may let it 
 dry spontaneously, in a vertical position, resting on blotting paper. 
 
 The picture is now finished. It cannot be rubbed off with the 
 finger ; it is fast and permanent. Put it at once in the passe-par- 
 tout. But before sealing it up, blow off any dust there may be upon 
 it, by means of an indian rubber syringe, kept for the purpose. 
 
 It sometimes happens that the plate on drying becomes covered 
 with minute black spots. These may be removed by a very simple 
 method. Immerse the plate in water, put it on the levelling stand, 
 and pour on it a dilute solution of cyanide of potassium. This will 
 remove the spots. Wash it in water, and dry it as before. Some 
 operators fix with cyanide instead of hypo. 
 
 The room in which the plate is sensitized should not be colder 
 than that in which the picture is taken, or than the external air when 
 taking a view ; for if a cold plate be taken into a warm situation 
 the dew condenses upon it, and although a picture may be obtained 
 in the mercury box, (and apparently a good picture,) it is neverthe- 
 less impossible to fix or tone that picture properly, and it assumes a 
 greenish tint, and is easily rubbed off the plate. 
 
DAM 
 
 159 
 
 In order fully to appreciate the beauty of a daguerreotype, it 
 should be viewed in sunshine with your back to the light. The 
 ultimate tint depends in great measure on the observance of the pro- 
 portions of iodine and bromine in the sensitive coating. 
 
 A daguerreotype is permanent ; the picture cannot be rubbed off 
 the plate ; the plate cannot be broken by accident ; the picture will 
 bear microscopic investigation. Other processes have their merits, 
 but this is the triumph of the photographic art, and a boon to 
 science. 
 
 The theory of this process is so exceedingly obscure and uncer- 
 tain that at present any attempt at explanation of it must involve 
 much that is hypothetical. 
 
 The sensitive film is supposed to be at first in an amorphous state, 
 but to be crystallized and roughened by the action of light. The 
 mercurial vapour adheres to this roughened surface and forms the 
 lights of the picture by amalgamating with the silver. The iodine 
 and bromine are removed by the hyposulphite of soda. The boiling 
 solution of sel d’or appears to act by elective affinity, in the follow- 
 ing manner : — Sel d’or is a double hyposulphite of gold and soda. 
 An atom of silver of the plate changes places with an atom of gold 
 of the solution. The sel d’or therefore becomes converted into a 
 double hyposulphite of silver and soda, and gold is precipitated. 
 The gold, silver, and mercury form an amalgam which adheres 
 tightly to the plate, assisted perhaps by the expansion and subse- 
 quent contraction of the metal on cooling. This forms the lights of 
 the picture, purple under one aspect, and golden under another. The 
 shadows, where no mercury exists, are blackened and enriched by the 
 precipitation of gold. 
 
 . Such appears to be the theory of this very beautiful process. 
 
 Damaskeening. This is a process for ornamenting the surface of 
 polished iron, steel, or other metals, with various devices in a differ- 
 ent metal either deposited upon, or inlaid in the surface to be orna- 
 mented. It has been attempted lately in Trance to employ photo- 
 graphy in this process, but as yet the results obtained are too imperfect 
 to render any account of the experiments necessary or desirable in this 
 place. Still, this application of photography may one day be suffi- 
 ciently perfected to open an important branch of industry. 
 
 Dammak. A white resin brought from India, where it is used 
 for various economical purposes. It is the produce of the Finns 
 dammar a or Sliorea robusta , and contains two parts, one soluble 
 in alcohol, the other not. It is soluble in benzole, and makes 
 
160 
 
 DAE 
 
 DEP 
 
 an excellent varnish for positives and negatives on glass. See 
 “ Varnish.” 
 
 Dark Lines of Spectrum. See “ Spectrum.” 
 
 Decoloration. Solutions which have become discoloured by 
 organic matter may be decolorized by mixing with them animal 
 charcoal, or kaolin, shaking up well together, then allowing the kao- 
 lin to settle, and carefully decanting the liquid. The nitrate 
 bath used in printing positives, particularly on albumenized paper, 
 soon acquires a brown tint, which may be removed in this way. 
 Kaolin is the best substance to employ. In the case of charcoal, 
 certain physical conditions are necessary, in order that it may possess 
 decolorizing properties. It should be porous, minutely divided, 
 and of a dull aspect. The decolorizing property is chiefly possessed 
 by charcoal obtained from animal matters, vegetable charcoal having 
 but little efficacy in this respect. There is, however, an important 
 objection to the use of animal charcoal, for it not only withdraws 
 organic colouring and odorous matters from solutions, but has also 
 the property of abstracting certain inorganic salts from their solutions. 
 It should therefore be used with caution. Kaolin is certainly pre- 
 ferable to it. Pipe clay, and fullers’ earth, have also been used as 
 decolorizers ; q. v. 
 
 Deliquescence. When a salt becomes moist by exposure to 
 the air, it is said to “ deliquesce.” 
 
 Depth of Chemical Eocus. Suppose a small direct pencil of 
 white light to be refracted through a single convex lens. Since 
 white light is not homogeneous, but composed of rays of different 
 refrangibilities, the pencil will be decomposed by refraction through 
 the lens into pencils of the various colours of the spectrum, red, 
 orange, yellow, green, blue, indigo, violet ; and each of these 
 pencils will have its particular focus, the red being furthest from 
 and the violet nearest to the lens. Let r, 0, y, y, b , i } v , be these 
 various foci arranged in the order in which they occur. Then, as 
 the red, orange, and yellow rays have little or no chemical action, 
 that action commencing with the green and terminating with certain 
 invisible rays beyond the violet, the space between g and a little 
 beyond v , is called the depth of chemical focus of the lens. 
 In a single lens this depth of chemical focus is so considerable that 
 the lens cannot be said to have any true chemical focus, and a sen- 
 sitive plate may be placed anywhere within a certain space lying 
 
161 
 
 DEV 
 
 between b and v, and equally good (or equally bad) pictures may be 
 obtained. 
 
 When a lens is corrected, as it is called, for chromatic aberra- 
 tion, by combining two lenses of different kinds of glass, two 
 of the foci, y and v, for instance, are United, and the other foci are 
 brought nearer to this combined focus y v. The depth of chemical 
 focus is, therefore, greatly diminished, and the point where the 
 maximum of chemical action takes place is also that of the maxi- 
 mum of luminosity of the image; When three lenses of three 
 different kinds of glass are used, three of the coloured foci may be 
 combined ; and with n lenses n foci. 
 
 When the greatest possible amount of sharpness of a photograph 
 is desired, depth of chemical focus must be considered a defect, but 
 when a tolerable average of definition has to be struck between 
 the images of objects at very different distances, a little depth of 
 chemical focus in the lens may be tolerated, because a picture in 
 moderately good focus all over may be considered better than one 
 which is very good in one part, and very bad in another. In 
 general, however, depth of chemical focus must be considered a 
 defect. A single lens is comparatively useless for photographic 
 purposes, because of its great depth of chemical focus. 
 
 Development. It is somewhat difficult to give an exact defi- 
 nition of this term, as used in photography. If it is defined to be the 
 production of a visible picture from an invisible impression, then 
 the term would not include the case of the printing processes in which 
 a faint visible image produced in the pressure frame is intensified or 
 developed with gallic acid. If, on the other hand, we consider deve- 
 lopment to be merely an intensifying process, then all the various 
 methods of toning would come under the head of development , which 
 would lead to confusion, and be objectionable. Suppose, then, 
 we define development to be the process which immediately follows 
 exposure, and which renders the picture visible in all its details. 
 
 In all the common development processes the image acquires fresh 
 material. In the case of the daguerreotype process that material is 
 mercury ; in the negative processes with the salts of silver, it is silver, 
 or silver combined with organic matter ; in the chrysotype process 
 it is gold ; and so on. But the principle on which development pro- 
 ceeds is different in different processes. This will be understood by 
 a few examples. 
 
 If a design be traced with the finger on a window pane, and we 
 then breathe on the glass, the dew condenses on those parts which 
 have not been touched by the finger. The design is therefore deve- 
 
 M 
 
162 
 
 DEV 
 
 loped by steam, and rendered visible, dew being deposited on some 
 parts of the glass and not on others ; in other words, material is 
 added to some parts and not to others. If an iodized silver plate is 
 exposed in the camera, and then submitted to the vapour of mercury, 
 the vapour adheres to those parts of the plate which have been modi- 
 fied in some way by the action of light, but not to the parts where 
 light has not acted. The lights of the picture are therefore “ brought 
 out ” or “ developed ” by the mercury ; that is to say, the lights 
 acquire fresh material, which renders them visible. 
 
 In the case of a oollodion negative, light affects in some way 
 those parts of the film on which it acts ; a solution of a deoxydizing 
 body, such as proto-sulphate of iron, or pyrogallic acid, is then poured 
 over the plate ; this mixes at once with the free nitrate of silver on 
 the plate, and would at once decompose it and precipitate silver 
 indiscriminately all over the film, were not that effect prevented for a 
 time by acidifying the developing solution with acetic acid. We 
 have then an acidulated mixture of the proto-salt of iron or pyro- 
 gallic acid with nitrate of silver floating upon a film, parts of which 
 have been acted on by light. Now these parts have the property of 
 decomposing the unstable fluid in contact with them, and of attract- 
 ing in this way a precipitate of silver (or of silver combined with 
 organic matter), whioh is thrown down upon them only ; so that a 
 visible picture is produced by the addition of fresh material to the 
 parts where light has acted. 
 
 The same thing happens in the chrysotype process, although in 
 this case the developer is not a deoxydizing but an oxydizing sub- 
 stance. In this process a sheet of paper is first impregnated with 
 a persalt of iron or uranium, (the per-oxalate, or ammonio-citrate of 
 iron, or the nitrate or tartrate of the sesquioxide of uranium,) and 
 then exposed to light. The metallic salt is deoxydized by light, 
 and reduced to a proto-salt, a faint visible image being produced. 
 The picture is then immersed in a solution of chloride of gold, which 
 is an oxydizer, for it parts with oxygen from the water to the reduced 
 iron salt. The liberated hydrogen unites with the chlorine, and 
 metallic gold is precipitated of a dark purple colour. Here then 
 again the picture is produced by the addition of material to the parts 
 where light has acted. 
 
 Should, however, any process be discovered in which a developer 
 acts by withdrawing material from the sensitive film, that would 
 not invalidate the definition we have given of development. 
 
 In the collodion process, both positives and negatives may be pro- 
 duced by development. In the case of a positive the picture is 
 looked at, and the precipitated metal which forms the lights must 
 
DEV 
 
 DEW 
 
 163 
 
 be white. This is effected by acidifying the nitrate bath and deve- 
 loper with nitric acid, which causes pure white silver to be thrown 
 down uncontaminated with organic matter. In the case of a negative 
 the picture is not to be looked at but printed through , and the pre- 
 cipitated material must possess sufficient density to stop the light 
 from passing through too readily. In this case it is found that 
 organic matter must be combined with the silver thrown down. 
 This is accomplished by using an organic acid, such as acetic or 
 citric, instead of an inorganic acid, as nitric. In the negative pro- 
 cess the dark parts of the picture should at first be red, and should 
 then slowly acquire density and pass ultimately to a brown or purple 
 black. This red tint, in the early stage of the development, indi- 
 cates the presence of a reduced organic salt of silver, which acts 
 powerfully in attracting more material to itself. W r hen the picture 
 comes out grey at first instead of red, this indicates that the mate- 
 rial is more metallic and contains less organic matter ; it cannot 
 then be easily intensified to a proper degree. When organic matter 
 is introduced too freely in the positive collodion process the lights of 
 the picture are brown or drab, instead of white, which interferes 
 greatly with its beauty, although in detail and gradation of shade it 
 may leave nothing to be desired. 
 
 Deviation. When a ray of light is refracted or reflected at the 
 surface of any medium, and follows a different course to that which 
 it had before, the angle through which it is turned out of its original 
 path is called its deviation. Eor instance, if QR be a ray of light 
 incident on the surface of a medium at R, and RS its path after 
 refraction or reflection at R, then, if QR be produced to T, the 
 angle TRS is the deviation of the ray. 
 
 The term “deviation” must not be confounded with “aberration.” 
 
 Dew ; Dew Point. — When a glass of iced water is brought into 
 a warm room in summer, dew is deposited upon it, from the condens- 
 ation of the aqueous vapour in the air by the cold glass. Eor the 
 same reason, if a cold glass lens be taken into warm air, it becomes 
 covered with dew, and this interferes with the production of a sharp, 
 clear picture. Sometimes a patch of dew on the middle of a lens 
 will produce a patch of indistinctness and fogginess in the centre of 
 the picture. It sometimes happens also in the paper processes, that 
 when the excited paper is placed in the slide behind a glass, instead 
 of being attached to th s front of it, the cold glass becomes covered 
 with dew on being taken into the warm air, which of course ruins 
 the picture if exposed before the dew has evaporated. In the 
 
 M 2 
 
164 
 
 DEX 
 
 DIA 
 
 daguerreotype process also, if a cold plate is placed in summer over 
 a warm solution of bromine, or if a cold iodized plate is taken into 
 the warm air, dew condenses upon it, and this interferes with the N 
 production of a fine picture. 
 
 The dew point is the temperature at which dew begins to be de- 
 posited ; it varies according to circumstances. The temperature of 
 the lens, at the time of using it, should be above the dew point. 
 
 Dextrine. (Latin, dexter, right). When starch is heated care- 
 fully until vapour rises from it, it becomes soluble both in cold and hot 
 water, and loses its gelatinous character. Starch thus altered in 
 character is called “ dextrine/’ or “ British gum,” or “ torrefied 
 starch.” It has the property, when acted on by polarized light, of 
 turning the plane of polarization to the right ; hence its name. 
 
 Dextrine of a pure form and very white may be made by moist- 
 ening 1000 parts of dry starch with dilute nitric acid, composed of 
 2 parts concentrated acid, and BOO parts of water. The mixture is 
 made into small blocks, which are first dried in the air, and then in 
 a drying stove exposed to hot air, at a temperature of 150°. They 
 are then pulverized, and again dried at a heat not exceeding 230°. 
 
 Another mode of producing dextrine is by the action of diastase. 
 Dextrine is also produced by the protracted action of dilute acids at 
 a high temperature upon starch. 
 
 Dextrine is of a pale buff colour, insoluble in alcohol, and not 
 blued by iodine. It is a valuable substitute for gum, being more 
 flexible and less brittle when dry. It is applied to the back of 
 postage stamps. 
 
 Dextrine and starch have the same chemical composition. 
 
 Diamond Cement; Chinese Cement. Soak isinglass in water 
 until it is soft ; then dissolve it in the smallest possible quantity of 
 proof spirit, with the aid of a gentle heat. In 2 drachms of this 
 dissolve 10 grains of ammonia cum,* and while still liquid, add a 
 solution of 2 drachms of mastic in 3 drachms of rectified spirit ; 
 stir them well together, and put the mixture into small bottles, 
 which are to be kept covered. 
 
 When used, the cement is to be liquefied by putting the bottle 
 into hot water. The oftener it is heated, the better it gets. It 
 resists the action of water. 
 
 Diaphanoscope. (Gr. $ia<pavoQ, transparent ; (tkottecj, I see.) 
 
 * Ammoniacum is made by mixing 8 ounces of carbonate of ammonia with 2 
 drachms of ethereal animal oil. 
 
DIA 
 
 DIP 
 
 165 
 
 A dark box for exhibiting transparent pictures, with or without a 
 lens. 
 
 Diaphragm. (Gr. 8 Lcuppaypa , a partition). A partition with a 
 hole in it, used in various optical instruments. 
 
 Diapositive. A positive intended to be viewed by transmitted 
 light. 
 
 Diathermanous. (Gr. <ha, through ; Oeppr /, heat.) Heat can 
 be transmitted through certain bodies almost without impediment, 
 just as light through glass. These bodies are not necessarily trans- 
 parent, or diaphanous, but are sometimes partially opaque to light. 
 They are said to be “ diathermanous.” 
 
 Eock salt is the most diathermanous substance known ; it trans- 
 mits 92 per cent, of the heat rays, from whatever source they may 
 be emitted. Sulphate of copper, which is transparent to light, is 
 “ athermanous,” or opaque to heat. 
 
 Eock salt is the “ true glass of radiant heat.” It can be cut 
 into lenses and prisms, and used for decomposing heat by double 
 refraction, &c. 
 
 Diffraction. See “ Light.” 
 
 Dimorphous. Substances which exist in two different forms 
 are said to be “ dimorphous.” Solids sometimes change their 
 molecular arrangement, and become dimorphous. The axles of rail- 
 way carriages are made of tough fibrous wrought iron, but by fric- 
 tion, vibration, and changes of temperature, they become crystal- 
 line in structure, and brittle. This is an example of dimorphism. 
 
 Dipper. The holder for the glass plate when immersed in the 
 nitrate bath. It is sometimes made of silver wire, with hooks at 
 the bottom to hold the plate, but more commonly of glass. For 
 this purpose a strip oi plate glass should be used, about one-third 
 the width of the plate, and an inch or two longer than the depth of 
 the bath, and having a strip of glass cemented across it at the 
 bottom to receive the glass plate. The cement generally used is 
 marine glue, but shellac, or sealing wax, answers very well, and if 
 these are not at hand, spirit varnish may be used. The end of the 
 dipper, and also the cross piece, should be heated with a spirit 
 lamp, or before the fire, and the cement applied, the pieces being 
 pressed into close contact while the cement is hot and perfectly fluid. 
 
166 
 
 DIS 
 
 Dispersion. This term is used in optics to denote the separa- 
 tion of a ray of heterogeneous light, by refraction, into its component 
 rays of different refrangibility. 
 
 Different transparent media have different dispersive powers, that 
 is, different powers of widening the angle between the red and violet 
 rays, after the refraction of a ray of white light through a prism of 
 given angle, at a given angle of incidence. In optical formulae the 
 measure of the dispersive power of any medium is a fraction, the 
 terms of which are determined as follows : — 
 
 Let fi v be the refractive index for the violet ray. 
 fx r for the red ray. 
 
 fj. for the ray of mean refrangibility. 
 
 Then, the measure of the dispersive power of the medium is the 
 
 4 
 
 128 
 062 
 052 
 045 
 038 
 034 
 029 
 026 
 022 
 
 In an achromatic lens, formed of two single lenses in contact, the 
 focal lengths of the single lenses are proportional to the dispersive 
 powers of the respective glasses. 
 
 Dispersion, Irrationality of. “ By measuring the spectra 
 in Fraunhofer’s manner, it is found that the distance between any, 
 the same two fixed lines of the spectrum has not a constant ratio 
 to the distance between the extreme fixed lines where different 
 media are used. This circumstance is called the c Irrationality of 
 Dispersion.’ 
 
 “ If refracting media had no irrationality, then, in providing a 
 combination such that two given lines should not be separated, we 
 should at the same time unite lights of all species. But, since the 
 colours are disproportionately dispersed in different media, the other 
 
 fraction 
 
 Pv A*r 
 
 ix — l 
 
 The dispersive power of chromate of lead is 
 „ phosphorus . 
 
 „ oil of cloves . 
 
 „ flint glass . 
 
 „ Canada balsam 
 „ diamond . . 
 
 „ crown glass „ 
 
 „ alcohol 
 
 „ rock crystal . 
 
 „ fluor spar . . 
 
dis 
 
 167 
 
 lines will, in such a case, be very nearly, but not exactly united.” 
 (Griffin’s “Optics/ 5 pages 95 and 100.) 
 
 Dissolving Views * The transparent slides for dissolving views 
 may be printed by photography-, flee “ Transparencies 55 and “ Magic 
 Lantern. 55 
 
 Distortion. When the image formed by a lens on the focussing 
 screen of a camera obscura does not satisfy the conditions imposed 
 by the rules of perspective, it is said to be distorted. 
 
 Distortion of the image in the camela obscura is in general pro- 
 duced by the excentrical incidence of the oblique pencils. Its effect 
 is to render all straight lines which do not pass through the centre 
 of the picture curvilinear, and also to exaggerate the dimensions of 
 certain objects in the picture, with respect to others, in a way which 
 is opposed to the principles of true perspective. We shall explain 
 how distortion arises when a large view lens is used with a dia- 
 phragm in front, as this arrangement offers an illustration of the 
 evil in a very marked degree. 
 
 Suppose P Q E to be a triangle placed in front of a large view 
 lens l m, having a diaphragm in front at H. First, let the lens be 
 removed, and the size of the aperture at H reduced indefinitely. 
 Then, images A, B, C, of the points P, Q, E, will be formed on the 
 focussing screen by producing the lines P H, Q H, E H, until they 
 cut it, and the triangle A B C on the focussing screen will be a 
 correct perspective view of P Q E. 
 
 Now introduce the lens l m . 
 
168 
 
 DOU 
 
 The small oblique excentrical pencil 
 P H, incident at m, is bent out of its 
 course by refraction through the mar- 
 gin of the lens, and comes to a focus 
 at a. Similarly, the pencils from Q, E, 
 have their foci at 5, c. The image given 
 by the lens is, therefore, the triangle 
 a b c. This triangle is smaller than 
 ABC; but, if the distances A a> B b, 
 
 Cc, are proportional to 0 A, O B, O C, 
 the two triangles will be similar figures, B 
 and there will be no distortion. But 
 it happens that the decrements A a , 
 
 BA, C c, are not proportional to the 
 radial lines OA, O B, O C, but increase 
 more rapidly than these do, There is, 
 therefore, distortion of the image, as 
 shown in the figure. 
 
 Double Iodide. By this term is meant “ double iodide of 
 silver and potassium,” — that is, a solution of iodide of silver in 
 iodide of potassium ; a very strong solution of iodide of potassium 
 being capable of dissolving a small quantity of iodide of silver. 
 
 Double iodide is used in the calotype process, and is made thus : — 
 
 Dissolve in one measure 20 grains of nitrate of silver in about an 
 ounce of distilled water (the quantity is not material), and in another 
 measure dissolve 20 grains of iodide of potassium in about the same 
 quantity of distilled water. Mix the solutions. The mixture im- 
 mediately becomes yellow and turbid, and a yellow precipitate, which 
 is iodide of silver, soon subsides. Pour off as much as possible of 
 the liquid (which contains nitrate of potass), then add an ounce of 
 distilled water to the precipitate, and as much iodide of potassium 
 in crystals as will completely dissolve it, and render the solution 
 colourless, but no more. The quantity required will be about 200 
 grains. Shake well after each addition of fresh crystals, and wait 
 till they are dissolved before adding more, proceeding cautiously 
 towards the last. 
 
 Double iodide is rendered turbid by the addition of water, but 
 the turbidity may be removed by adding crystals of iodide of 
 potassium. 
 
 Pouble Eefraction. See “ Light.” 
 
 C 
 
DRA 
 
 DRY 
 
 169 
 
 Dragon’s Blood. A deep red resin brought from the East 
 Indies. It is soluble in ether and alcohol, and also in alkaline solu- 
 tions. It combines with metallic oxides, forming red compounds. 
 The alcoholic solution is much used in lacquers, to which it imparts 
 a red colour. 
 
 Dry Collodion Process. If a collodion plate, excited in the 
 usual way, is allowed to get dry, without having previously washed 
 off the free nitrate of silver, the yellow layer of iodide of silver dis- 
 appears, and the film becomes transparent and insensitive. This 
 arises from the fact that iodide of silver is soluble in a concentrated 
 solution of nitrate of silver; so that, as the moisture evaporates 
 from the film, the free nitrate becomes concentrated, and dissolves 
 the iodide of silver, forming a double salt, which has been called 
 iodo-nitrate of silver. In order, therefore, to use the collodion film 
 in a dry state, it is found necessary in the first place to remove the 
 greater part of the free nitrate of silver by washing ; and another 
 condition is, that the collodion shall adhere tightly to the plate when 
 dry, and not contract, or form blisters when the solutions are 
 applied to it a second time, after having been once dried. 
 
 All these conditions are fulfilled by the simple and excellent pro- 
 cess of Dr. Hill Norris, of Birmingham, which is as follows : — 
 
 The plate is collodionized and excited in the usual way, and then 
 washed with distilled water, until as much as possible of the free 
 nitrate of silver is removed. This done, a solution of gelatine is 
 poured over it, made by dissolving 80 grains of gelatine in 20 
 ounces of distilled water at the boiling point, filtering while hot, 
 and then boiling down to half the quantity, stirring well with a glass 
 rod, and, when cooled down a little, adding 1 \ ounces of alcohol. 
 Before applying this gelatine preservative solution, it should be 
 heated by immersing the bottle containing it in hot water, and 
 poured over the plate hot. The plate is then left to dry spontane- 
 ously, or it may be dried by artificial heat. 
 
 Plates so prepared may be kept sensitive for many months ; Dr. 
 Norris says indefinitely. 
 
 The exposure in the camera must be much longer than in the 
 wet process ; as long, perhaps, as for waxed-paper. 
 
 The development is accomplished thus : — The plate is first im- 
 mersed for five minutes in a dish of distilled water; then the usual 
 mixture of pyrogallic and acetic acid is made in a chemically clean 
 measure, and a few drops of nitrate of silver added to it ; this is 
 poured over the plate, and the picture developed in precisely the 
 same way and as quickly as in the wet process. 
 
170 
 
 DEY 
 
 DYE 
 
 The picture is fixed either with hyposulphite of soda, or cyanide 
 of potassium, in the usual way, and varnished like an ordinary 
 negative. 
 
 When the chemicals are in good order, this process is as certain, 
 and yields as good results, as wet collodion. 
 
 Another excellent dry collodion process, due to Mr. E other gill, is 
 as follows : — 
 
 The excited plate, after removal from the nitrate bath, is washed 
 once with distilled water ; then albumen is poured over it ; it is then 
 washed again. The remaining treatment is the same as Dr. Hill 
 Norris’s. The rationale of this process appears to be that the albu- 
 men forms, with the free nitrate in the film, a minute quantity of 
 albuminate of silver. These plates do not keep so well as Dr. Norris’s, 
 but are more sensitive. The process was tried by him and aban- 
 doned, long before Mr. Eothergill brought it forward. 
 
 In all dry collodion processes it is important to use a powdery non- 
 contractile collodion, made with acids at a temperature not lower than 
 150°, and of the minimum strength that is consistent with a struc- 
 tureless film. 
 
 Drying Oils. These oils are distinguished from greasy or 
 fatty oils, by their property of becoming oxidized when exposed in 
 thin layers to the air, and forming a dry resinous varnish ; an effect 
 which is accelerated by dissolving in them a small quantity of oxide 
 of lead. The principal drying oils are — poppyseed oil, linseed oil, 
 and nut oil. Eatty oils which do not dry become rancid under 
 similar circumstances. Palm oil is an instance of this property. See 
 “ Oils.” 
 
 Dutch Varnish. Used for paper, parchment, &c. Mix, with 
 the aid of heat : — 
 
 Sandarach . . . . .120 parts 
 
 Mastic ..... 120 „ 
 
 Venice turpentine . . . 120 „ 
 
 Amber . . . . . 30 „ 
 
 Linseed oil . . . . 250 „ 
 
 Turpentine . . . . 250 „ 
 
 Dyeing. The principles of dyeing and calico printing may be 
 included shortly in those of photography, from the fact that certain 
 chemicals are capable of becoming mordants by exposure to light. 
 The object of dyeing is to produce a permanent combination between 
 the material to be dyed and the colouring matter. This combination 
 
DYH 
 
 EAU 
 
 17 L 
 
 is effected by means of a mordant, which combines chemically with 
 the stuff to be dyed, and at the same time with the colouring 
 matter, forming, so to speak, a triple combination. Without the 
 mordant, the colouring matter might of course be removed from 
 the fabric by washing it in water, since it would offer a mere case of 
 mixture, and not of chemical combination. 
 
 It is found that bichromate of potass has the property of be- 
 coming a mordant by exposure to light, and this property has already 
 been taken advantage of by calico printers in Paris, to print designs 
 upon textile fabrics by means of photography. If a piece of paper is 
 rubbed over with a little indigo mixed with bichromate of potass, 
 dried in the dark, and exposed under a negative, the parts acted on 
 by light are permanently fixed, while from those not acted on, the 
 colour may be removed by washing the paper in water. 
 
 Processes of this kind offer a new and important field of 
 research, for the want of permanence of ordinary photographs printed 
 with the salts of silver is a very serious evil.t 
 
 The principal dyes are indigo, cochineal, madder, quercitron, log- 
 wood, annatto, lake, prussian blue, various salts of iron, copper, lead, 
 chromium, cobalt, &c., and the principal mordants proto-chloride of 
 tin, alum, sulphate of iron, &c. 
 
 Dynactin ometer. An instrument for measuring the intensity of 
 actinic power, or for comparing the “ quickness” of lenses. See 
 “ Hunt’s Treatise on Photography.” 
 
 Earthenware, Glaze for. As earthenware vessels are much 
 used in photography, the mode of glazing them should not be dis- 
 regarded, as some of the chemicals, (nitrate of silver, for instance,) 
 act on the glaze. 
 
 The common white dishes are glazed with 
 
 White lead . . . . . . 53 parts. 
 
 Cornish stone 16 „ 
 
 Ground flints 36 „ 
 
 Elint glass 4 „ 
 
 Stone ware is glazed with a mixture similar to the above, but 
 containing also red lead. The materials are pulverized, and then 
 mixed into a paste with water ; this is spread upon the vessel to be 
 glazed, which is then submitted to the furnace, which vitrifies the glaze. 
 
 Eau Bromee. Bromine water, formerly used in the daguerreo- 
 
172 
 
 EFF 
 
 ELE 
 
 type process, but now superseded by bromide of lime, because in 
 hot weather the vapour from the bromine water condenses on the 
 cold plate, and interferes with the production of a good impression. 
 
 Efflorescence. When a crystallized salt parts with its water 
 of crystallization by exposure to dry air, and crumbles into a 
 powder, it is said to “effloresce. 5 ’ Sulphate of soda is an instance 
 of this property. 
 
 Eggs. As the albumen obtained from eggs is much used in 
 photography, it becomes important to know something about eggs. 
 
 The albumen contained in eggs has been described in the article 
 “Albumen . 55 The yelk of the hen’s egg contains about 30 per 
 cent, of oil, in which phosphorus is present. New-laid eggs are 
 heavier than water, but gradually become lighter by the evaporation 
 of water through the shell. Of an egg which weighs 1000 grains, 
 106-9 parts constitute the shell, 604-2 the white, and 288*9 the 
 yelk. 
 
 Electrical Images. See “Moser’s Experiments.” 
 
 Electro-Chemical Action of the Solar Rays. Mr. Robert 
 Hunt has, from some experiments described at pages 291, and fol- 
 lowing, of his “ Researches on Light,” arrived at the following con- 
 clusions : — 
 
 1st. — “ That electro-metallic precipitation is prevented by the sun’s 
 
 rays.” 
 
 2nd. — “ That light is not the retarding agent, but the exercise of 
 electrical force is negatived by the direct influence of actinism.” 
 
 These conclusions must be received with caution, as being purely 
 hypothetical, and very probably erroneous. 
 
 Elements, The elements, or elementary bodies, are such as have 
 hitherto resisted all attempts to reduce them into simpler forms of 
 matter. They are fifty-five in number, and will be found arranged 
 in alphabetical order in the Table at the end of the work. 
 
 Elemi. This is a species of resin much used in varnishes. There 
 are several varieties of it. The principal are obtained from the 
 'Canarium balsamiferum of Ceylon, and the Idea icicariba of the 
 Brazils. Elemi is at first soft and viscid, but becomes hard and 
 brittle by cold and age. It is yellow, translucent, and of a peculiar 
 odour resembling fennel. It contains 60 parts per cent, of an acid 
 
ELL 
 
 ENA 
 
 173 
 
 resin soluble in cold alcohol, and 20 parts of an indifferent crystal- 
 lizable resin soluble in hot alcohol. 
 
 Elliotype Process. A process patented by Mr. Robinson Elliot, 
 of South Shields, for enabling artists to obtain an indefinite number 
 of photographic copies of their works. 
 
 A painting is made upon glass in a body colour, more or less trans- 
 parent according to its density, and the thickness with which it is 
 applied. The painting is then treated as a negative, and positives 
 are printed from it in the usual way. 
 
 Emery. A dark coloured mineral, found in abundance near Cape 
 Emeri, in the island of Naxos. It occurs also in the Channel Islands, 
 Poland, Sweden, Persia, &c. According to Mr. Tennant’s analysis, 
 it is composed of alumina 80 parts, silica 3 parts, iron 4 parts. 
 When reduced to powder emery varies in colour from dark grey to 
 black. Examined under the microscope the powder is seen to be 
 composed of two crystals, viz., corundum, and oxide of iron. This 
 powder is so hard as to scratch quartz and many precious stones. 
 It is used in the arts for grinding, polishing, and scouring articles of 
 metal, glass, &c. 
 
 Emery Paper; Emery Cloth. Emery powder maybe spread 
 upon paper or cloth by first applying a coating of thin glue, and then 
 dusting the powder over it from a sieve. There are six degrees of 
 coarseness. To use emery paper, it should be wrapped round a stick 
 and a little oil applied, which makes it cut better. Emery cake is 
 made by mixing emery powder with bees-wax. 
 
 Razor-strop paper is made by mixing the finest emery powder and 
 a little finely powdered glass with paper pulp, and making it into sheets 
 in the ordinary way. This should be glued to a piece of wood, and 
 a little oil applied occasionally. The emery and glass constitute more 
 than half the weight of the paper. 
 
 Empyreuma. The offensive smell produced by the combustion 
 of organic matters, chiefly vegetable, in closed vessels. 
 
 Enamelled Photographs. In the “Comptes Rendus” of June 
 11, 1855, various methods of producing enamelled photographs are 
 described by M. A. Lafon de Camarsac. The following translation of 
 this article is extracted from the Journal of the Photographic Society 
 for September 21st, 1855 
 
 “ I select for grounds either metals or substances used for pottery ; 
 
174 
 
 ENA 
 
 I use verifiable compounds for tracing the image on them ; and I 
 operate both on the images obtained by the use of metallic salts and 
 those furnished by the resins. 
 
 “With pictures produced by the aid of collodion, albumen, gela- 
 tine, and by the ordinary processes of the salts of silver, I develop the 
 image by nitrate of silver until the half tints are overdone and obscured, 
 and the deep shades are covered with a thick deposit presenting the 
 appearance of a bas-relief. The proof is then placed in an enamel- 
 ler’s muffle ; the organic matters vanish under the action of a suitable 
 temperature. The fire cleans the image and restores all its delicacy. 
 
 “ I operate on white grounds, or black, or coloured grounds. On 
 tinted porcelain, coloured glass, or on brown or black enamel, the 
 whites of the image are formed by the deposit of reduced metal, 
 which acquires great brilliancy in the furnace. On white porcelain, or 
 enamel, or on transparent glass, the blacks of the image will be formed 
 by the metallic deposit, which I then treat with solutions of salts of 
 tin, salts of gold, or salts of chromium. In the latter case I obtain 
 various colours, very vigorous when removed from the muffle, pre- 
 senting a peculiar semi-metallic brilliancy. A very thin layer of an 
 appropriate and very fusible flux fixes the image on the ground* as 
 in the case of applying gold or silver to porcelain. On enamel, the 
 fusion of the matrix itself fulfils the same office. 
 
 “With regard to images obtained by the action of light upon salts 
 of chromium, — as soon as the image is cleaned by distilled water I 
 expose it in a muffle to temperature which destroys the gelatine ; 
 the metallic deposit remains alone upon the ground, Salts of silver 
 and of lead laid on this give yellow tints after baking ; salts of tin 
 and of gold produce violets and purples. These colours are developed 
 under a layer of flux which here covers the metallic deposit. The 
 image presents the appearance of a painting on porcelain. 
 
 “ The pictures furnished by resins are treated differently. I com- 
 pound a coating capable of receiving application of a negative, and 
 of being easily rendered sticky after the exposure to light. Solution 
 of bitumen of Judaea in essence of turpentine, with the addition of 
 resin, fulfils this requirement. The exposure to light being finished 
 and the solvent having performed its office, I proceed to the substitu- 
 tion of ceramic colours for this varnish, which must be destroyed by 
 the fire. Metallic oxides and their fluxes, ground very fine and dried, 
 are deposited on the surface of the image, while a gentle and gra- 
 duated heat restores to the coating the adhesive property it had lost 
 in drying. These enamel powders, carried over the entire surface of 
 the picture, follow with great delicacy all the inequalities of the sub- 
 ject, which they partially penetrate, and translate faithfully all its 
 
me 
 
 eng 
 
 175 
 
 depths and delicacies. The piece is then ready for the fire ; the 
 organic matters are destroyed, and the image, formed of indestructible 
 substances, remains fixed by vitrification. 
 
 “ One of the remarkable characters of these images is the aspect of 
 under-enamel ( sous-email ) which they present, which cannot be fur- 
 nished in the same degree of perfection by any other kind of 
 painting. 
 
 “ The heliographic image thus treated may receive any kind of 
 colouring : it may be transformed into gold or silver as well as into 
 blue or purple ; it may even be burnt into porcelain with the furnace 
 colours. 
 
 “ Observing that, in the same image, the light, in tracing the bright 
 parts, left a faithful image of the shadows, and that the entire negative 
 cliche might be transformed into a positive cliche.-r-Y was led to com- 
 bine two opposite and successive impressions of the same image. By 
 taking from one of these impressions all the light tints and from the 
 other all the dark ones, I obtained the image of the lights (defined) 
 by the shades, and of the shades (defined) by the lights, with the 
 infinite variety of tints resulting from the combination. 5 5 
 
 Encaustic Painting. This method of painting, which is of 
 great antiquity, consists in using wax with the colours, in order to 
 give them a gloss, and preserve them from injury. Proceed thus : — 
 
 Melt together equal quantities of white wax and powdered mastic, 
 stirring well. Pour the melted mixture into cold water, which 
 hardens it; then pound it in a mortar. This powder is to be 
 mixed with strong gum arabic, and added to the common water- 
 colours, which are then to be used at once in the ordinary way, 
 diluted with water as occasion may require, and applied with a brush. 
 The pale colours require least, and the dark ones most of the 
 powder to be added to them. The picture must be very highly 
 and carefully finished. It is then to be varnished with a mixture 
 of one part white wax, and two parts turpentine, applied hot, and 
 the superfluity allowed to run off. 
 
 The wood, or cloth, or paper, to be painted upon must first be 
 waxed. 
 
 Engeaving, Heliogeaphic. The processes of heliographic 
 engraving have not yet been brought to sufficient perfection to be 
 worthy of more than a few brief remarks in this place. 
 
 M. Nicephore Niepce availed himself of the property possessed 
 by bitumen of Judaea, of being rendered insoluble in the usual 
 menstrua by the joint action of light and oxygen. A metal plate 
 
176 
 
 ENG 
 
 was coated, in the dark, with a solution of bitumen in a solvent, 
 dried, and exposed to light under an engraving, or in the camera. 
 It was then washed with a solvent, which removed the bitumen 
 where light had not acted, and allowed it to remain where light 
 and oxygen had rendered it insoluble. The bare metal thus ex- 
 posed was then etched with an acid, and an engraving produced. 
 
 This process was followed up by M. Niepce de St. Victor, and 
 some improvements made in it, which will be found described at 
 different times in his communications to the French Academy of 
 Sciences. 
 
 Mr. Eox Talbot, a few years ago, obtained some little success 
 with a rather different, and, we imagine, a better process. He 
 coated a steel plate with a mixture of gelatine and bichromate of 
 potass, and exposed it to light under a negative. Where the light 
 acted on this mixture, it reduced the salt of chromium, and caused 
 it to combine with the gelatine, thereby producing a compound 
 which was insoluble in water. The plate was then washed with 
 water, which removed the unaltered gelatine only. A picture was 
 in this way produced upon the plate, which was etched by a solution 
 of bichloride of platinum, which attacked only those parts where 
 the metal was not protected by the gelatinous compound. 
 
 MM. Salmon and Gamier have suggested a process of helio- 
 graphic engraving different from either of the preceding, and which 
 appears very ingenious. 
 
 “ If a polished plate of brass, previously submitted to the action 
 of iodine vapour, is exposed to diffused light, and then rubbed with 
 wadding, charged with globules of mercury, the following pheno- 
 mena will be observed — the plate will not be amalgamated, the 
 mercury refusing to attach itself wherever the iodine has been 
 influenced by light. If, instead of proceeding thus, the plate is 
 covered in places with an opaque body, and we then try to amal- 
 gamate it as before, it will bp observed that the mercury takes 
 perfectly on the parts where light has not acted, while it refuses to 
 adhere in other places. This property, discovered by MM. Salmon 
 and Gamier, indicates the possibility of reproducing photographic 
 images upon a plate of brass. A negative on glass, or paper, is 
 applied to the iodized brass plate, and left in contact from ten 
 minutes to two hours in diffused light. On mercurializing the plate, 
 the mercury is seen to adhere to those parts which correspond to 
 the blacks of the negative, leaving the other parts intact. If an 
 ink-roller is then passed over, the untouched parts take the ink, 
 while the mercurialized parts do not, so that the picture is black 
 upon a white ground. 
 
ENG 
 
 177 
 
 “ An etching may then be produced by first dissolving out the 
 mercury by means of a solution of nitrate of silver acidified with 
 nitric acid, and then biting in the plate still deeper by acid alone. 
 If, on the other hand, an engraving is wanted for printing in a 
 lithographic press, the plate is immersed for a few minutes in a 
 galvanic bath charged with chloride of iron, so as to deposit a thin 
 layer of metallic iron in the places previously occupied by the mer- 
 cury, and where the brass is now bare, namely, on the lines of the 
 drawing. The utility of this deposit of iron will soon be perceived. 
 The brass plate being removed from the bath, the greasy ink is dis- 
 solved off with spirits of turpentine. The entire plate is then 
 exposed again to the vapour of iodine, and rubbed with wadding 
 bearing globules of mercury ; whence it results that the plate, as 
 before, acquires a white colour, from the amalgam of mercury ; but 
 as this metal does not amalgamate with iron (for mercury is preserved 
 in iron vessels), a mere gentle rubbing of the plate removes it from 
 the places covered with iron, that is to say, from the drawing itself ; 
 so that after the second operation the drawing has the lines covered 
 by a thin layer of iron, while all the rest of the brass plate is coated 
 with mercury. 
 
 “ Things being in this state, the ink roller is passed over the plate ; 
 only the drawing itself takes the ink, while those parts coated with 
 mercury repel it. This is just what was required As many impressions 
 as desired may now be printed, only taking -the precaution to rub the 
 plate afresh with mercury after a certain number of impressions have 
 been' thrown off. If it be wished indeed, the two last operations 
 might be omitted : it would suffice to wet with water the plate 
 once inked with greasy ink, as done by the lithographic printers ; 
 under these circumstances the parts free from the drawing would 
 be isolated by water, whica would prevent the greasy ink from 
 touching them. 
 
 “ Supposing now that instead of a plate to be printed in a litho- 
 graphic press, it is desired to have one that may be printed 
 with letter press, the following will be the mode of procedure : — 
 Taking the plate at the moment when it is about to be immersed 
 in the galvanic bath, a preparation of gold should be substituted for 
 the salt of iron, and a thin layer allowed to deposit over the drawing 
 (it will take the gold because it resists the action of the acids better) ; 
 the plate is then inked and eaten away all round the lines, which will 
 be preserved by the gold, the surrounding copper only being attacked, 
 so that the drawing itself will be found in relief.” 
 
 Engraving Photographs upon Glass and Porcelain. A 
 
 N 
 
178 
 
 ENG 
 
 process of engraving photographs upon glass and porcelain was de- 
 scribed by Mr. Thomas Sims, at a meeting of the Photographic 
 Society, on January 1st, 1857. The following is the account given 
 by him of this process : — 
 
 “ Wishing to offer as much information as possible, without oc- 
 cupying too much space, I think I cannot do better than give the 
 process of engraving photographs on glass, by means of hydro- 
 fluoric acid gas, in the form of notes, which have been penned at 
 various times, generally after making the experiments. Previous to 
 this, however, it is necessary to give a general idea as to the mode by 
 which 1 have taken the kind of photograph most suitable for the pur- 
 pose. First, then, the collodion I use is thin ; it has as much alco- 
 hol as it will bear, adding ether only when it is too gelatinous, 
 iodized with iodide of silver dissolved in an excess of iodide of po- 
 tassium. The glass here is a desideratum, as it is not only necessary 
 to obtain it free from striae and specks, but its component parts will, 
 I believe, occupy the attention of photographers in time to come as 
 much as paper has done for calotype. I have given crown, sheet, 
 and several kinds of plate-glass very fair trials, and as yet I find 
 none to yield so readily to the action of the hydro-fluoric acid gas 
 as British patent plate. 
 
 “ Some idea of the care I have taken to arrive at this conclusion 
 may be perceived, when I state the manner in which I tested the 
 quality. I have cut 9 -inch plates of various kinds into four or six 
 squares, putting each lot carefully apart ; J have then taken one of 
 each kind and marked it ; collodionized and developed each with the 
 same material, and photographed from the same model, such as a 
 chalk bust or engraving, and have applied the hydro-fluoric acid in 
 the same manner to each, and this two or three separate times, and 
 in every case the patent plate showed itself to be most easily acted 
 upon. Previous to putting on the collodion, and in order to cleanse 
 the plate, I wash it in sulphuric acid and water. 
 
 “Bath . — 30 grains of silver to the ounce of water, made in the 
 usual way. 
 
 “ The time in the camera is of consequence : it must be timed to 
 a nicety, and developed well. ( See Notes.) 
 
 “ The developing solution is made with different samples of proto- 
 sulphate of iron, i pound of three or four samples, put together in 
 rather a wide-mouthed bottle, and the crystals covered with distilled 
 water, to which are added 2 drachms of acetic acid. 
 
 “ When a developing solution, fit for use, is required, take li 
 ounce of this saturated solution and put to it 6 ounces of distilled 
 water and 2 drachms of acetic acid. If this does not flow ev enly over, 
 
ENG 
 
 179 
 
 add a drachm of alcohol ; but if it can be done without it is better, 
 as the alcohol is liable to produce grey pictures. 
 
 “ I invariably fix with hyposulphite of soda ; and too much care 
 cannot be taken to get out the iodide of silver (and the same may be 
 said of the hyposulphite of soda), with abundance of water and a 
 wash or two with ammonia and water, and this, again, washed away 
 with water. 
 
 “ The plate must be dried with as much care as a daguerreotype 
 and in the same manner, from the top downwards, evenly and regu- 
 larly : the water must not concentrate itself upon any particular 
 part, so as to dry in patches ; for though the impression may pass 
 through the burning process without showing the stain, it certainly 
 will not stand the test of the hydro-fluoric acid gas, for the stains 
 themselves will be engraved. 
 
 “ It will be found that if the most minute particle of the iodide or 
 hypo remains in the impression after washing, it will turn black 
 quickly while burning ; and if heat be long applied the impression 
 will evaporate altogether, and this at not a very high temperature. 
 This is a fact that has so many times come under my notice, often 
 to my sorrow, that I have been led to conclude, reasoning from 
 analogy, that the hypo must be the great enemy to the durability of 
 paper photographs ; for if a collodion film, which we have so much 
 more power over in the way of cleansing, will yet, after all the 
 washing, retain sufficient quantity of hypo-sulphite to be the des- 
 truction of the photograph, how much more is paper likely to do so ! 
 
 “ Burning the Photograph . — The only requisites necessary for this 
 purpose are, boxes of sheet iron or tin of various sizes, and a stove , 
 the top of which easily becomes slightly incandescent when the fire 
 is lighted in it. The new boxes must be burnt to get rid of any 
 grease in the iron ones, and of the tin and grease in the tin ones. 
 They must be made a quarter of an inch larger than the size of the 
 plate to be burnt in them ; and they must also be rounded slightly 
 at the bottom. If the first precaution be not taken, the box on 
 cooling will contract and crack the glass. The reason for having 
 the box a little rounded at the bottom is, to allow the operator to 
 place the impression face downwards, which will enable him to stove 
 the picture without having a cover on the box (provided the room be 
 free from cold draughts), and to see how the process goes on ; this 
 may be easily done by holding the box with pincers in such a posi- 
 tion as to get the reflected ray to the eye from the back of the im- 
 pression ; for, as the silver becomes reduced it assumes a mirror- 
 like appearance. 
 
 “ TV hen speaking of the reduction of silver, it is associated in most 
 
 n 2 
 
180 
 
 ENG 
 
 minds with the idea of great heat, when the reduction is said to be 
 by heat ; this is liable to mislead : a great heat is not only unneces- 
 sary but objectionable ; a dull red heat is the most required, for a 
 low heat and a longer time does the work more effectually. (See 
 Notes.) 
 
 44 That the silver does become so reduced is very curious ; it shows 
 the metallic deposit to be in a peculiar state, and I imagine that the 
 contraction of the film must take a meehanical part in the opera- 
 tion. 
 
 44 If the operator allows the burning process to go on slowly, (say 
 half-an-hour or more, as it varies with different impressions,) he can- 
 not fail in this part of the process. 
 
 “ When sufficiently burned, push the box containing the picture 
 to a cool part of the stove, and, when cold enough to be taken hold 
 of by the hand, the plate may be removed, and placed in a plate-box. 
 
 44 It is better to engrave the picture as quickly as possible, or a 
 film of oxide will soon damage it if not used. 
 
 “ Materials and Apparatus for the application of Hydro-fluoric 
 Acid Gas : — 
 
 44 Fluor spar, 
 
 44 Common sulphuric acid, 
 
 44 A leaden pan and cover. 
 
 44 Grind the fluor spar into fine powder, and put \ ounce into the 
 leaden pan, upon which pour 2 ounces of common sulphuric acid, 
 and stir with a leaden rod. The size of a pan for a picture 3f X 2 J 
 inches, should be 4 x 3 inches in width and breadth, and 4 inches in 
 depth. 
 
 44 The frames holding the plates over the vapour should be waxed 
 or varnished, otherwise the wood' absorbs the vapour ; and it acts 
 upon the edges of the plate before it receives the vapour from the 
 pan, which should be placed upon a stand high enough to admit a 
 spirit-lamp underneath. The stand holding the pan of hydro- fluoric 
 acid should be placed upon the hob of a kitchen range, when there 
 is a fire and the draught generally good, as this will carry away the 
 fumes, which are suffocating unless the operator is fortunate enough 
 to have a laboratory with all the requisites. 
 
 44 If an ounce or so of water be used with the sulphuric acid, in the 
 pan 'with the fluor spar, the spirit-lamp will not be much required ; 
 but the action is so rapid, that I prefer adding water sometime pre- 
 viously, and letting the mixture become cool before putting in the 
 fluor spar. Using the spirit-lamp, and not adding water gives the 
 operator most control, and is the method I have adopted ; but, in 
 one or two cases, when water was added to raise the temperature, 
 
ENG- 
 
 181 
 
 there was so much energy as to eat half through the glass. But 
 this plan might really prove the best, provided it could be so modi- 
 fied as to be a little more manageable. 
 
 “ When the hydro-fluoric acid is made to act upon the plate, it 
 permeates the transparent coat first, except in the false action 
 (mentioned in the notes) ; and in proportion as the silver, forming 
 the opaque parts of the picture, increases in density from the half- 
 tone to the liigh light, so is the time the acid takes to act ; the high 
 lights being the most opaque, resist the attack longest. When this 
 part yields to the action of the acid, the whole of the picture is 
 loosened from the surface of the glass, and any further action of the 
 hydro-fluoric gas tends only to destroy the engraving which has 
 been made, so that a few seconds will form an engraving, and a few 
 seconds more, if the action be continued, will entirely destroy it, by 
 eating too far into the body of the glass. It will thus be seen how 
 great a nicety is the timing of the plate over the hydro-fluoric gas ; 
 it varies according to the nature of the glass, and the strength of 
 the impression and of the gas, the latter being regulated by the size 
 of the flame of the spirit-lamp, so that, like the timing of a picture 
 in the camera, it would be difficult to state any time nearer than 
 from five seconds to five minutes. 
 
 “ The best plan is to take the plate up after about ten seconds, 
 and if the picture has a uniform grey appearance by transmitted light, 
 and a prismatic appearance by reflected light, there is every reason 
 to suppose it will bear the action no longer. It is now to be washed 
 off with clean cold water ; but should the plate not be sufficiently 
 cool for this, use warm water, and wipe dry. The engraving will 
 be distinctly seen, and if strong will be easily felt ; but if only a 
 surface impression, it will be necessary to hold the glass in the same 
 position as you would a daguerreotype, that the picture may be seen. 
 
 “ It is evident that this paper, so far, applies to direct camera 
 pictures : the lights of the picture being formed by the metallic de- 
 ppsit, the process of engraving being founded on the well-known 
 resistance of silver, as well as other metals, to the action of hydro- 
 fluoric acid on the one part, and the affinity which that acid has 
 for glass on the other. If, therefore, the law holds good, the trans- 
 parent parts of the proof will be bitten out, and the lights of the 
 picture will be untouched, or nearly so. This is the kind of proof 
 required to produce a plastic cast in relief, from which a plate for 
 the press may be obtained by the electrotype process. 
 
 “ To apply this process to porcelain, upon which material a direct 
 picture is wanted as a permanent article, the process is the same, 
 but the requirements differ. 
 
182 
 
 ENG 
 
 “ Some idea of the plan may be obtained when I state the neces- 
 sity which brought it about ; it was the desire to burn-in a photo- 
 graph on enamel or porcelain : but a positive collodion picture will 
 not sink into hard enamel ; it will burn away before the enamel ob- 
 tains heat enough to become soft. It will sink into soft enamel. 
 The result, however, is so like a daguerreotype that I did not think 
 it worth the trouble of continuing the work. 
 
 “ When a collodion photograph is taken from a negative on white 
 porcelain, the metallic deposit forms the dark part of the picture, 
 and the white surface of the porcelain the whites of the picture. 
 
 “ It is impossible to obtain good collodion pictures on unglazed 
 porcelain, and the photograph will not stand the heat, as before 
 mentioned, which is required to sink-in the impression, so that, to 
 make the picture pass through the kiln, it is necessary to go over 
 the photograph with enamel colours. 
 
 “ On engraving this kind of picture on porcelain, I have found, 
 especially with Mr. Minton’s beautiful white tiles, that when the 
 acid had acted too much, the picture still retained its hold of the 
 porcelain and did not wash off as it does after the same action on 
 glass ; and when it was dried, all the glaze was found to be eaten 
 off, but still the lines of the photograph were fixed as fast as ever. 
 This is worthy the attention of persons who are anxious to apply 
 photography to porcelain. 
 
 “ The point of interest, however, for the photographer is the follow- 
 ing : — If the porcelain is acted upon properly, those parts under the 
 metallic lines are protected from the action of the acid, while the other 
 parts are acted upon, so that the picture, after being engraved, is 
 formed by the dull surface of the unglazed porcelain on the one part, 
 and the brightness of the glazed on the other part. This is suffi- 
 cient to convince me, that if we had white tiles ‘ flashed,’ as it is 
 technically termed, with a dark verifiable colour, we should be able 
 to obtain an engraving with hydro-fluoric acid, a white 'picture on a 
 dark ground , the acid eating out the bare parts of the proof, while 
 those protected by the metallic deposit would retain the surface 
 colour. 
 
 NOTES. 
 
 “1. This acid acts most uniformly when there is a large proportion of sulphuric 
 acid covering the bottom of the pan. 
 
 “ There seems a very strong analogy between the working of this gas and the ob- 
 taining a deep sensitive coat, with the iodine on the daguerreotype plate. 
 
 “2. Whenever the hydro-fluoric acid acts through the silver, the impression is only 
 on the surface, and is analogous to a weak collodion positive (if the impression en- 
 graved is a direct positive) and in appearance is very similar to a weak daguerreo- 
 type : as an engraving it is worthless. In the other case, when the operation is sue- 
 
ENG 
 
 183 
 
 cessful, and the silver has been so reduced as to resist the action of the hydro-fluoric 
 acid the picture appears in the form of a negative on the glass ; everything which, 
 previous to engraving, was white will now appear the reverse, and when seen in a 
 proper light, the picture has much the effect of an etching on steel ; and when a hard 
 substance, such as a needle or pin, be passed over it, the lines may be sensibly felt. 
 
 “3. The lines of demarcation, which can now be drawn between these very different 
 results, are at present ill-defined, but are accumulating so fast as to promise that we 
 shall be able to work with as much certainty as in iodizing a daguerreotype plate. 
 
 “ 4. The characteristics of an impression likely to turn out a superficial engraving 
 are these : — 
 
 “ A feeble and grey impression. 
 
 “ A solarized impression. 
 
 “An undeveloped impression. 
 
 “ If the impression be burnt too suddenly or too much, the result is the same. The 
 reason why an overbumed picture is likely to be affected by the hydro-fluoric acid 
 through the silver, or not engraved at all, is probably because the film of collodion be- 
 comes carbonized ; the silver also, under these circumstances, becomes granular and 
 porous, and will rub off with a brush ; the film of carbon will therefore resist the 
 action of the acid as much or more than the granular silver. 
 
 “ 5. When the opaque parts of the impression are firm and solid, and the blacks rich 
 and transparent, without a stain or mottle from the effects of impurities, either left by 
 the chemicals used, or from the want of cleansing in the plate, the operator may de- 
 pend upon a legitimate engraving, the only question being the time of exposure to the 
 action of the hydro-fluoric acid gas ; and in this part of the process nothing but expe- 
 rience will avail, as it depends upon a number of secondary conditions, which the 
 novelty of the work has made impossible hitherto to reduce to rule. 
 
 “6. When the effect of the fluoric acid seemed very sudden, it immediately reminded 
 me of the early days of daguerreotyping, when pure and undiluted bromide and 
 chlorine was attempted, and I added three or four times more common sulphuric acid 
 than was necessary for the evolution of the fluoric acid. 
 
 “ 7. The first portion of hydro-fluoric acid which rises in vapour is seldom or never 
 of any good in the way of engraving ; it destroys the picture without showing any 
 lines of drawing ; the same happens if the mixture is used too long ; the latter may be 
 easily accounted for, the former not so. Dr. Ure, in his ‘ Dictionary of Chemistry,’ 
 suggests it to be silicated fluoric acid, supposing the fluor spar experimented with 
 might contain a portion of silica. This is also a sufficient reason why the most paltry 
 experiment should not be conducted with glass vessels ; glass covers should not be 
 used to the leaden pan, for the same reason. 
 
 “ 8. If the proof becomes oxidized by lying about exposed to the air, before or after 
 burning , it will not resist the action of the fluoric acid. 
 
 “ 9. If, in burning, the plate assumes a dark and charred appearance, it never be- 
 comes white again ; but will be completely destroyed by further burning, and is occa- 
 sioned by some salts which should have been washed out. 
 
 “ 10. The difficulty of obtaining a strong metallic deposit on the lines of the draw- 
 ing, without getting the transparent parts of the picture grey, might suggest itself as 
 a great objection. In answer to this, I have found it quite possible to augment these 
 lines to a great extent, provided the impression is perfect, by the use of bichloride of 
 mercury and ammonia; chloride of gold to precipitate it; another weak wash of 
 bichloride of mercury ; and this acted upon by a weak solution of iodide of potassium 
 and common water, is used after each application to keep the precipitate from the 
 transparent parts of the film. After which a 30-grain solution of nitrate of silver is 
 poured evenly over the plate and washed with water ; the plate is then exposed to the 
 light, when the metallic parts turn intensely black, and the lines become so raised that 
 it will itself yield a plate by the electrotyping process.” 
 
184 
 
 EQU 
 
 Equivalent. On referring to the table of Elementary Sub- 
 stances, the reader will observe that against every substance a 
 certain number is placed, called its “ Equivalent,” or sometimes its 
 “atomic weight.” Eor instance; silver has the equivalent 108, 
 iodine 126, carbon 6, sulphur 16, and so on. The meaning of 
 these numbers we will endeavour to explain. 
 
 It is found by experiment that substances combine with one 
 another only in certain definite proportions by weight ; and, reasoning 
 on this fact, it has been supposed that matter is not indefinitely 
 divisible, but .that bodies are composed of atoms incapable of 
 being divided into smaller fragments, and therefore called “ ultimate 
 atoms,” these ultimate atoms being of different weight in different 
 bodies. On this hypothesis, the equivalent of any elementary body 
 is the weight of an ultimate atom of that body, the unit of weight 
 being of course indefinitely small, but the same for all substances ; 
 and the equivalent of a compound body is the total weight of all the 
 single atoms composing the compound atom. If, then, we say that the 
 equivalent of silver is 108, we mean that an ultimate atom of silver 
 weighs 108 units of weight ; and since in the same scale the 
 equivalent of hydrogen is 1, it appears that the unit of weight in 
 that scale is the weight of an ultimate atom of hydrogen. 
 
 But this explanation of the meaning of the table of equivalents 
 involves a hypothesis which, however reasonable it may appear, may 
 nevertheless be false and is after all but a hypothesis. If then 
 we reject the explanation given, according to this hypothesis, and 
 confine ourselves to the facts of the case, the table of equivalents 
 becomes merely a table of the proportions by weight in which 
 different bodies are found to combine. 
 
 In order to show the utility of this table, take the following 
 examples : — 
 
 1st. In an ounce (480 grains) of chloride of silver, how many 
 grains of silver and how many of chlorine are there ? 
 
 The equivalent of silver is 108, of chlorine 36 ; the equivalent of 
 chloride of silver is therefore 144. If, then, we multiply 480 by 
 108, and divide by 144, we get the number of grains of silver in 
 the ounce of chloride, viz. 360 ; and if we multiply 480 by 36, and 
 divide by 144, we get the number of grains of chlorine, viz. 120. 
 There are, therefore, in an ounce of chloride of silver 360 grains of 
 silver, and 120 grains of chlorine. 
 
 2nd. - Bequired to know how much iodide of potassium will 
 exactly decompose one ounce (480 grains) of nitrate of silver, neither 
 of these salts containing any water of crystallization. 
 
 Nitric acid is composed of 1 part nitrogen and 5 parts oxygen by 
 
ETH 
 
 185 
 
 EQU 
 
 weight. The equivalent of nitrogen is 14, of oxygen 8. The 
 equivalent of nitric acid is therefore 54, and of oxide of silver 
 108 + 8, viz. 116. The equivalent of nitrate of silver is therefore 
 116 + 54, viz. 170. 
 
 Iodide of potassium is a salt containing no water of crystalliza- 
 tion. The equivalent of iodine is 126, and of potassium 40 ; and 
 therefore of iodide of potassium 166. 
 
 In the decomposition of nitrate of silver by iodide of potassium, 
 there are formed nitrate of potass and iodide of silver. If, then, we 
 multiply 480 by 170, and divide by 166, we obtain the exact 
 number of grains of iodide of potassium necessary to effect the 
 decomposition, viz. 4914+. 
 
 A Table of Equivalents of the simple substances will be found at 
 the end of the work. 
 
 Equivalent Eocus. See “ Eocus.” 
 
 Ether. Oxide of ethyle. C 4 H 5 0=Ae. O. 
 
 Alcohol is supposed to be a hydrate of ether, two volumes of 
 alcoholic vapour being composed of one volume of aqueous vapour, 
 and one of the vapour of ether. If, then, alcohol be decomposed, 
 and the elements of water removed, the result is the production of 
 ether. Such is the theory of etherification. 
 
 Ether, or as it is frequently called, “ sulphuric ether,” is an 
 extremely volatile, colourless, combustible, fragrant liquid, obtained 
 by distilling together alcohol and sulphuric acid. It is of great use 
 in photography as a solvent of pyroxyline. 
 
 The manufacture of ether is an extremely dangerous operation, 
 and should not be attempted by the amateur. The mode in which 
 it may be made, either on a large or small scale, with the proper 
 precautions, is as follows : — 
 
 A large flask contains the mixture of alcohol and sulphuric acid. 
 It has a stopper with three holes in it : through one is inserted a 
 thermometer tube, through another the pipe which leads to the 
 condenser, and through the third a tube through which a fresh 
 supply of alcohol is kept up. The flask is placed in a sand bath, 
 and heat applied, which may be regulated as required. 
 
 In using the apparatus, a mixture of 8 parts by weight of con- 
 centrated sulphuric acid, and 5 parts of alcohol S.G\ *834, is put 
 into the flask and heated till it boils and rises to a temperature of 
 300° Fahrenheit. Eresh alcohol is then introduced as required, and 
 this temperature evenly maintained, the liquid in the flask being kept in 
 rapid ebullition. The bulk of the liquid in the flask is in this way kept 
 up for any length of time, and every drop of alcohol introduced is 
 
186 
 
 ETH 
 
 converted into ether and water, the mixed vapours of which go over 
 together through the condenser into the receiver, where the ether 
 floats upon the surface of the water. When the operation is pro- 
 perly conducted, the same quantity of sulphuric acid is sufficient for 
 any amount of alcohol that may be added to it. The limits of the 
 ether-producing temperature are between 260° and 310° : if the boil- 
 ing point should fall below 260° by using too much alcohol or too 
 weak acid, little else than unchanged alcohol distils over ; and if, by 
 using too much sulphuric acid, the boiling point rises above 320°, 
 olefiant gas is generated, besides variable quantities of other 
 impurities. 
 
 Ether will take up one -ninth part of its bulk of water, and will 
 mix with alcohol in any proportion. In order to obtain pure ether, 
 free from both alcohol and water, it is first mixed with about twice 
 its volume of water, and well shaken ; this separates the alcohol, which 
 leaves the ether, and goes to the water. The ether, which floats on 
 the water, is then carefully decanted, and a sufficient quantity of 
 fresh-burned lime added to it : this abstracts the water. The 
 mixture is then distilled with great care, the receiver being sur- 
 rounded with ice and carefully stopped. The first third which 
 distils over is pure ether. 
 
 The S.Gr. of pure ether at 60° Fahrenheit is *724. It boils 
 at 96° Fahrenheit, at the mean pressure of the atmosphere. It 
 is neither acid nor alkaline, has a high refractive power in regard 
 to light, and is a non-conductor of electricity. It is extremely 
 volatile, and its vapour very dense and highly elastic. Ether burns 
 without leaving a residue, and produces carbonic acid and water. 
 It should be kept in well-stoppered bottles, as it turns acid by con- 
 tact with air, from the production, first, of acetic ether, and subse- 
 quently, acetic acid. 
 
 Iodine and bromine are soluble in ether, and gradually react upon 
 and decompose it, producing hydriodic and hydrobromic acids. 
 
 Ammoniacal gas and hydrochloric acid gas are readily absorbed 
 by ether. Potassa and soda act feebly upon it, and give rise, among 
 other products, to acetic and formic acids, which unite to the 
 alkali. Many salts are soluble in ether, and especially the chlorides 
 of gold, platinum, iron, and uranium. 
 
 The fixed and volatile oils, many of the resins, caoutchouc, various 
 forms of extractive, the alkaloids, and some other vegetable prin- 
 ciples, are more or less soluble in ether. 
 
 The chief use of ether in photography is to dissolve pyroxyline 
 and form collodion. 
 
 By adding a variety of different acids to alcohol, and distilling the 
 mixture, corresponding varieties of ether are produced. Hence we 
 
ETH 
 
 EXP 
 
 187 
 
 have nitrous ether, oxalic ether, hydrochloric ether, acetic ether, 
 and so on. As none of these ethers are at present of any use in 
 photography, it would be out of place to describe them in this 
 work. 
 
 A table, indicating the quantity of alcohol contained in ether of 
 variable specific gravity, will be found among the tables at the end. 
 
 Ethyl. Ae. = C 4 H 5 . Ether is the oxide of ethyl, C 4 H 5 0 ; and 
 alcohol the hydrate of ether, C 4 H 5 O, HO. Ethyl was for some 
 time a hypothetical substance, but a substance having the composi- 
 tion C 4 H 5 has lately been obtained by Dr. Erankland, by exposing 
 iodide of ethyl in sealed tubes to the action of metallic zinc, at 320°. 
 On opening the tubes, and allowing the gas to pass into a freezing 
 mixture kept at — 9°, the ethyl condenses to a colourless liquid. 
 Chlorine acts upon it in the light, but not in the dark. 
 
 No compound ethers have yet been produced from ethyl, and at 
 present the ethyl theory must be considered as hypothetical. 
 
 Exposure. The exposure of a sensitive surface to light, either 
 in the camera or pressure frame, is one of the most important 
 features of every photographic process ; and we shall endeavour to 
 describe in this place the effects due both to over and under expo- 
 sure in the principal processes, and as far as possible to account for 
 them satisfactorily. 
 
 The simplest case is that of sun-printing on chloride paper. Here 
 a sensitive surface consisting of chloride of silver, nitrate of silver, 
 and organic matter is exposed to direct light until a visible image of 
 the required strength is obtained. The effect of light on such a 
 surface is to decompose it and produce a dark material composed 
 of subchloride of silver, together with a compound of suboxide of 
 silver and organic matter. When the image thus obtained, and 
 which is of a purple brown tint, is put into the bath of hypo- 
 sulphite of soda, in order to fix it by removing the unaltered chloride 
 of silver, the purple sub chloride in the image is also decomposed 
 into chloride of silver and metallic silver, and the former dissolved 
 out. The fixing bath therefore not only removes the superfluous 
 chloride in the paper, but also enfeebles the dark tint of the shadows 
 of the picture, by changing it from a deep purple to a red or brown. 
 When sufficient allowance is not made for this change which occurs 
 in fixing a print, it is of course “under-exposed” or “under- 
 printed.” When, on the other hand, the exposure is carried too 
 far, the reduced organic compound of silver presents a metallic 
 lustre, or bronzed appearance, and the details in the shadows are 
 
188 
 
 EXP 
 
 obliterated, and buried beneath a crust which is not easily removed 
 in the fixing bath. 
 
 In printing by development on chloride paper, when the print is 
 under-exposed, the details in the lights cannot be brought out until 
 the development has been carried so far as either to stain the paper 
 all over, or bury the details in the shadows. When the print is 
 over-exposed, the details in the lights come out so rapidly in the 
 gallic acid, and darken so quickly, that the development has to be 
 stopped before the blacks have time to acquire sufficient depth and 
 vigour. The picture is therefore red and feeble, and the lights and 
 shades do not exhibit sufficient contrast. 
 
 It will be observed that in printing on chloride paper by direct 
 light, the longer the exposure the darker the picture becomes, up 
 to a certain point, and then the reduced material afterwards assumes 
 a metallic appearance. So also in development printing on chloride 
 paper, the longer the exposure and the longer the development the 
 darker the blacks become, up to a certam point , and then, as in the 
 other case, they assume a metallic appearance. By carrying the 
 process too far, the same effect happens in both cases. And not 
 only so, but this effect happens in every other photographic process 
 with metallic salts. Excessive exposure produces an amount of 
 reduction which defeats its object by exhibiting the metallic or 
 solarized appearance in the case of a positive print, or, in the case 
 of a negative, that metallic condition of the reduced salt which 
 interferes with its property of becoming a centre of attraction for 
 the matter which should accumulate upon it from the developer. 
 
 But in these remarks we are anticipating the cases of the 
 Collodion and Daguerreotype processes, in which iodide of silver 
 takes the place of chloride. In these cases a curious effect occurs 
 through over-exposure, which has been called, we think improperly, 
 the “ reverse action of light.” In the Collodion process the sen- 
 sitiveness of the film is due to the presence of an excess of free 
 nitrate of silver, which it is impossible to remove by any amount of 
 washing with water, because it probably combines with the organic 
 matter of the film. This free nitrate, existing in minute quantity 
 in presence of iodide of silver, is decomposed by light with extra- 
 ordinary rapidity, and when not over-exposed and rendered metallic, 
 it furnishes the proper centres of attraction for the developing pyro- 
 gallo -nitrate. But if rendered metallic by over-exposure, it no 
 longer acts in this way, and therefore no dark material is precipi- 
 tated upon it from the developer. In order, therefore, to gain the 
 greatest intensity in the image, there is a certain amount of exposure 
 which must not be exceeded. 
 
PAD 
 
 189 
 
 In the daguerreotype process over-exposure produces in the 
 iodized surface an effect which is shown by the solarized part look- 
 ing blue, or sometimes quite metallic. In an over-exposed collodion 
 positive a very similar effect of blueness is produced, which shows 
 the analogy existing between the processes. 
 
 It appears, therefore, that by over-exposure not only is more 
 material reduced, but the reduction of the atoms which are first 
 acted on by light is carried to such a stage, that they look metallic 
 instead of black in positive prints, and in negatives cease to become 
 fit centres of attraction for the production of a picture by a de- 
 veloping or intensifying process. 
 
 Fading. It is unfortunately by no means an uncommon thing 
 for photographs to become altered by time, even when preserved 
 with the utmost care. In the case of daguerreotypes, or collodion 
 positives or negatives, or negatives upon paper, this misfortune is, 
 however, comparatively rare; but in the case of paper positives 
 which have been printed by direct exposure to light, and fixed and 
 toned with hypo-baths, fading may unfortunately be said to be the 
 rule, and permanence the exception. 
 
 Experience has sufficiently proved that the black material of a 
 developed argentine photograph on glass or paper is permanent when 
 the fixing agent (hyposulphite of soda or cyanide of potassium) is 
 removed by copious and thorough washing in water many times 
 renewed. The same may be said of the thin white metallic precipi- 
 tate which forms the lights of a collodion positive. It is however 
 necessary, in the case of a collodion positive or negative, to varnish 
 the film, for if this be not done, the pyroxyline is liable to be decom- 
 posed, and give off an oxide of nitrogen which destroys the image. 
 
 The most serious case of fading is therefore that which occurs 
 where the material of the image is a red compound of organic 
 matter and a low oxide of silver, as in a sun-print, or a red 
 developed print stopped in too early a stage of the development, — 
 the picture being then fixed in a hypo-bath, containing sulphurating 
 salts. Positives produced in this way may be considered certain to 
 fade in from two or three months to as many years, no matter how 
 carefully they may be preserved, or how thoroughly washed after 
 their fixation. The faded appearance is produced by the dark brown 
 tints of the shadows turning yellow. 
 
 The chemistry of fading is at present very obscure, but the 
 phenomena may be produced at pleasure by immersing a fixed red 
 sun-print in a weak bath of hydrosulphate of ammonia. The red 
 tones first darken to a brown or purple brown, then to a purple 
 
190 
 
 FAH 
 
 FEB 
 
 black, and ultimately to a yellow, the entire series of changes only 
 occupying a few minutes. If then the hypo-bath contains, as it 
 always does, a sulphurating compound analogous to the sulphide of 
 ammonium, and any of this should remain, as it probably must do, 
 in the pores of the paper, it is easy to understand why the thin red 
 material of the print becomes yellow by time. 
 
 As nothing certain has yet been proved with respect to the com- 
 position of this yellow substance, we will not offer any conjectures 
 with respect to it. 
 
 When sel d’or is used to tone a print, it is more likely to be 
 permanent, because the effect of this mode of toning is to substitute 
 gold for silver in the material of the image. This is done most 
 effectually by toning the print in a bath of sel d’or before fixing it 
 in hypo. When chloride of gold is added to hyposulphite of soda 
 in excess, and a toning and fixing bath made in this way to act 
 together, the deposition of gold takes place simultaneously with the 
 sulphuration of the silver, and the image is less permanent than in 
 the former case. 
 
 But even metallic gold will not entirely resist the action of an 
 alkaline sulphide, for this combines with it, and forms a double sul- 
 phide of gold and the alkali. This is probably why prints toned 
 with gold have sometimes been known to change from a purple tint 
 to reddish yellow, particularly in the finer details of the lights. 
 
 The liability of argentine photographs to fade when sulphur is 
 present in the paper renders it therefore very desirable that a pro- 
 cess of printing in carbon, or some permanent pigment, or dye, 
 should be brought to sufficient perfection to be substituted for the 
 present methods. 
 
 Fahrenheit’s Thermometer. (Greek, Oepfirj, heat, pLETpov, 
 a measure.) In this thermometer the freezing point of water, or 
 temperature of melting snow, is marked-!- 32°, and the boiling point 
 of water in a thin clean metallic vessel, at an atmospheric pressure of 
 30 inches, 212°. The space, therefore, between the freezing and 
 boiling points of water is divided into 180°. In the Centigrade 
 Thermometer this space is divided into 100°, the freezing point being 
 0 and the boiling point 100°. There is a table at the end in which 
 Fahrenheit’ s thermometer and the Centigrade are placed side by side, 
 and the scales compared. 
 
 Fermentation. This is a peculiar metamorphosis brought 
 about in solutions of certain organic substances, such as sugar, by 
 the introduction of a decomposing azotized body, called a “ ferment.” 
 
PER 
 
 191 
 
 The particular case of fermentation with which the photographer is 
 most concerned is that of the conversion of sugar into alcohol, which 
 is called the “ vinous fermentation.’ 5 A solution of sugar in water 
 is permanent, but if a putrescible animal substance, such as albumen, 
 in the proper stage of decay , be added to it, at a proper temperature, 
 the sugar is converted into alcohol, and carbonic acid given off. The 
 formula by which this decomposition is represented is as follows : — 
 C 12 H 13 0 12 =2 (C 4 H.O + H 0) + 4 C 0 2 : i. e. grape sugar =2 
 alcohol + 4 carbonic acid. 
 
 The most convenient ferment to employ is yeast, (q. v.,) in an active 
 state. This sets up the action in the sugar, by which it is first con- 
 verted into grape sugar, and then into alcohol and carbonic acid. 
 The exact mode in which the ferment acts is not clearly understood. 
 
 When the fermentation is completed the liquid becomes clear, and 
 contains the alcohol, which is obtained from it by distillation. The 
 yeast which has been destroyed is converted into a grey amorphous 
 substance, insoluble in water, scarcely azotized, and quite inert. The 
 proper temperature at which fermentation should be conducted is 
 about 75°. 
 
 It has been supposed that the corpuscles composing ferment are 
 living beings, which nourish themselves with sugar and albumen, and 
 secrete alcohol and carbonic acid. 
 
 Ferrotype. A name given by Mr. Hunt to a process in which 
 an argentine photograph is developed with proto-sulphate of iron. 
 The term is no longer in use. 
 
 Eerrid-cyaNogen. Ee. 2, Cy. 6 = Cfdy. This assumed tribasic 
 salt-radical has not been isolated. It is isomeric with ferro-cyanogen, 
 being formed by the coalescence of two atoms of that compound. 
 
 Eerrid-cyanide of Potassium ; or Red Prussiate of Potass. 
 This magnificent salt is produced by passing chlorine through a solu- 
 tion of yellow prussiate of potass, (ferro-cyanide of potassium,) until 
 the liquid ceases to precipitate prussian blue from the joer-salts of 
 iron. It is then filtered and evaporated, and yields right rhombic 
 prismatic crystals of a ruby-red colour, which are anhydrous. 
 
 Eerrid-cyanide of potassium is soluble in about 4 parts of cold 
 water, and nearly insoluble in alcohol. 
 
 This salt forms no precipitate with the j!?er-salts of iron, but is a 
 most delicate test of the proto- salts, with which it forms a blue pre- 
 cipitate, (Turnbull’s blue.) 
 
 Eerrid-cyanide of potassium, or as it is sometimes called Eerri- 
 
192 
 
 EEE 
 
 EIL 
 
 cyanide of potassium, contains one atom of Eerrid-cyanogen and three 
 atoms of potassium. 
 
 Eerro-cyanogen. Ee., Cy. 3 =Cfy. A hypothetical bibasic 
 salt-radical, supposed to contain 1 atom of iron and 3 atoms of 
 cyanogen. 
 
 Eerro-cyanide of Potassium ; or Yellow Prussiate of Potass ; 
 K 2 , Cfy in the anhydrous state, or K 2 Cfy+3 H 0 when in yellow 
 crystals. 
 
 This salt is soluble in 4 parts of cold and 2 parts of hot water ; 
 it is insoluble in alcohol, which throws it down in yellow flakes from 
 its aqueous solution. The crystals are four sided tables derived 
 from a primary octahedron. The salt is not poisonous. When in 
 crystals, the oxygen and hydrogen of the water of crystallization are 
 exactly in such proportions as are required to convert the metals into 
 protoxides and the cyanogen into hydrocyanic acid (prussic acid). 
 
 Eerro-cyanide of potassium is prepared as an article of commerce 
 by putting chips of hoofs, animal horns, woollen rags, greaves, &e. 
 into an iron pot, and burning them at a very high heat with potash, 
 so as to form what is called “ prussiate cake.’ 5 This, when cold, is 
 lixiviated with water, and evaporated. The resulting crystals are an 
 impure ferro- cyanide of potassium. These are purified by being 
 redissolved and recrystallized. The vessels and stirrers used in the 
 operation should be of iron, as they then supply the iron contained 
 in the salt. If not in sufficient quantity, iron filings should be added. 
 
 Eiltration. This is a process for separating a liquid from the 
 insoluble matter which it may contain. The liquid to be filtered is 
 made to pass through a porous substance, such as unsized paper, 
 porous earthenware, cloth, sand, &c. When common blotting paper 
 is used it should first be washed with dilute muriatic acid, in order to 
 remove some lime and iron which it generally contains. Eilter 
 papers are generally cut round, and the sides folded in puckers 
 like a fan. They are then placed in a glass funnel, the diameter 
 of which should be about three-fourths of its height, measured 
 from the neck. The liquid should be poured into the funnel very 
 gently along a glass rod. A filter covered with sediment may 
 be conveniently washed by squirting water against it from a 
 small syringe. Linen or calico should be used for filtering weak 
 alkaline liquids, and flannel or felt-stuff for weak acid ones. These 
 filter bags are made like a fool’ s cap, and have a wooden hoop at the 
 top. Cotton wool, put into the neck of a glass funnel, makes a good 
 
FIX 
 
 FLI 
 
 193 
 
 filter for many purposes. Strong acids and alkalies should be filtered 
 through a layer of pounded glass, quartz, clean sand, or bruised char- 
 coal. 
 
 Sometimes the liquid is made to ascend in the filter by hydro- 
 static pressure. This is often a very good plan. 
 
 Volatile liquids, such as ether, collodion, benzole, &c., should be 
 filtered under an air-tight vessel ; or very rapidly, by exhausting the 
 air from the vessel which is to contain the filtered liquid, and into 
 which the end of the funnel is fitted air-tight. 
 
 Fixing. By this term is meant, in the daguerreotype process, the 
 real fixation of the image to the plate by means of a boiling hot solu- 
 tion of sel d’or. The image can then no longer be rubbed off by 
 the finger. In the ordinary collodion and paper processes the term 
 “ fixing ” is improperly used to denote the removal of the sensitive 
 material from the tablet, when the picture itself is in other respects 
 completed. 
 
 Flame. The combustion of an inflammable vapour mixed with 
 air ; or according to some, “ luminous gaseous matter.” The lumi- 
 nosity of a flame depends chiefly on the presence of particles of solid 
 matter. For instance, the flame of burning hydrogen is intensely 
 hot and very feebly luminous ; but if a little lime be dusted into 
 it, the particles become intensely luminous. In general, it appears 
 that the greater the heat of a flame the less the light , and conversely. 
 If the top of the glass chimney of an oil lamp be contracted there is 
 less escape of smoke, more combustion of solid matter, and the light 
 is increased with a diminution of the oil consumed. In the case of 
 the flame of a candle or spirit lamp, combustion only takes place at 
 the outer surface of the flame and not in the centre ; this may be 
 proved by inserting a tube into the hollow of the flame, when the 
 inflammable vapour will pass up it, and may be lighted at its other ex- 
 tremity. In the Argand lamp the wick is cylindrical, and the inside 
 of the flame is supplied with air. 
 
 Flame can only exist at a very high temperature. If a piece of 
 wire gauze be laid across a flame it conducts away some of the heat, 
 and the combustible vapour, cooled by passing through the the gauze, 
 passes off on the upper side without flame. This is the principle on 
 which Davy’s safety lamp is constructed. 
 
 Flint Glass. The composition of a good flint glass, S.G. 32, 
 is as follows : — 120 parts of fine clear white sand, 40 purified pearl- 
 
 o 
 
194 
 
 FLO 
 
 FLU 
 
 ash, 35 litharge, (oxide of lead, or “ minium,”) 13 nitre, and a small 
 quantity of black oxide of manganese. 
 
 Flint glass is distinguished from crown glass by the absence of 
 colour, and its higher refractive power. The lead, which is absent 
 in crown glass, renders the flint glass more fusible, and increases its 
 refractive power, giving it great brilliancy when cut, but it has some 
 disadvantages, for glass containing lead is softer and more easily 
 scratched ; and it is difficult to obtain it of equal density throughout, 
 and free from wavy marks or striae. 
 
 When borate of lead is added to glass the density and refractive 
 power are raised to the maximum at present known. See “ Glass.” 
 
 Flowers, Coloured Juices or. Some of the coloured juices 
 of flowers have been shown by Sir John Herschel and M. Chevreul 
 to be sensitive to light, but none of them have yet been employed 
 in photography. The reader is referred to Sir John Herschel’s 
 memoir on this subject in the “ Philosophical Transactions,” Part 2, 
 for 1842, for further information. 
 
 Fluorescence. This term has been introduced by Professor 
 Stokes to denote a remarkable property possessed by certain sub- 
 stances with respect to light, and it has been adopted from the fact 
 that fluor spar exhibits the phenomenon in a marked degree. 
 
 Suppose a trough, the sides and ends of which are made of plate 
 glass, to be filled with a solution of sulphate of quinine. A ray 
 of sunshine is admitted through a small hole in a shutter, and 
 passed through a prism so as to be decomposed into rays of the 
 prismatic colours. The trough, with its solution, is then placed 
 so as to receive and transmit the solar spectrum. On looking 
 through the ends of the trough, the luminous and least refrangible 
 rays are seen to be transmitted, while the extreme violet rays are 
 absorbed, and seen to penetrate only to a certain depth in the liquid ; 
 and in addition to this, rays beyond the violet, which were before 
 invisible, are now rendered visible, and appear of a celestial blue 
 colour, penetrating to a certain depth, and then disappearing. 
 
 If a piece of sensitive photographic paper be placed so as to receive 
 the spectrum transmitted through the fluid, it is found that the 
 usual darkening at and beyond the violet end of it is wanting. 
 
 A block of yellow uranium glass possesses a similar property to the 
 solution of sulphate of quinine. So do sesculine, and other substances. 
 
 For further particulars of this curious phenomenon, the reader is 
 referred to Mr. Stokes’s original paper in the “ Philosophical Tran- 
 sactions” for the year 1852. 
 
FLU 
 
 FOC 
 
 195 
 
 It has been said that fluorescent bodies have the property not only 
 of rendering the invisible rays visible, but of imparting chemical 
 action to the luminous rays. This assertion must, however, be 
 received with caution, and will probably turn out to be erroneous. 
 
 Mr. Stokes is of opinion that 44 the phenomena of internal dis- 
 persion oppose fresh difficulties to the supposition of a difference of 
 nature in luminous, chemical, and phosphor ogenic rays, but are per- 
 fectly conformable to the supposition that the production of light, 
 of chemical changes, and of phosphoric excitement, are merely 
 different effects of the same cause.” 
 
 Fluorine. F=7. This is a hypothetical elementary body, 
 which has not yet been isolated. 
 
 Fluoride of Potassium. KF = 59. This salt is sometimes 
 used in photography, but its good effects are very questionable. It 
 is composed probably of one atom of potassium, and one of fluorine. 
 It is deliquescent, and bears intense heat without change. Its 
 solution acts upon glass. 
 
 Fluoride of Silver is a soluble compound which does not 
 crystallize. It fuses when heated, and is reduced by exposure to 
 light. 
 
 Focal Lines. When a small oblique pencil is reflected or 
 refracted at a spherical surface, or refracted at a plane surface, the 
 reflected or refracted pencil does not come to a <c geometrical focus,” 
 or 4 4 least circle of aberration,” but all the rays composing it pass 
 through two straight lines (or elongated figures of 8), situated in 
 planes at right angles to each other, and called 44 primary, and 
 secondary focal lines.” 
 
 For instance, suppose the circle at A, situated in a plane perpen- 
 dicular to that of the paper, to be the base of a small pencil which 
 
 o 2 
 
196 
 
 roc 
 
 has suffered oblique reflection or refraction. Then all the rays 
 composing this pencil will first passthrough the “primary focal 
 line” q v which is perpendicular to the plane of the paper, and after- 
 wards through the “ secondary focal line” q 2 , which is in the plane 
 of the paper. The reason of this it would be out of place to discuss 
 in the present work ; the fact must be taken for granted ; the de- 
 monstration of it will be found in Coddington’s “Treatise on 
 Optics.” If a section be made of the curious solid formed by the 
 pencil between the focal lines, at a point q exactly midway between 
 them, that section will be a circle, and it is the nearest approach 
 that the pencil can have to a focus. This circle is called the “ circle 
 of least confusion.” 
 
 The proportions of the figure are, of necessity, greatly exagge- 
 rated. The distance between the focal lines is in general very small 
 compared with the distance of either of them from A; but the 
 distance q Y y 2 , and also the diameter of the circle of least confusion, 
 increase as the obliquity of the incident pencil increases. 
 
 Tocimeteu. This is an instrument employed by M. Claudet 
 4 for finding the chemical focus of a lens which has not been properly 
 achromatized. 
 
 A number of boards are arranged in a spiral about a horizontal 
 axis, at different distances, and each board is painted white, and 
 inscribed with a black letter. These boards are so placed as to be 
 all visible together from the lens. If, when one of them is in exact 
 visual focus, another is found to come out more distinctly in the 
 photograph, it is evident that the visual and chemical foci do not 
 coincide, and the lens should then be rejected as unfit for use, 
 for it will be seen by referring to the article on Depth of Focus, 
 that a lens whose chemical and visual foci do not coincide cannot 
 give such perfect definition as one in which they do, other qualities 
 being equal. 
 
 Focus. A focus is either “ real” or “ virtual.” A “ real” focus 
 is a point through which an assemblage of rays actually pass. A 
 “virtual” focus one through which their directions if produced 
 forwards or backwards would pass. 
 
 A lens is said to have “ positive focus ” when the focus of a 
 parallel pencil refracted through it is on the same side of it as the 
 origin of light. A concave lens has therefore positive focal length. 
 
 A lens is said to have “ negative focus ” when the focus of a 
 parallel pencil refracted through it is on the opposite side of it to the 
 origin of light. A convex lens has therefore negative focal length. 
 
foc 
 
 FOE 
 
 197 
 
 In the former case the focus of the pencil is “ virtual/’ in the 
 latter case “ real.” 
 
 The principal focus of a lens, whether positive or negative, is the 
 focus of a pencil of parallel rays after refraction through it. 
 
 The equivalent focal length of a combination of lenses having 
 negative focus, is equal to the focal length of a single lens which, 
 when presented to the same objects, gives an image of the same 
 size as the combination. 
 
 Focussing Glass. This is a magnifier used for the purpose of 
 magnifying the image on the ground glass, and enabling the 
 operator to get it into better focus. A good form is that of the 
 Eamsden’s Eye Piece of a telescope, which consists of two equal 
 single plano-convex lenses, placed with their plane sides outwards, 
 and at a distance apart equal to two-thirds of the focal length of 
 either. When in use the image on the ground glass must be in the 
 principal focus (or thereabouts) of the eye-piece or focussing glass. 
 
 It must be remembered that Eamsden’s Eye Piece is not achromatic. 
 
 Fog. Fogging. When a precipitate is thrown down over the # 
 entire plate by the action of the developer, so as to obscure the 
 transparency of the glass when looked through, that precipitate is 
 called “ fog,” and the picture is said to be “fogged.” The prin- 
 cipal causes of fog are — want of sufficient acid in the process, chemi- 
 cals out of order, rough or dirty plates, and diffused light either in 
 the camera or dark room. A frequent cause of fog is the want of a 
 protecting tube in front of the lens, and a diaphragm within the 
 camera. 
 
 Foumic Acid. C 2 H 0 3 = 37. 
 
 This acid was first discovered by Fisher in the red ant, (formica 
 rufa) ; hence its name. 
 
 Formic acid and the alkaline formiates are reducing agents, 
 and reduce the oxides of the noble metals, the acid being oxidized 
 into carbonic acid. 
 
 Formic acid was originally obtained by distilling the ants with 
 water ; but the principal processes now employed for producing it 
 are as follow : — 
 
 1st. 10 parts of starch, 37 of binoxide of manganese, 30 of 
 water, and 30 of sulphuric acid are distilled in a retort (the capacity 
 of which should be at least ten times that of the mixture), until 38 
 parts have passed over. 
 
 2nd. Concentrated formiate of soda and sulphuric acid distilled 
 
198 
 
 FOB 
 
 FUL 
 
 together yield very strong fuming formic acid, containing only 1 
 atom of water, but the sulphuric acid must not be in excess. 
 
 Formic acid is a colourless and slightly fuming liquid of a pungent 
 odour, and acting as a caustic upon the skin. It freezes at 80°, and 
 its S. G. at 60° is 1* 23 5. It boils at 210°, yielding an inflammable 
 vapour, which burns with a blue flame. 
 
 Formiates. The affinity of formic acid for bases far exceeds 
 that of acetic acid. Most of the formiates are soluble in water. 
 
 Formiate of Silver. By mixing concentrated solutions of 
 neutral nitrate of silver and slightly acid formiate of potass, nitrate 
 of potass and formiate of silver are produced. The latter salt is 
 thrown down as a white crystalline precipitate, which soon blackens 
 in the light, and when neutral is converted into metallic silver, 
 carbonic acid, and formic acid. 
 
 Fraunhofer’s Lines in the Spectrum. See “ Spectrum.” 
 
 Freezing Mixtures See “ Table of Freezing Mixtures ” at the 
 end. 
 
 French Cement. Gum-water thickened with powdered starch. 
 A little lemon juice is sometimes added. It keeps good for a long 
 time. 
 
 French Polish. Dissolve with a gentle heat 22 ounces of 
 shellac in 80 ounces of rectified alcohol. 
 
 French polish is mixed with a few drops of oil at the time of 
 application, and applied to the wood with a ball of cotton-wool ; it 
 is then rubbed briskly in the direction of the fibre, and finished, after 
 drying by friction, with tripoli and oil. 
 
 Fuller’s Earth. This is a greenish or yellowish grey powder 
 found in different parts of England. It is used for removing grease 
 spots, and also for decolorizing old nitrate baths, which is effected 
 by filtering them through it. It is composed of silica, 53 parts; 
 alumina, 10 ; red oxide of iron, 9*75 ; magnesia, 125 ; lime, 5, and 
 a trace of potash. 
 
 Fulminating Gold. Subaurate of ammonia. 2NH 3 , Au. 0 3 . 
 
 When liquid ammonia is added to a concentrated solution of 
 chloride of gold diluted with three times its weight of water, a 
 
FUL 
 
 GAL 
 
 199 
 
 yellowish brown precipitate is formed, which, when collected upon a 
 filter, washed with water, and carefully dried at 212°, is fulminating 
 gold. Acids and alkalis have little elfect upon this substance. It 
 is highly dangerous, and explodes, when dry, on the slightest 
 friction. 
 
 Fulminating Silver. 2Ag. 0, Cy. 2 , 0 3 . 
 
 Liebig’s process of preparing this dangerous substance is as 
 follows : — 
 
 Dissolve 1 part of silver in 10 parts of nitric acid, S.G. 1*36 
 to L38. Pour the solution into 20 parts of alcohol, S.G. 
 about ’830, and heat the mixture to ebullition. As soon as it 
 begins to boil, remove the heat and let it cool; it then becomes 
 turbid, and deposits fulminate of silver in the form of brilliant white 
 needles. These must be washed and dried in very small quantities 
 at a time, and with the greatest care. They should then be kept in 
 a wide-mouthed bottle covered with paper. 
 
 This substance explodes with the slightest friction. 
 
 Fusel Oil. Oil of potato-spirit. An impurity contained in 
 the alcohol obtained from potatoes, or beet-root treacle, or corn. It 
 is supposed to be the alcohol of the amylic series, the base of which, 
 arnyle, (C 10 H 11 ), has not, as yet, been isolated. This impurity in 
 alcohol, when introduced into collodion, is believed to be injurious. 
 To avoid it, the alcohol used in photography should be obtained 
 from the juice of the grape. 
 
 Fusel oil is a colourless liquid of a peculiar nauseous odour and 
 acrid taste. It mixes in all proportions with alcohol and ether, 
 but not with water. Its S.G. is ‘812. It boils at 270°, and 
 freezes at 4°. 
 
 Fusible Metal. This metal fuses at the temperature of boiling 
 water. It is composed of — 
 
 Bismuth ... 8 parts. 
 
 Lead . . . . 5 „ 
 
 Tin . . • . 3 „ 
 
 and sometimes 1 part of mercury is added. 
 
 Gall. Ox-gall. The bile of the ox. Bile is a secretion which 
 is separated from the venous blood in the liver. It is supposed to 
 be a saponaceous compound in which an organic acid is combined 
 with soda. Ox-gall is of a dingy green colour, transparent, and 
 viscid ; having a peculiar odour and a nauseously bitter taste. It 
 
200 
 
 GAL 
 
 has a slightly alkaline reaction, and mixes in all proportions with 
 water. 
 
 Ox-gall may be clarified in the following manner : — Take the 
 gall of newly killed oxen, and let it settle for 12 or 15 hours in a 
 basin. Pour the supernatant liquor off the sediment into an eva- 
 porating dish, and boil it until it is somewhat thick. Then, spread 
 it upon a dish, and put it before a fire until it becomes nearly dry. 
 In this state it maybe kept for years in jelly pots covered- with paper. 
 When required for use a piece of it the size of a pea is to be dissolved 
 in a table-spoonful of water. 
 
 Ox-gall may be rendered perfectly colourless in the following man- 
 ner : — To a pint of gall, boiled and skimmed, add one ounce of fine 
 powdered alum, and leave the mixture on the fire till the alum is 
 dissolved. When cold pour it into a bottle, and cork it loosely. 
 Next, treat another pint of gall in exactly the same way, only sub- 
 stituting salt for alum. In about three months these preparations 
 will deposit a thick sediment. Then decant the fluid portion of each, 
 and mix them. A precipitate is immediately formed which takes 
 down the colouring matter, and the supernatant liquid may then be 
 filtered, and is as transparent and colourless as water. 
 
 Clarified ox-gall combines readily with colouring matters or pig- 
 ments, and gives them solidity, either by being mixed with them, or 
 passed over them upon paper. It increases the brilliancy and durability 
 of ultramarine, carmine, green, and in general of all delicate colours, 
 while it contributes to make them spread more evenly upon paper, 
 ivory, &c. When mixed with gum arabic it thickens the colours 
 without making them glisten, and prevents the gum from cracking, 
 and fixes the colours so well that others may be applied over them. 
 Along with lamp black and gum it forms a good imitation of Indian 
 ink. When a coat of ox-gall is put upon drawings made with black 
 lead, or crayons, the lines can no longer be effaced, but may be painted 
 over with a variety of colours previously mixed with the same ox- 
 gall. Miniature painters find great advantage in using it. When 
 passed over ivory it removes the unctuous matter from its surface ; 
 and when ground with the colours makes them spread with the 
 greatest ease, and renders them fast. It serves also for transpa- 
 rencies ; being first passed over the oiled, or waxed, or varnished 
 paper, and allowed to dry. The colours mixed with the gall are 
 then applied, and cannot afterwards be removed by any means. It 
 is useful to the photographer in blackening the skies of waxed-paper 
 negatives, for when mixed with the Indian ink it causes it to flow 
 more readily on the greasy surface of the wax. 
 
 Ox-gall is used for taking out spots of grease and oil. 
 
GAL 
 
 201 
 
 This substance has been described at some length because it is 
 not only very useful to the colourist of photographs, but may also 
 turn out to be of some utility in the new methods of photographic 
 printing in which pigments are fixed to the paper by means of bi- 
 chromate of potass reduced by light. 
 
 Gallates. The salts formed with gallic acid ; which, according 
 to Liebig, is bibasic. 
 
 The basic salts of gallic acid when exposed to air absorb oxygen, 
 and acquire various shades of yellow and brown, till they ultimately 
 blacken, in consequence of the formation of tanno-metanic acid, (C 14 
 H 4 0 7 + *2 H 0). In consequence of this property some of the gall- 
 ates are more energetic developers in photography than gallic acid. 
 Gallate of lead is an instance of this ; for if acetate of lead be added 
 to gallic acid an image may be developed very speedily which would 
 not be brought out by gallic acid alone. 
 
 Gallate of Ammonia. A white crystalline powder obtained 
 by passing gaseous ammonia into an alcoholic solution of gallic acid. 
 It is soluble in water. 
 
 Gallate of Potass. When an alcoholic solution of potass is 
 dropped into an alcoholic solution of gallic acid till green streaks 
 remain on the surface, a white crystalline precipitate falls, which as- 
 sumes a greenish hue by exposure to air, and becomes brown when 
 dissolved in water. 
 
 Gallate of Soda. A white crystalline powder which forms a 
 brown aqueous solution, and is obtained in the same way as gallate 
 of potass. 
 
 Gallate of Iron. When gallic acid is added to cold solutions 
 of the persalts of iron they become dark blue, which colour disap- 
 pears on the application of heat, the peroxide losing oxygen, and car- 
 bonic acid being evolved. 
 
 The gallate of protoxide of iron is a colourless soluble compound, 
 but becomes red, violet, and finally dark blue by absorption of oxygen, 
 still however remaining soluble, till at length it blackens and is pre- 
 cipitated. It is then a gallate of the black oxide of iron, (Fe. 3 0 4 ). 
 It forms the principal part of the colouring matter of writing ink. 
 
 Gallate of Lead. When acetate of lead is added to a hot 
 solution of gallic acid, leaving the latter in excess, a grey crystalline 
 
1 
 
 202 
 
 GAL 
 
 powder falls. When a solution of gallic acid is added to a boiling 
 solution of acetate of lead, the latter being in excess, a yellow 
 crystalline basic gallate falls. 
 
 Collodion negatives may sometimes be fully developed, after a 
 very short exposure, by the following process : — 
 
 First pour over the plate a saturated solution of gallic acid. Bet 
 it remain a minute or so. No image appears. Then pour it off 
 into the measure, and add to it a few drops of a solution of acetate 
 of lead. The mixture becomes milky from the formation of gallate 
 of lead. Now pour it over the plate, and the picture immediately 
 appears, and may be developed in the course of a few minutes to a 
 dense and good negative, adding a few drops of nitrate of silver, if 
 required. 
 
 Gallate of Silver. If gallic acid be added to a solution of 
 nitrate of silver, the mixture soon becomes discoloured, and if left 
 undisturbed for some hours, a dark brown powder is precipitated. 
 This however does not appear to be gallate of silver, and it is ques- 
 tionable whether there is any such salt, for the gallic acid most 
 probably reduces the oxide of silver to suboxide, and becomes 
 itself oxidized. The brown powder is most probably metagallate 
 of silver, but its exact composition has not yet been ascertained. 
 
 If gallic acid be added to ammonia-nitrate of silver (oxide of 
 silver dissolved in ammonia or nitrate of ammonia), the solution is 
 immediately decomposed, and a brown powder precipitated, which, 
 as in the former case, is of unknown composition. 
 
 If an acid be added to the solution of nitrate of silver, the decom- 
 position of the solution is in general retarded for some minutes, or 
 even for many hours, but it eventually takes place, even in the dark. 
 
 Gallic Acid. C 7 H0 3 +2H O. 
 
 This acid was discovered by Scheele in 1 786. It may be obtained 
 as follows : — 
 
 Powdered galls are mixed with water, and the paste exposed for 
 some weeks to the air, at a temperature of from 70° to 75°, adding 
 water occasionally to prevent the paste from drying. The powder 
 swells and becomes mouldy ; and when the magma is exposed to 
 pressure, a quantity of coloured liquor may be squeezed out. The 
 residue or cake is then boiled in water, and the solution filtered 
 while hot. On cooling, it deposits crystals of gallic acid, which 
 may be purified by re-dissolving and boiling with a little animal 
 charcoal. The filtered solution then deposits the gallic acid in white 
 silky crystals. These are of a slightly sour and astringent taste, 
 
GAL 
 
 GAM 
 
 203 
 
 soluble in 100 parts of cold, and three parts of boiling water; 
 readily soluble in alcohol, and sparingly soluble in ether. 
 
 Gallic acid is obtained in the above process by the oxidation of 
 the tannic acid contained in the galls, by exposure to air. It is a 
 feeble acid, and scarcely reddens litmus paper. It is a most im- 
 portant agent in photography, from its deoxidizing property. The 
 products of its oxidation have not yet been thoroughly investigated. 
 
 The aqueous solution of gallic acid becomes mouldy and dis- 
 colour* hby keeping. 
 
 Galich acid gives no precipitate with gelatine. 
 
 Galls. The gall-nut is an excrescence which forms upon the 
 branches and shoots of the Quercus infectoria , being produced by 
 the puncture of the female of the Cynips gallce tinctorice , or gall 
 wasp, which insect deposits its ovum in the puncture, and occasions 
 the excrescence, or gall, within which the larva is developed, and 
 when the insect is perfect it eats its way out. The best galls, 
 known in commerce as black or blue galls, are gathered before the 
 insect has escaped ; the white galls are those from which the insect 
 has departed, and are consequently perforated with a small circular 
 hole. They are of a brownish or dingy yellow colour, but less 
 heavy and astringent than the others. Gall-nuts are chiefly im- 
 ported from Turkey, and have long been used as a source of black 
 dye, and in the manufacture of writing ink. They contain a larger 
 quantity of tannic acid than any other known vegetable product. 
 
 The insect which produces the gall-nut is about a quarter of an 
 inch in length, and when the wings are expanded, nearly an inch in 
 breadth. It is to be found chiefly in Asia Minor and Turkey, and 
 is of a dirty yellowish-brown colour. 
 
 The Quercus infectoria, or gall oak, is found chiefly in Asia 
 Minor. 
 
 Infusion of galls is a chemical antidote in cases of poisoning by 
 opium, or nux vomica. 
 
 Galvanized Iron. Iron dipped into melted zinc, the surface 
 of which is protected from the air by a layer of sal-ammoniac. On 
 removing the iron it is found coated with a thin layer of zinc, which 
 protects it for a considerable time from rusting. 
 
 Gamboge. A gum resin, concreted from the milky juice which 
 exudes from the Gambogia gutta , and Guttrefera vera , trees native 
 in Ceylon and Siam. It consists of about 85 parts of a red resin 
 
204 
 
 GEL 
 
 soluble in alcohol, and 15 parts of gum. It is a valuable pigment, 
 and may be used for painting out the skies of negatives. 
 
 Gelatine. C 13 H 10 0 5 N 2 . This substance is produced by the 
 action of hot water on the membranous tissues of animals. To 
 obtain it, such substances as clippings of hides, hoofs, horns, calves* 
 feet, cow’s heel, sheep’s trotters, pig’s pettitoes, certain membranes, 
 &c., are cleansed in cold water and then boiled. The solution so ob- 
 tained is freed from fat, and any deposit, by skimming and straining, 
 and allowed to form a jelly on cooling. This is called size, and when 
 cut into slices and dried is called glue. 
 
 The purest form of gelatine is isinglass, which is obtained in 
 Eussia from the air bladder and sound of a species of sturgeon. 
 
 Size is sometimes obtained from the waste of vellum, parchment, 
 and some kinds of white leather, and also from bones. It may be 
 rendered inodorous, tasteless, and colourless, by the careful applica- 
 tion of sulphurous acid. It is then called patent gelatine, gr eme- 
 tine, &c. 
 
 Gelatine gradually softens and swells in cold water but does not 
 dissolve without heat. It absorbs three or four times its weight of cold 
 water. 1 part of isinglass dissolved in 100 of hot water gelatinizes on 
 cooling, but in 150 parts remains liquid: the effect, however, varies 
 with the temperature. 
 
 When a solution of gelatine is repeatedly boiled and cooled it loses 
 its power of gelatinizing on cooling, and remains soluble and delique- 
 scent. In this state it has been called by photographers metagelatine. 
 
 Gelatine is insoluble in absolute alcohol, and ether, and also in 
 fixed and volatile oils. When alcohol is added to a warm and strong 
 aqueous solution of gelatine, the gelatine separates as a white viscid 
 substance ; and if a drop of the same solution of gelatine be added 
 to alcohol, ether, or collodion, the gelatine immediately rolls itself 
 up into a white ball, and sinks to the bottom of the bottle. 
 
 Gelatine is soluble in all the dilute acids, differing essentially in 
 this respect from albumen. Of thesfc the acetic solution only gela- 
 tinizes on evaporation. 
 
 The dilute caustic alkalis, and ammonia, do not prevent the gela- 
 tinization of gelatine, but often throw down a portion of phosphate 
 of lime. When gelatine is dissolved in a cold dilute solution of 
 caustic potass, and exactly neutralized with acetic acid, the evapor- 
 ated liquor does not gelatinize on cooling ; it leaves a residue of 
 altered gelatine combined with acetate of potass, which is soluble in 
 alcohol. This substance might in certain cases be added to collodion 
 
GEE 
 
 GLA 
 
 205 
 
 to increase the density of the negative ; or it might be used as a 
 preservative solution to be applied to a sensitive plate. 
 
 Tannin precipitates gelatine from its solution as a dense white 
 curdy precipitate, called tanno-gelatine. It is on this principle that 
 leather is produced by the long soaking of hides in infusion of oak 
 bark. Tannin is a very delicate test of gelatine, for when added to 
 a solution of 1 part of gelatine in 5000 of water a cloudiness is pro- 
 duced. Sulphate of platina produces the same effect, and is the 
 better test of the two, as it does not act in a similar way on albumen, 
 as tannin does, A mixture of salt and alum also forms a white 
 precipitate with gelatine. 
 
 Gelatine is capable of combining with some of the metallic oxides ; 
 for instance, with the oxides of iron, chromium, lead, tin, mercury, 
 silver, gold, and platinum, its combination with silver and gold being 
 effected by means of light, for it does not readily alter the colour of 
 their solutions, if kept in the dark. 
 
 Gelatine has many important uses in photography, and enters 
 largely into the sizing of English photographic papers. 
 
 German Silver. This is an alloy of nickel. The common sort 
 is composed of copper 8, nickle 2, zinc . A better sort has one 
 part more of nickel ; and the best sort, called Electrum , has two parts 
 more of nickel. This last has a tinge of blue like very highly polished 
 silver, and tarnishes less easily than silver. Objects may be elec- 
 tro-plated with nickel in the same way as with copper; using 
 either the sulphate or chloride of nickel. German silver is superior 
 to brass for optical instruments. 
 
 Gtlding Dissolvent. This liquid is used for obliterating the 
 images which have been fixed by sel d’or upon daguerreotype plates. 
 Its exact composition has not been published, but it, no doubt, 
 contains nitro-hydrochloric acid, disguised in some way. A mix- 
 ture of salt and dilute nitric acid, or of nitre and dilute hydrochloric 
 acid, will answer the purpose equally well. 
 
 Glass. Glass is a mixture of various insoluble silicates with 
 excess of silica, and devoid of crystalline structure. The alkaline 
 silicates, when in a state of fusion, have the power of dissolving a 
 large quantity of silica. 
 
 There are two principal varieties of glass. The first comprises crown 
 and plate glass, and is composed principally of silica, potass, and 
 lime ; the second, called flint glass, contains, in addition, silicate of 
 
206 
 
 GLA 
 
 lead ; the lead promoting fusibility, and increasing the density, lustre, 
 and refractive power. 
 
 The principle of the glass manufacture is very simple. Silica, in 
 the shape of fine sand, is heated with carbonate of potass or soda, 
 and slaked lime, or oxide of lead. At a high temperature, fusion 
 and combination occur, and carbonic acid is expelled. When the 
 melted mass has become clear and free from air bubbles, it is left to 
 cool slowly until it assumes the peculiar tenacious condition suitable 
 for working. 
 
 Crown and plate glass have a greenish colour; flint glass is 
 colourless, and has a higher refractive index. Plate glass is cast 
 upon a flat metal table, rolled, and, after very careful annealing, 
 ground true and polished by machinery, ( See “ Plate Glass.”) The 
 large circular tables of crown glass are made by the glass blower. 
 
 A long iron tube, called the blow-pipe, with a wooden mouth- f 
 piece, has its end dipped into the tenacious, soft, semi-fused glass in 
 the glass pot. The lump of glass thus removed is then blown into 
 a flask. An iron rod, called a pontil, is then dipped into the glass 
 pot, and applied to the bottom of the flask so as to form a handle to it. 
 The blow-pipe is then detached from the neck of the flask, which 
 is again heated in the furnace, and then suddenly turned with great 
 rapidity, so that the centrifugal force causes it to assume the shape 
 of a flat disc. This done it is removed from the pontil, leaving a 
 great knob or bull’s eye of glass in the centre of the sheet, and put 
 into the annealing oven, where it is allowed to cool slowly. Unless 
 glass is annealed , or allowed to cool very slowly in this way, it 
 becomes exceedingly brittle. 
 
 Tables of crown glass made in this way exhibit an exceedingly 
 fine polished surface, although not strictly flat, like that of plate 
 glass. Sheet glass is made by blowing a hollow cylinder, then cut- 
 ting it lengthwise with a red hot iron, and spreading it open upon a 
 flat table. 
 
 Glass is coloured by the addition of various metallic oxides. 
 Oxide of cobalt gives deep blue ; oxide of manganese, amethyst or 
 black ; sub -oxide of copper, ruby-red ; black oxide of copper, green ; 
 the oxides of iron, dull green or brown ; oxide of uranium, yellow ; 
 oxide of silver, yellow ; purple of cassius, a ruby tint ; oxide of 
 antimony, yellow ; oxide of chromium, fine reds and greens. 
 
 Opaque white glass, called white enamel, is made by adding 
 oxide of tin. 
 
 The colouring oxide is either added to the glass in the glass pot, 
 or applied afterwards to the surface, and burnt in. 
 
GLA 
 
 207 
 
 Kane gives the following tabular view of the composition of 
 several kinds of Glass. (Elem. Chem., 720.) 
 
 Constituents. 
 
 Hard 
 
 White Glass. 
 
 1 2 
 
 Crown Glass. 
 
 3 4 
 
 Bottle Glass. 
 
 5 6 
 
 Crystal. 
 
 7 
 
 Flint Glass. 
 
 8 9 
 
 Silica 
 
 71 *7 
 
 69-2 
 
 62-8 
 
 69-2 
 
 60*4 
 
 53-5 
 
 59 2 
 
 51 9 
 
 42*5 
 
 Potass 
 
 127 
 
 15*8 
 
 221 
 
 8‘ 
 
 3*2 
 
 55 
 
 9* 
 
 13-8 
 
 117 
 
 Soda 
 
 25 
 
 3‘ 
 
 
 3* 
 
 
 
 
 
 
 Lime 
 
 10-3 
 
 7-6 
 
 125 
 
 13- 
 
 207 
 
 292 
 
 
 
 •5 
 
 Alumina 
 
 •4 
 
 ‘ 1*2^| 
 
 
 36 
 
 10*4 
 
 6‘ 
 
 
 
 1*8 
 
 Magnesia 
 
 
 2- l 
 
 26 
 
 •6 
 
 •6 
 
 
 
 
 
 Oxide of Iron 
 
 *3 
 
 •5 J 
 
 
 re 
 
 3-8 
 
 5-8 
 
 •4 
 
 
 
 Manganese 
 
 •2 
 
 
 
 
 
 
 r 
 
 
 
 Lead 
 
 
 
 
 
 
 
 8-2 
 
 33-8 
 
 43-5 
 
 
 98-1 
 
 99-3 
 
 100- 
 
 99- 
 
 99*1 
 
 100- 
 
 97-8 
 
 99- 
 
 100- 
 
 No. 1, is the difficultly fusible Bohemian glass ; No. 2, ordinary 
 Bohemian glass ; No. 3, English, and No. 4, German glass ; Nos. 5 
 and 6, Erench glass; Nos. 7 and 8, English; No. 9, the celebrated 
 optical glass of M. Guinaud, of Brennets, near Geneva. 
 
 All common glass, when reduced to fine powder is more or less 
 acted on by boiling water, which separates the alkali ; glass cannot 
 therefore be considered as insoluble in water. If finely powdered 
 glass be laid upon a piece of reddened litmus paper and moistened 
 with water, the moisture restores the blue colour of the paper by dis- 
 solving out the alkali in the glass. The power of glass to resist the 
 action of water, alkalis, acids, air, and light, is in general greater 
 the higher the temperature employed in its manufacture, the smaller 
 the proportion of its fluxes, and the more exact the chemical ratios 
 of its constituents. Most crystal glass is affected by having water 
 boiled in it for a considerable time : but crown glass, being poorer 
 in alkali, and containing no lead, resists that action much longer 
 and is better adapted for chemical purposes. 
 
 Air and light act upon glass, probably by their oxidizing pro- 
 perty. Blueish and greenish coloured glasses become colourless 
 by exposure, in consequence of the per-oxidation of the iron they 
 contain. Glass containing manganese becomes purple-red by the 
 per-oxidation of that metal from the same cause. Elint glass, 
 which contains lead, is acted on by sulphuretted hydrogen, and the 
 surface rendered opaque and iridescent. Achromatic lenses should, 
 therefore, be carefully preserved from the action of sulphur. 
 
 Charcoal colours glass of a yellow or brownish tint, so that it is 
 impossible to make glass in furnaces which smoke. 
 
208 
 
 GLA 
 
 The discovery of glass is said by Pliny to be due to the following 
 accident : — A merchant ship laden with natron, (soda,) being driven 
 in a gale upon the coast of Syria at the mouth of the river Belus, the 
 crew were compelled to cook their victuals ashore, and having placed 
 lumps of the natron upon the sand as supports to the kettles, found 
 to their surprise masses of transparent stone among the embers. 
 According to Pliny and Strabo the glass works of Sidon and Alexan- 
 dria were famous in their times, and produced beautiful articles which 
 were cut, engraved, gilt, and stained of the most brilliant colours in 
 imitation of gems. Some of the windows of Herculaneum appear to 
 have been glazed. 
 
 Por an account of the glass used in optical instruments See further 
 particulars in the articles “ Optical Glass,” and “ Plate Glass and 
 for the method of grinding lenses, See “ Lens.” 
 
 Glass, Soluble. When silica is melted with twice its weight of 
 carbonate of potass or soda, and the product treated with water, the 
 greater part dissolves, yielding a solution from which acids precipi- 
 tate gelatinous silica. This is the soluble glass mentioned by some 
 chemical writers. Its solution has been used for rendering muslin 
 and other cotton fabrics incombustible. 
 
 If stone or brick work be first washed over with the above solution 
 of silicate of soda, and then with a solution of chloride of calcium, a 
 double decomposition takes place by which common salt and silicate 
 of lime are produced. The latter is an insoluble salt and forms a 
 glaze on the surface to which it is applied, filling up the cavities and 
 protecting it from the weather. It has been proposed to employ this 
 process for architectural purposes, and it may perhaps find some 
 uses in photography. 
 
 Glass, Drayton’s method of silvering. Make an edging 
 of wax all round the glass plate. Pour into the dish thus formed, 
 a solution of ammonio -nitrate of silver to which a reducing agent, 
 such as grape sugar, is added. In a short time the glass will be 
 covered with a beautiful metallic coating of silver. 
 
 Glass Dishes. Dishes are sometimes made of plate glass for 
 holding the nitrate bath. The bottom is formed of one piece of plate 
 glass, and the sides of strips cemented to it with marine glue. The 
 edges and ends of these strips should be accurately ground. To 
 cement them together the pieces are heated with a spirit lamp, and 
 the glue rubbed on while still hot, and the pieces immediately applied 
 together, and pressed into contact. It is by no means an easy oper- 
 
GLA 
 
 GLY 
 
 209 
 
 ation, and dishes made in this way are rather dear. If an iron mould 
 were made of the required size and shape, glass dishes might be cast, 
 and these would be very useful in photography. 
 
 Dishes of plate glass should be enclosed in a wooden case, pro- 
 vided with a lid. 
 
 Glazier’s Putty. Whiting, or white lead, made into a paste 
 with boiled linseed oil. 
 
 Glucose. See “ Grape Sugar.” 
 
 Glue. See “ Gelatine.” 
 
 Glycerine. C 6 H 8 0 6 . (Gr. yXvicvQ, sweet.) Glycerine is a 
 sweet substance which may be extracted from fatty bodies and oils. 
 In its purest form it is obtained in the following manner : — 
 
 Take equal parts of olive oil and finely ground litharge ; put them 
 into a basin with a little water, and set it on a sand bath moderately 
 heated, continually stirring the liquid and adding fresh boiling water 
 when necessary. In this way in a short time a soap or plaster of 
 lead is formed. Add more water to this and remove the vessel from 
 the fire, decant the liquor, filter it, and pass sulphuretted hydrogen 
 through it to separate the lead ; then filter again and concentrate 
 the liquid as much as possible by evaporation without allowing it to 
 be burned on the sand bath. Evaporate what remains under the 
 receiver df an air pump. This is Glycerine It is a transparent 
 liquid without colour or smell, and of a syrupy consistence. Its S. G. 
 at 60° is 1*27. Water combines with it in all proportions, and 
 alcohol dissolves it readily, but it does not mix with ether or oils. 
 
 Glycerine is a neutral substance, and exhibits no tendency to 
 combine either with acids or bases. It has but little action upon 
 nitrate of silver, even in the light. Glycerine, if spread upon glass 
 or paper, does not dry, but retains its moisture. Prom this circum- 
 stance, and its perfect neutrality, it may be employed in photography 
 as a means of preserving the moisture of sensitive collodion films 
 and papers. 
 
 Glycerine is decomposed by nitric acid, with the formation of 
 oxalic acid, carbonic acid, and water. When, therefore, glycerine is 
 used as a preservative fluid in photography, the nitrate bath should 
 not contain free nitric acid. 
 
 A great many salts which are soluble in water, are also soluble in 
 glycerine. 
 
 In the manufacture of soap, which consists in boiling a fatty sub- 
 
 p 
 
210 
 
 GYL 
 
 GOL 
 
 stance containing glycerine with an alkali, the oil combines with the 
 alkali, and glycerine remains. See “ Saponification. 5 ’ 
 
 Glycyrrhizine. This substance has been recommended by 
 Mr. Hardwich to be added to collodion, in order, when necessary, 
 to increase the density of the blacks of the negative. In this respect 
 it acts like many other kinds of organic matter, in the mode described 
 in the articles on the “ Chemistry of Photography, 55 and “ Develop- 
 ment. 55 
 
 Glycyrrhizine (Greek, yXuKvg sweet, pi£ a a root) is a sugar, or 
 sweet resin, obtained from the liquorice root. A strong infusion of 
 liquorice root is slowly evaporated to a small bulk, and sulphuric 
 acid added : a precipitate falls, containing sugar and albumen. This 
 is to be washed in water acidulated with sulphuric acid, and after- 
 wards in water; then digested in alcohol, which dissolves the 
 glycyrrhizine and leaves the albumen. A solution of carbonate of 
 potass is then dropped into the alcoholic solution till its acid is 
 neutralized ; it is then filtered and evaporated, and the sugar remains 
 as a yellow transparent mass. 
 
 Liquorice sugar is easily soluble in water and alcohol, and has a 
 tendency to combine with acids, bases, and salts, and occasions 
 precipitates with most metallic salts. Its property of combining 
 with suboxide of silver renders it useful in the processes of deve- 
 lopment when increased density in the precipitate is required. 
 The best mode of using it is probably to add a drop or two of the 
 alcoholic solution to the collodion. The sensitiveness of the film 
 will then be diminished, and the intensity of the negative increased. 
 
 Gold. Symbol Au. ; Equivalent 200. 
 
 This valuable metal occurs in nature in a metallic state alloyed 
 with silver or copper, and is then called native gold. The veins of 
 gold are confined to primitive countries, but large quantities are 
 collected in alluvial soils, and the beds of rivers. It is generally 
 found in small nodules, and grains, but sometimes occurs in con- 
 siderable masses weighing several pounds. The principal supplies 
 of gold are from Australia and California, the IJral Mountains, the 
 rivers on the west coast of Africa, and of Peru, Brazil, and Mexico. 
 
 Gold may be obtained pure by dissolving standard gold in aqua- 
 regia (1 part by weight of nitric acid, and 2 of hydrochloric acid), 
 evaporating to dryness, redissolvihg the dry mass in distilled water, 
 filtering, acidulating with hydrochloric acid, and then adding a 
 solution of proto-sulphate of iron. A brown powder falls, which 
 after having been washed with dilute hydrochloric acid and distilled 
 
GOL 
 
 GBA 
 
 211 
 
 water is pure gold. It may be fused in a crucible with a little 
 borax, and a button of gold obtained. 
 
 Proto-sulphate of iron is a very delicate test of the presence of 
 gold, producing a blue tint in a solution containing not more than 
 the 60,000th part of the metal. It also throws down the whole of 
 the gold from its solutions. 
 
 Gold melts at a bright red heat. It is so malleable that a single 
 grain may be extended over 56 square inches of surface, and so 
 ductile that a grain may be drawn out into 500 feet of wire; 
 
 The pure acids have scarcely any action upon pure gold ; neither 
 has sulphur, nor sulphuretted hydrogen. Chlorine, iodine, and 
 bromine act upon it. Gj;old forms various interesting double salts, 
 and its oxide combines’ with the alkalies. These compounds are 
 described in their proper places. Sulphide of potassium is a solvent 
 of gold, and forms with it a double sulphide of gold and potassium. 
 Metallic gold, in a stafe of tine division, has sometimes a purple 
 tint. 
 
 Gold, Oxide of. There are two well defined oxides of gold, 
 viz., the protoxide, Au. O, and the peroxide, An. 0 3 , sometimes 
 called auric acid. 
 
 Protoxide of gold is produced by evaporating a solution of the 
 perchloride to dryness, and stirring the residue till it acquires a 
 yellow colour, taking care that the temperature never rises above 300°. 
 Caustic potass is then added in solution, and protoxide of gold 
 separates, the liquid becoming deep yellow in consequence of the 
 solution of a part of the oxide in the alkali. If this be filtered and 
 nitric acid added so as exactly to saturate the potass, hydrated 
 protoxide of gold falls. It is of a deep violet colour, almost ap- 
 proaching to black, insoluble in water and alcohol, and not decom- 
 posed till heated to 400°, when it parts with oxygen. It is not 
 soluble in nitric, sulphuric, or acetic acids, only in aqua-regia. 
 Hydrochloric acid converts it into metallic gold and perchloride of 
 gold. Potass and soda dissolve it in its nascent state, and with 
 ammonia it forms a purple detonating compound. The protoxide 
 of gold is important in photography as it is possible that, combined 
 with organic matter, it may be the compound which gives the violet 
 colour to positives that are toned with gold. 
 
 The peroxide of gold, or auric acid, is obtained by adding 
 carbonate of potass to neutral chloride of gold, and digesting at 
 170°. Carbonic acid escapes, and the hydrated peroxide subsides. 
 
 Grape Sugar; Glucose. C 24 H 28 0 28 . 
 
 p 2 
 
212 
 
 GEO 
 
 GITA 
 
 This modification of sugar, sometimes called “ Glucose,” is inter- 
 esting in photography from the fact of its reducing the oxides of 
 the noble metals to the metallic state without the aid of light ; a 
 property which is not possessed by cane sugar. It forms the hard 
 saccharine concretion found in honey, raisins, dried figs, etc., and is 
 contained abundantly in the juice of the grape. It may also be 
 produced from starch by the action of acids. It is neither so sweet, 
 nor so soluble in water as cane sugar. 
 
 The expressed juice of the grape contains 30 or 40 per cent, of 
 grape sugar- To obtain it, the juice is neutralized with chalk, 
 strained and filtered, mixed with a little white of egg, boiled, 
 skimmed, and evaporated till of the S. G. 1*32. It is then allowed 
 to cool, and in a few days it concretes into a solid mass, which is 
 drained, and then dried by pressure. It affords about 3 parts of 
 dried grape sugar and 1 of syrup. The sugar may be rendered 
 white by boiling it with charcoal. 
 
 Grape sugar is sometimes made by acting upon potato-starch 
 with dilute sulphuric acid. The process is hardly of sufficient 
 interest to the photographer to be worthy of description in this 
 place. 
 
 Ground Glass. A sheet of ground glass is generally used for 
 the focussing screen of the camera. It should be as finely ground 
 as possible, and the best way to secure this is to use plate glass 
 which has not received the final polishing in the manufacture. A 
 sheet of plate glass may be ground by hand, by rubbing with a 
 smaller piece of thick plate, and the finest powdered emery moistened 
 with water between. It takes three or four hours continuous labour 
 to do this properly. The last and finest grinding may be given 
 with a mixture of colcothar (red oxide of iron, iron rust,) and water, 
 instead of emery. 
 
 The ground glass screen should be placed in the camera with the 
 ground side next to the lens ; and it will be found very useful to 
 mark two diagonals across it, and also one central horizontal line, 
 and an odd number of vertical lines, with a lead pencil. 
 
 A good substitute for a ground glass focussing screen is to coat a 
 piece of plate glass with spirit varnish applied to a cold plate, and 
 allowed to set without heat. When dry, it forms a semi-opaque 
 film upon the glass. Benzole varnish with a little wax in solution 
 also answers the purpose. 
 
 Guaiacum. A resinous exudation from the Guaiacum officinale , a 
 lofty tree, native in Jamaica and St. Domingo. It is soluble to the 
 
GUM 
 
 GUT 
 
 213 
 
 extent of 90 per cent, in absolute alcohol. If a piece of paper be 
 dipped in tincture of guaiacum, and exposed to the prismatic spec- 
 trum, it is oxidized, and becomes of a green tint in the violet rays ; 
 but in the red rays the green colour is said to be destroyed. 
 
 Gum. Gum is a substance which occurs largely in the vegetable 
 kingdom, and is characterized by forming a viscid adhesive solu- 
 tion with water, which precipitates by the addition of alcohol a 
 white magma. It occurs in the form of an exudation upon the 
 bark of trees, and collects into drops, which gradually harden by 
 exposure. 
 
 There are two principal modifications of gum, one of which is 
 well represented by gum arabic, and the other by gum tragacanth. 
 
 Gum Arabic. This is the produce of various species of acacia , 
 and is imported from the Levant, Barbary, Senegal, the Cape of 
 Good Hope, and India. It is soluble in cold water, but more 
 rapidly in boiling water. Its solution is acid, and reddens litmus 
 paper, from the presence of permalate of lime. It is insoluble in 
 alcohol, ether, and oils. It enters into combination with some of 
 the metallic oxides. 
 
 Gum Dragon. Same as Gum Tragacanth. 
 
 Gum Tragacanth. This gum is obtained from the Astralagus 
 tragacanther , which grows in Crete and the surrounding islands. 
 It looks like twisted ribands, and is of a reddish white colour, nearly 
 opaque, and a little ductile. When plunged into water it dissolves 
 in part, swells considerably, and forms a thick mucilage, which when 
 boiled with water resembles a solution of gum arabic. It is very 
 difficult to pulverize, and should be heated to 212°, and pounded in 
 a hot mortar. 
 
 Gum Dammar, or, more properly, Dammar Besin. This is a 
 white resin brought from India, and obtained from the Finns 
 dammara. It is soluble in benzole, and makes a tolerably good 
 varnish for photographs upon glass, which dries very quickly without 
 heat. A portion only of Dammar is soluble in alcohol. 
 
 Gun Cotton, or Pyroxyline. The manufacture of this im- 
 portant substance is described in the article “ Pyroxyline.” 
 
 Gutta percha. This useful article is the produce of a very large 
 
214 
 
 GUT 
 
 and lofty forest tree called the Isonandra gutta , which is native in 
 the islands of the Indian Archipelago. Some of its properties were 
 first pointed out by Dr. W. Montgomerie in a letter to the Bengal 
 Medical Board in 1 843. When an incision is made in the bark of the 
 above tree, a white substance exudes which becomes shortly, by expo- 
 sure to air, hard and tough ; this is pure gutta percha. Neither the 
 wood nor fruit of the tree appear to be of any great value ; and a full 
 sized tree, when cut down and the gutta percha collected in bamboos, 
 yields about 30 or 40 pounds of gutta percha. 
 
 Gutta percha is sometimes contaminated with organic matters, 
 bits of bark, and foreign substances. To purify it, it is rasped in cold 
 w r ater, which removes the greater part of the soluble organic matters 
 and salts, and also facilitates the removal of portions of wood and 
 earthy matters, for gutta percha does not combine w 7 ith any substance 
 of this kind, but merely holds it mechanically. The raspings are 
 then washed and left to soak in warm water for several hours, and are 
 finally dried, heated to about 230°, and kneaded into lumps. 
 
 Gutta percha may be softened by hot water, and in that state 
 moulded into any form, stretched into sheets or straps, drawn out 
 into tubes or threads, &c., and on cooling completely it hardens and 
 retains the form given to it. It does not possess at any temperature 
 the peculiar elasticity of india-rubber. 
 
 Gutta percha resists the action of cold water and damp, and all 
 those agents which promote fermentation. It is not acted on by 
 alkalis, even when caustic and in their most concentrated form ; nor 
 by ammonia, saline solutions, water containing carbonic acid, the 
 variojis vegetable and mineral acids, and alcoholic liquors. 
 
 Olive oil dissolves a small portion of it when hot, but precipi- 
 tates it on cooling. Sulphuric acid with one equivalent of water 
 colours it brown, and disintegrates it with a sensible evolution 
 of sulphurous acid. Hydrochloric acid attacks it slowly, and 
 renders it brittle at a temperature of 68°. Monohydrated nitric 
 acid attacks it rapidly, with effervescence and an abundant evolu- 
 tion of fumes of hyponitrous acid. 
 
 Only a small portion of gutta percha can be dissolved, even with 
 the aid of heat, in absolute alcohol or ether. Benzole and spirits 
 of turpentine dissolve it partially when cold, and nearly completely 
 when hot. Sulphide of carbon, and chloroform, dissolve it completely 
 when cold ; the solution becoming perfectly clear and almost colourless 
 w T hen filtered under a bell glass. 
 
 Gutta percha is sometimes vulcanized with sulphur, by a process 
 patented by Mr. Hancock in 1847. It is sometimes treated with 
 chloride of zinc. The object of these processes is to give it a hard 
 
HAL 
 
 HEL 
 
 215 
 
 glazed surface free from stickiness. Vulcanized gutta percha maybe 
 varnished with a mixture of oil and resin, and polished till it acquires 
 the lustre of japanned wares. 
 
 Gutta percha is sometimes adulterated with the gums of otner 
 trees, plaster of paris, &c. 
 
 It is an exceedingly valuable material for the baths and dishes used 
 in photography, but when employed for these purposes should be 
 scrupulously pure. 
 
 It has a porous texture, and its cells contain air. They may be 
 clearly seen in a thin film of gutta percha placed under a powerful 
 microscope. 
 
 Gutta percha is of great value to the surgeon for forming splints for 
 fractured bones, since it can easily be moulded to any shape when 
 hot, and becomes sufficiently hard and rigid on cooling. It is also 
 very useful for many surgical instruments. 
 
 Halo. Sometimes a halo, or line of light, is seen to surround the 
 edges of dark objects in a photograph. It is produced by some pe- 
 culiar action of light on iodide of silver, which is not yet understood. 
 The effect is not due to the lens, since it occurs in positives taken 
 by superposition and printed by development upon iodide of silver, 
 without any indication of the kind existing in the negative. The 
 effect may be frequently observed in daguerreotypes and collodion 
 positives, and in collodion and paper negatives it shows itself as a 
 black line surrounding the light parts of the negatives. 
 
 This singular halo is never observed when chloride of silver is used 
 instead of iodide. It may be removed in some measure by adding a 
 chloride to the iodizing solution. The chlorides most soluble in al- 
 cohol are those of magnesium, calcium, (not chloride of lime,) and 
 zinc. This addition diminishes the sensitiveness of the plate or pa- 
 per, but increases the density of the negative, and renders it more 
 uniform in the opacity of the blacks. 
 
 Halogens. Haloid Salts. (Greek, a\g sea salt, eitiog form.) 
 Haloid salts are such as are formed by the combination of a salt ra- 
 dical such as chlorine, iodine, bromine, &c., with a metal such as 
 sodium, cadmium, &c. Chloride of sodium, iodide of potassium, 
 bromide of cadmium, &c., are haloid salts ; and chlorine, bromine, 
 iodine, &c. are called “ halogens . 55 
 
 Helioguaphy. (Greek r/Atoe the sun, to delineate.) 
 
 Another name for photography. 
 
216 
 
 HEB 
 
 HOE 
 
 Hermetical Seal. A vessel is said to be “ hermetically sealed” 
 when its lips are closed together perfectly by means of heat, as in 
 the case of a thermometer tube. The term is derived from Hermes, 
 the fabulous founder of Egyptian chemistry. 
 
 Honey. A sweet viscid liquor extracted by bees from the nec- 
 taries of flowers, and deposited by them, apparently without under- 
 going chemical change, in the waxen cells of their combs. 
 
 Honey is of two kinds, viz., virgin honey, and common honey. 
 The former is that which flows spontaneously with a very gentle heat 
 from the comb, — the latter that which is obtained by submitting the 
 comb to pressure. The former is the purest, and is nearly colour- 
 less ; the latter is thicker, darker coloured, and not so agreeable to the 
 taste. 
 
 Virgin honey should be used in photography, and not the common 
 kind, which is frequently adulterated. 
 
 Honey contains two distinct kinds of sugar ; viz, grape sugar, and 
 an uncry stallizable species of syrup. The former possesses consider- 
 able reducing properties, hence its use as a reducing agent in photo- 
 graphy. The syrup only possesses these properties to a limited ex- 
 tent, and its value in photography chiefly consists in its not drying 
 or crystallizing, but retaining its moisture for a long time when ex- 
 posed to the air. Honey has therefore been employed as a means of 
 preserving the moist condition of a sensitive collodion film ; but it is 
 evident that a simple syrup which does not contain a strong reduc- 
 ing agent, such as grape sugar, would be preferable to honey ; golden 
 syrup, for instance, which is a pure form of treacle, and free from 
 grape sugar, is far better than honey ; and perhaps glycerine may 
 be found still better than any form of syrup. But this question 
 is discussed under the head of “ Preservative Processes q. v. 
 
 If inspissated honey be treated with absolute alcohol, the thin portion 
 contains principally the uncrystallizable syrup, and the grape sugar 
 remains for the most part undissolved, being less soluble in alcohol. 
 
 Honey is soon decomposed in contact with nitrate of silver. 
 
 Honey-preservative Process. See “ Preservative Processes.” 
 
 Horn Silver. Eused chloride of silver, sometimes called “ luna 
 cornea;” a grey semi-transparent concrete mass, which is darkened 
 to a slaty tint by exposure to light. The melting point of horn silver 
 is 500°, Faht. If heated much above that point it volatilizes in dense 
 white fumes. When cooled slowly after fusion it has a tending to 
 octahedral crystallization. 
 
HUN 
 
 HYD 
 
 217 
 
 Hungarian Solution. This liquid is sometimes used in the 
 daguerreotype process for exciting the plate at one operation. Its 
 composition has not been published, but it is supposed to be a dilute 
 alcoholic solution of iodine, bromine, and chlorine, in certain propor- 
 tions. Its use is now nearly superseded by that of bromide of lime; 
 q. v. 
 
 Hydrate. When water enters into direct combination with a 
 body the compound is called a hydrate of that body. For instance, 
 water combines energetically with slaked lime, and the compound is 
 called “ hydrate of lime.” Sometimes water combines less energetic- 
 ally with bodies, and merely in such a way as to constitute what is 
 called their water of crystallization, which may be expelled by heat ; 
 in this case the term hydrate is not employed. 
 
 Hydriodates. Salts formed by the combination of hydriodic. 
 acid with bases. Sometimes a salt is called a hydriodate of a base 
 when it ought more properly to be termed an iodide of a metal. 
 Iodide of potassium for instance contains no water of crystallization, 
 and the term hydriodate of potass is therefore incorrectly applied to 
 it. The same may be said of many hydrochlorates, hydrosulphates, 
 &c. 
 
 Hydriodic Acid. HI = 127. This acid is a combination of 
 equal volumes of hydrogen and iodine, which unite in the gaseous 
 form without decrease of volume. It is obtained by gently heating 
 1 part of phosphorus, 14 of iodide of potassium, 20 of iodine, and 
 a little water. The formula of the action is, according to Grmelin, 
 2 KI+5 I + P+7 HO=2 KO, P0 5 +7 HI. 
 
 Hydriodic acid may also be formed by passing a mixture of 
 hydrogen and the vapour of iodine, through a red hot iron tube. 
 The acid gas may be collected over mercury, but is soon decomposed 
 by the mercury, which takes a part of the iodine. 
 
 Hydriodic acid gas is extremely sour and reddens litmus paper, 
 colourless, and exhales fumes in the air. It extinguishes flame, 
 and is not inflammable. It may be liquefied under pressure, and 
 becomes solid at a temperature of — 60° Paht. It is not permanent 
 at a red heat. Chlorine decomposes hydriodic acid, producing 
 hydrochloric acid and iodine, and chloride of iodine. Nitric acid 
 decomposes it with great violence, and in general, oxidizing agents 
 decompose it by forming water with the hydrogen, and liberating 
 iodine. 
 
218 
 
 HYD 
 
 Hydriodic acid gas mixes readily with water. The S. G. of the 
 strongest liquid acid is 1*7. It becomes dark coloured when kept 
 in contact with air, in consequence of a partial decomposition. 
 Aqueous hydriodic acid may readily be produced by passing sul- 
 phuretted hydrogen through a mixture of iodine and water ; the 
 sulphur is precipitated, and hydriodic acid produced. On heating 
 and filtering the liquor a pure solution of hydriodic acid is obtained. 
 
 Hydriodous Acid ; HI 2 . A solution of iodide of potassium, 
 or zinc, will take up a quantity of iodine equal to that which it 
 already contains. These salts have been sometimes called “ ioduretted 
 iodides,” and the acid has been termed “ hydriodous acid.” 
 
 Hydro-Carbon. Hydrogen and carbon combine in a great 
 many different proportions, forming compounds many of which 
 possess considerable interest. It rarely happens, however, that 
 these compounds can be obtained synthetically by actually mixing 
 the ingredients ; they are in general produced by the decomposition 
 of organic matters. 
 
 Some of the principal hydro-carbons are as follows : 
 
 Light-carburetted hydrogen . 
 Olefiant gas 
 
 Bicarbide of hydrogen . 
 Etherine .... 
 Naphtha . . , . 
 
 Naphthalin 
 
 and also common coal gas, oil gas, resin gas, coal tar, and many 
 analogous substances. 
 
 The characteristic properties of the hydro-carbons are their 
 combustibility, and the energy with which they are in general 
 decomposed by chlorine when exposed to light, the result being the 
 production of hydrochloric acid, and the liberation of carbon. Some 
 of the hydro-carbons are isomeric, and yet possess distinctive 
 peculiarities. 
 
 Light-carburetted hydrogen may be produced artificially, by mix- 
 ing 40 parts crystallized acetate of soda, 40 parts solid hydrate of 
 potass, and 60 parts of powdered quick lime. When this mixture 
 is strongly heated in a retort, the gas passes over, and may be 
 collected over water. It is the fire damp of coal mines, and the 
 marsh damp which proceeds from stagnant and decomposing mud. 
 
 Common coal gas is composed chiefly of carbon, hydrogen, and 
 
HYD 
 
 % 
 
 219 
 
 sulphur, and is produced by the destructive distillation of coal, the 
 volatile substances passing over, and coke remaining in the retort. 
 The volatile matters are first passed through a condenser kept cold 
 by immersion in water, and here the water, tar, ammoniacal liquor 
 and other condensable vapours are retained. The gases which pass 
 over, and which consist chiefly of carburetted hydrogen, hydrogen, 
 sulphuretted hydrogen, carbonic oxide, carbonic acid, nitrogen, and 
 cyanogen, are then passed through a mixture of quick lime and water 
 in a vessel called a purifier, by which a large portion of the sulphur- 
 etted hydrogen and carbonic acid are absorbed ; the remaining mixed 
 gases are then sufficiently pure for the ordinary purposes of illumina- 
 tion. 
 
 Olefiant Gas, and Naphtha are described in their proper places ; 
 
 q. v. 
 
 i 
 
 Hydrochloric Acid; H Cl. = 37. This acid is sometimes 
 called “ muriatic acid,” and “ chlorhydric acid.” It is formed by 
 combining equal volumes of hydrogen and chlorine, which unite with- 
 out diminution of volume, the result being a strongly acid gas. The 
 simplest mode of effecting this combination is to expose the mixed 
 gases to light. In diffused daylight they combine gradually, but in 
 strong sunshine at once, and with explosion. If a mixture of equal 
 volumes of hydrogen and chlorine be collected over water in a tall 
 graduated glass jar, and exposed to diffused light, the gases gradually 
 combine and form hydrochloric, acid ; this is absorbed by the water, 
 which has a strong affinity for it, as fast as it is formed, and the water 
 therefore rises in the tube. It is found by this experiment that the 
 quantity of acid produced is proportional to the time of exposure to 
 light. 
 
 Hydrochloric acid is readily obtained by adding diluted sulphuric 
 acid to common* salt in a retort, and applying heat to the mixture. 
 The chlorine of the salt combines with the hydrogen of the water to 
 form hydrochloric acid, the sodium is oxidized, and sulphate of soda 
 left in the retort. 
 
 Dry hydrochloric acid gas is colourless, and fumes in the air from 
 it s^ab sorption of moisture. It is intensely sour, and combines readily 
 with water. The S. G. of the strongest liquid acid is 1*210, and 
 water takes up 500 times its bulk of the gas. It boils at 112°, and 
 freezes at — 60°. 
 
 The impurities in commercial liquid hydrochloric acid, and which 
 give it a yellowish colour, are principally organic matter (from bits 
 t of cork, lute, &c.), a trace of bromine, chloride of iron, sulphuric 
 acid, and sometimes nitric and sulphurous acids, and also a trace of 
 
220 
 
 HYD 
 
 arsenic. By adding chloride of barium, and redistilling the acid, 
 the principal impurities are removed. 
 
 Hydrochloric acid acts on spme of the metals, and produces 
 corresponding chlorides. It precipitates chloride of silver from the 
 soluble salts of the oxide. 
 
 Hydrochlorates. The salts of hydrochloric acid, and also 
 the chlorides which contain water of crystallization, are called 
 “ hydrochlorates.” 
 
 Hydrocyanic Acid; Prussic acid; C 3 NH, or H Cy.=27. 
 Hydrogen and cyanogen have no tendency to direct combination, 
 but hydrocyanic acid may be produced by acting on cyanide of 
 mercury with hydrochloric acid, distilling the mixture with a gentle 
 heat, and surrounding the condenser with ice. Anhydrous hydro- 
 cyanic acid thus obtained is a limpid liquid having a strong odour 
 like that of bitter almonds. It scarcely affects the blue of litmus, 
 and is decomposed by light, with the evolution of ammonia. It is 
 so extremely volatile that a drop of it let fall upon a glass plate 
 freezes itself. It combines readily with water. 
 
 Hydrated hydrocyanic acid may be readily obtained by distilling 
 a mixture of equal parts of cyanide of potassium and sulphuric acid ; 
 the cyanide being dissolved in twice its weight of water, and the 
 sulphuric acid diluted with three times its weight of water. 
 
 This acid is highly poisonous ; the most effective antidotes are 
 ammonia, and chlorine. Its S. Gr. is not a correct criterion of its 
 strength. Added to a solution of nitrate of silver it throws down 
 insoluble cyanide of silver. 
 
 Hydrofluoric Acid. HP. This acid is obtained by distilling 
 finely powdered fluor spar (fluoride of calcium), with twice its weight 
 of sulphuric acid. The acid which passes over is a highly volatile 
 and corrosive liquid, which acts powerfully on glass ; the retort 
 should therefore be made of lead, and the tube and receiver of 
 platinum, and the latter surrounded with ice. This acid must be 
 kept in a leaden or platinum bottle. It is colourless, fumes in the 
 air, and has a most powerful affinity for water, exceeding that of oil 
 of vitriol. When dropped into water it hisses and evolves great 
 heat. It acts vigorously on the skin, and produces bad sores and 
 dangerous ulcers, if carelessly handled. 
 
 The characteristic property of hydrofluoric acid is the energy 
 with which it attacks glass, producing heat and effervescence when 
 dropped upon it. Diluted with 6 times its weight of water it is 
 
HYD 
 
 221 
 
 used for etching upon glass, See “ Etching by Photography.” The 
 fumes of hydrofluoric acid destroy the polish and dim the surface 
 of glass. Focussing screens are sometimes made in this way instead 
 of grinding the glass. 
 
 Hydrogen. H=l. (Greek, water, and yevveiv to pro- 
 duce.) 
 
 Hydrogen is the lightest of all known bodies, and its atomic 
 weight is therefore taken as the unit of the scale of equivalents. It 
 may be obtained in a somewhat impure form by acting on some bits of 
 zinc, or iron filings, with dilute sulphuric acid. Water is decomposed, 
 its oxygen going to the metal to form a sulphate, and hydrogen 
 being given off. The best proportions appear to be, zinc 3 parts, 
 sulphuric acid 5 parts, water 25 parts. Pure hydrogen may be 
 obtained by acting on pure water with an amalgam of potassium or 
 sodium. Hydrogen may also be obtained bypassing steam through 
 a red-hot gun-barrel. 
 
 Hydrogen is inflammable, but extinguishes flame. When mixed 
 with oxygen or atmospheric air in sufficient quantity it explodes with 
 violence. It has a high power of refracting light. When pure it is 
 colourless, tasteless, and nearly inodorous. Water only takes up about 
 1 per cent, of it. 
 
 Hydrogen plays an important part in many of the chemical changes 
 produced by light. This matter will be found discussed in the article 
 on the “ Chemistry of Photography ;” q.v. 
 
 Hydrogen is supposed by many chemists to be a metal in the form 
 of vapour, and to be the acidifying principle of some acids. Take, 
 for instance, sulphuric acid ; this substance, so powerful an acid in 
 its hydrated or common form, does not when pure and perfectly an- 
 hydrous redden litmus paper, or exhibit any acid properties. Other 
 similar cases might be quoted. 
 
 Hydrogen, Oxide of. There are two oxides of hydrogen, viz., 
 water, which is the protoxide ; and oxygenated water, or peroxide 
 of hydrogen, or ozone, about which very little is known at present. 
 See “ Water ” and “ Ozone.” 
 
 Hydrogen-Acids, or Hydracids. These are the acids which 
 hydrogen forms with chlorine, sulphur, iodine, &c.; and which are 
 described under their various heads. The term “ hydracid,” is used 
 in contradistinction to “ oxacid,” in which oxygen in combination 
 with another element forms the electro-negative constituent of a 
 salt. 
 
222 
 
 HYD 
 
 Hydrometer. An instrument for measuring the specific gravi- 
 ties of liquids. Its commonest form is a long graduated tube loaded 
 at the bottom, and allowed to float in the liquid to be tested, the 
 depth to which it sinks being indicated by the scale. The principle 
 of the instrument consists in the fact, that a floating body displaces 
 a quantity of liquid exactly equal in weight to itself. The hydrometer 
 alfords, therefore, a measure of the volume of the liquid necessary to 
 counterbalance in weight the weight of the instrument; and by 
 comparing these measures of volume when different liquids are tested, 
 the specific gravities of the respective liquids may be compared and 
 determined. 
 
 The most convenient fcPrm of hydrometer on this principle is that 
 of Zanetti, manufactured at Manchester. It is sold in sets of six, 
 and the S. G. is got by adding a cipher to the number of de- 
 grees indicated ; the assumed temperature being 60° Faht. 
 
 The hydrometers of Twaddle, and Beaume, are constructed on the 
 same principle, but graduated differently. There are two forms of 
 Beaume’ s hydrometer; one for measuring the S. G. of liquids 
 heavier than water, and called a “ pese-acide,” or “ pese-sirop ;” 
 the other, for liquids lighter than water, and called a “ pese^esprit” 
 These instruments are much used both in France and England, and 
 are sometimes called “ Areometers.” 
 
 An entirely different principle of construction has been adopted in 
 the hydrometers of Nicholson, and Fahrenheit. In these instruments 
 the line of flotation in water at 60° is marked, and when immersed 
 in the liquid to be tested the instrument is loaded until it sinks to 
 the same level, — the weight required to be added or removed deter- 
 mining the S. G. of the liquid. 
 
 The hydrometer does not afford an exact test of the strength of 
 an old nitrate bath, because the bath acquires by use nitrate of potass 
 and other contaminations, which increase its specific gravity. 
 
 Hydrosulphate of Ammonia ; sulphide of ammonium : NH 3 , 
 HS. 
 
 The aqueous solution of this compound may be obtained by 
 dividing a portion of liquor ammonise into two parts, saturating one 
 with hydrosulpliuric acid, and then adding it to the other. It 
 forms a colourless liquid, which decomposes and turns brown in 
 the air, and has a most offensive smell, like rotten eggs. 
 
 When ammonia is saturated with hydrosulphuric acid, hydro- 
 sulpliate of sulphide of ammonium is produced, which is a double 
 sulphur salt, the formula of which is NH 4 S-f-HS=51. 
 
 If a few drops of sulphide of ammonium are added to a pint of 
 
HYD 
 
 HYP 
 
 223 
 
 water, and a fixed positive print immersed in it, the print darkens 
 from red to brown, thence to purple and black, and becomes 
 ultimately a pale yellow; exhibiting, in a few minutes, all the 
 changes which a fading photograph usually undergoes. Unless the 
 print is completely fixed before immersing it in this bath it is immedi- 
 ately discoloured all over with a brown coating of sulphide of silver. 
 
 Hydrosulphuric Acid; Sulphuretted hydrogen; HS=17. 
 Sometimes called hydro-thionic acid. 
 
 This gas is readily obtained by acting on proto-sulphide of iron 
 with dilute sulphuric acid ; or by heating in a flask equal parts of 
 tallow and powdered sulphur. In the former case water is decom- 
 posed, the hydrogen combining with the sulphur to form hydro- 
 sulphuric acid, and the oxygen going to the iron to form proto- 
 sulphate of iron. The second method yields the purest gas. 
 
 At ordinary temperatures hydrosulphuric acid is gaseous ; it is 
 liquefied under a pressure of 17 atmospheres, and becomes solid at 
 a temperature of —122° Eaht. It is inflammable, but extinguishes 
 flame ; highly poisonous, and its odour extremely offensive, resembling 
 that of rotten eggs. Cold water takes up about three times its 
 volume of this gas, and the solution is slightly acid to litmus 
 paper. 
 
 Hydrosulphuric acid throws down silver, gold, and many other 
 metals from their solutions, by forming insoluble sulphides, of 
 various colours ; See “ Sulphides. 55 It combines with many of the 
 basic sulphides and forms double sulphur salts.. 
 
 Hydrosulphuric acid is immediately decomposed by iodine, 
 chlorine, and bromine, and sulphur precipitated. It is also decom- 
 posed by nitric acid. In combination with potass, or soda, it is 
 capable of dissolving gold, and forming a double sulphide of gold 
 and potassium or sodium. Silver may probably be acted on in the 
 same way, and the yellow material of a faded photograph may be 
 a double sulphide of silver and sodium. 
 
 Hyposulphite of Gold and Soda. See “ Sel d 5 or. 55 
 
 Hyposulphite of Silver and Soda ; 2 (Ha. 0, S 9 0 9 ) 4* 
 Ag. O, S 2 0. 2 +2 HO. 
 
 When chloride of silver is dissolved in hyposulphite of soda, and 
 the solution evaporated, lamellar crystals of the above salt are 
 obtained. They are of a dirty grey colour, and contain two atoms 
 of water. They are unaltered by exposure to air and light, are 
 very soluble in water, and have a sweet taste. When the iodide, 
 
224 
 
 HYP 
 
 chloride, bromide, &c., of silver are dissolved in hyposulphite of 
 soda, this double salt is produced, together with iodide, chloride, bro- 
 mide, &c., of sodium. It is nearly insoluble in alcohol. 
 
 Hyposulphite of silver alone cannot be isolated. It is no sooner 
 formed than it undergoes decomposition into sulphide of silver 
 and sulphurous acid, unless excess of hyposulphite of soda be present ; 
 in which case a double salt is produced. 
 
 Hyposulphite of Soda. Na. 0, S 2 0 2 +5 HO=125. 
 
 This salt has been used largely in photography for some years, 
 but its use has had so much to do with the fading of photographs, 
 that it has now become very desirable to find a substitute for it. 
 Its uses in fixing and toning are discussed in the articles treating 
 of the various processes, and we shall therefore, in the present, 
 merely describe its mode of manufacture, and principal properties. 
 
 Take of crystallized carbonate of soda 8 parts, rain water 1 6 parts, 
 sublimed sulphur 1 part. Mix the ingredients, and pass through 
 the mixture sulphurous acid gas. As soon as this acid is in excess, 
 the liquid contains hyposulphite of soda in solution. Boil it gently 
 for some minutes, filter it, and then evaporate it to one third of its 
 bulk, and place it in a cool place to crystallize. The crystals are 
 four-sided prisms. 
 
 Hyposulphite of soda deliquesces in the air. Its taste is bitter 
 and nauseous. When heated it fuses, deflagrates, and takes fire, 
 burning with a yellow flame. It is insoluble in alcohol, which pre- 
 cipitates it from its aqueous solution. . It is a ready solvent of the 
 haloid salts of silver, and this, its principal use in photography, was 
 first suggested by Sir John Herschel. It is neutral to test paper, and 
 extremely soluble in water, with the production of cold. Large 
 quantities of this salt are now made in the neighbourhood of New- 
 castle-upon-Tyne. 
 
 The addition of an acid to a solution of hyposulphite of soda ren- 
 ders it turbid by causing the liberation of sulphur in a state of fine 
 division. 
 
 Hyposulphites. Many of the metallic oxides combine with 
 hyposulphurous acid and form hyposulphites, but at present none 
 of them posses any interest in photography except those of gold and 
 silver ; q. v. The hyposulphites of the alkalies, and alkaline earths, 
 mostly act in the same way as that of soda in dissolving the haloid 
 salts of silver. Hyposulphite of lime might probably be manufac- 
 tured at a cheaper rate than that of soda, and from experiments of Dr. 
 Alfred Taylor it appears to answer as well, or even better. 
 
HYP 
 
 IMA 
 
 225 
 
 Hyposulphurous Acid ; S 2 0 2 . This acid has not been isola" 
 ted. It only exists in combination. 
 
 Ice. Water solidified by cold. Ice is in general pure water, for 
 when water freezes it rejects all saline matters and impurities ; 
 melted ice may therefore be used in photography instead of distilled 
 water. This may sometimes be found a convenience to the photo- 
 graphic tourist on the continent, since ice can be obtained at the 
 hotels of all large continental towns. The melting point of ice is 
 32° Paht., and 0° Centigrade. Water may be frozen by artificial 
 cold produced by various freezing mixtures ; q. v. 
 
 Iceland Moss. Cetraria Islandica . A lichen which grows in 
 exposed places in Iceland, and is used by the natives as food. When 
 boiled with water, and allowed to cool, it forms a jelly. It is some- 
 times used in photography instead of gelatine, which it resembles in 
 its properties. 
 
 * 
 
 Iceland Spar. Native carbonate of lime, crystallized and pure. 
 The crystals exhibit the phenomenon of “ double refraction” {See 
 “ Light,”) and are used in experiments on polarized light. Their 
 form is an obtuse rhomboid of 105° 5' and 74° 55', according to 
 Hr. Wollaston. They are not always transparent, but sometimes 
 opaque, or snow white, or tinged of different hues. 
 
 Illumination. The illumination of a surface varies inversely as 
 the square of its distance from the source of light. The same quan- 
 tity of light which falls upon a certain area at a distance of I foot 
 from the luminous body will be extended over 4 times that area at a 
 distance of 2 feet, 9 times at a distance of 3 feet, 100 times at a 
 distance of 10 feet, and so on. This is evident from the considera- 
 tion of the sections of a pyramid made at distances 1, 2, 3, — 10 from 
 the vertex. Hence the rule stated above. 
 
 Image. A term used in optics to denote an assemblage or sys- 
 tem of foci. When the foci are “real” the image is “real,” and 
 when “virtual,” “virtual,” {See “Pocus.”) The image formed by 
 the lens of a camera obscura is a “ real ” image, and it illuminates 
 a screen placed to receive it ; the image formed by a looking-glass 
 is not a real but a virtual image. In the former case the rays ac- 
 tually pass through the image, in the latter it is only the directions 
 of the rays produced backwards which pass through the image. 
 
 In optics a luminous body is considered to be an assemblage of 
 
 Q 
 
226 
 
 IMP 
 
 luminous points, from each of which a pencil of light proceeds, these 
 pencils not interfering with one another. Each pencil being then 
 refracted through a convex lens, its rays are brought to a focus. 
 This focus is sometimes called the image of the luminous point 
 which emits the rays that are collected in it, and the assemblage of 
 all these foci constitutes the image of the object. 
 
 The distinctness of the image will depend upon the accuracy 
 with which the rays of each particular pencil are brought to a focus. 
 
 Since the objects are in general at different distances from the 
 lens, the image does not necessarily lie upon a plane or upon any 
 regular curved surface. 
 
 Imperial Measure. See “ Tables of Weights and Measures.” 
 
 Imponderable Agents. Light, Heat, and the various forms of 
 Electricity are called “ imponderable agents,” because they cannot 
 be weighed. When the philosopher is called on to give a definition 
 of matter, he replies, “ Matter is whatever can be proved to possess 
 weight, that is, to be subject to the law of gravitation.” Light and 
 Heat are probably the undulations of a medium so subtle that it 
 has not yet been weighed ; and neither light, heat, nor electricity 
 perceptibly affect the weight of bodies that are affected by them ; 
 hence they are called imponderable or immaterial agents ; the terms 
 “ imponderable ” and “ immaterial ” being strictly synonymous, 
 according to the definition of matter. 
 
 It is probable that light, heat, and the various forms of electricity 
 may all be undulations of an elastic medium pervading space, which, 
 although its weight may not be appreciable by our senses aided by 
 any form of apparatus, may still possess weight, and be material. 
 This subtle but material ether may be supposed to fill the interstices 
 between the atoms of bodies, and by its motions to disturb their re- 
 lative positions in such a way as to produce the phenomena due to 
 what are called the “ imponderable agents.” The difference between 
 light, heat, and electricity would depend, according to this hypothesis, 
 upon the kind and length of the undulation, and the velocity of its 
 propagation ; so that these agents may actually glide one into the 
 other, and become merely modified cases of one form of matter in mo- 
 tion. But these are speculations ; let them not be confounded with 
 ascertained facts. Still, speculation is a necessary part of philosophic 
 enquiry ; and as for the undulatory theory of light, that is now received 
 by scientific men with the same faith as the law of universal gravita- 
 tion. Both hypotheses rest upon equally strong evidence, and there 
 is the highest degree of moral certainty of their truth. 
 
INC 
 
 IND 
 
 m 
 
 Incidence, Angle of. When a ray of light is incident upon 
 the surface of any medium, the angle which it makes with the normal 
 to the surface at the point of incidence is called the “ angle of 
 incidence. 5 ’ 
 
 Incorrodible Ink, used for writing the lables of bottles contain- 
 ing strong acids or alkalis : — Dissolve one part of asphaltum in two 
 parts of oil of turpentine. 
 
 Indian Ink. Pure indian ink is manufactured in China. A 
 very good imitation may be made in the following manner : — 
 
 Grind together lamp black, (previously purified with caustic potass,) 
 and gelatine, the gelatinizing power of which has been partly destroyed 
 by long continued boiling. It may then be scented with musk and a 
 little camphor, and made into sticks. 
 
 Indian ink is used in photography for blackening the skies of col- 
 lodion and paper negatives. It may be made extemporaneously by 
 blackening the inside of a plate over the flame of a candle, and mixing 
 up the lamp black thus obtained with a little gelatine water. 
 
 Indian Eubber. See “ Caoutchouc.” 
 
 Indigo. This valuable dye and pigment is of vegetable origin, 
 and comes principally from India, being manufactured from the 
 leaves and stems of the indigo plant. There are also West Indian, 
 Egyptian, and Arabian varieties. Indigo is also obtained from 
 woad. 
 
 The seeds of the plant are sown in the spring, and the plant is cut as 
 it comes into blossom. Sometimes the indigo is obtained from the 
 fermentation of the fresh leaves and stems, at other times from the 
 dried leaves. An infusion is made which after fermentation is of a 
 yellowish or greenish colour. It is then decanted and beaten with 
 sticks for some time, which expels the carbonic acid from the liquid, 
 and brings the particles of indigo into contact with the oxygen of 
 the air ; they then separate in grains and fall to the bottom of the 
 vessel, the liquid becoming clear. 
 
 Indigo is a dark blue powder, devoid of taste or smell, insoluble in 
 water, cold alcohol, ether, hydrochloric acid, and fat oils, but soluble 
 in strong sulphuric acid, and creasote. The commercial sample con- 
 tains scarcely one half of pure indigo, the remainder being composed 
 of resinous substances, silica, alumina, oxide of iron, carbonate of 
 lime, &c. 
 
 When indigo is placed in contact with a substance having a strong 
 
 Q 2 
 
228 
 
 INF 
 
 INK 
 
 affinity for oxygen, it parts witli oxygen and takes hydrogen, becoming 
 hydruretted and losing its colour, and forming white indigo, (indigo - 
 tine) by exposure to air and heat. This again absorbs oxygen, and 
 acquires its former blue colour. White indigo is soluble in alcohol 
 and ether, and is very unstable. Although, therefore, many substances 
 may deprive indigo of its colour, yet it has always a tendency to 
 recover it by becoming again oxidized. 
 
 Indigo dissolved in strong sulphuric acid forms a deep blue liquid, 
 called sulphate of indigo. 
 
 Indigo is likely to prove a valuable substance in the photographic 
 process of printing in pigments. 
 
 Inflexion of Light. See “Light,” 
 
 Ink. Common black writing ink is tannate of iron. It is made 
 thus : 
 
 Take Bruised nut-galls . . .12 parts 
 
 Sulphate of iron . . . . 4 „ 
 
 Gum arabic 
 Water 
 
 Mix together in a stone bottle, and let them stand for two or three 
 weeks, shaking the bottle from time to time. Then, pour off the 
 clear liquor, and add a little creasote to prevent mouldiness. 
 
 If an ounce of treacle be added to a pint of common black writing 
 ink, the writing may be transferred by means of a copying machine. 
 
 Indelible Ink. Berzelius’s indelible ink is made by adding 
 a weak solution of vanadate of ammonia to an infusion of galls. A 
 very small quantity of the salt will produce a perfectly black ink. 
 
 Ink-Printing Process. By this process positive prints may 
 be obtained in common writing ink. 
 
 The paper is first immersed in a nearly saturated solution of 
 bichromate of potass, and dried in the dark. It is of a bright 
 yellow colour. 
 
 It is then exposed to light under the negative in the pressure 
 frame, until all the details of the picture are brought out. The time 
 required is less than that in ordinary sun-printing. The picture is 
 of a pale brown tint upon a yellow ground. 
 
 The picture is then washed in water, in order to remove the whole 
 of the undecomposed chromium salt. The water should be changed 
 several times, and the print left to soak in it for two or three hours. 
 This should be done in the dark room. 
 
INS 
 
 229 
 
 When the print has been sufficiently washed it may be taken into 
 the light, and the picture, which is now a pale brown upon a white 
 ground, is permanently fixed, the dark material being an oxide of 
 chromium in combination with the lignin of the paper. 
 
 It now remains to colour the print black or purple. This is done 
 by causing writing ink to adhere to the dark parts of the picture, 
 which act as a mordant. 
 
 Immerse the print for a few minutes in a weak solution of proto- 
 sulphate of iron, say 5 grains to the ounce of water. Then remove 
 it, and wash it as before in several changes of water, and let it soak 
 for two or three hours in water. The iron salt adheres to the 
 image or mordant, but in much less quantity to the bare lignin of 
 the paper. 
 
 Next, immerse it in a solution of tannic acid, of moderate 
 strength. The dark parts of the picture are rapidly blackened by 
 the formation of tannate of iron, or writing ink ; the lights remain- 
 ing unchanged. When fully developed wash the print well in water, 
 and dry it. It is now finished. 
 
 The difficulties of the process consist in thoroughly removing the 
 chromium and iron salts from the paper, so as to preserve the purity 
 of the whites ; and also in obtaining blacks of sufficient depth and 
 vigour. The prints are permanent, and uniform in colour* and the 
 detail very good ; the process is also very economical. The paper 
 may either be albumenized or gelatinized by mixing albumen or 
 gelatine with the bichromate of potass. These organic substances 
 are fixed and rendered insoluble by the reduction of the chromium, 
 and are washed out from the parts where light has not acted. The 
 details of the picture are therefore glazed upon a dead ground of 
 white paper. The most vigorous prints are obtained upon unsized 
 paper, but this is very liable to get torn in the operations of 
 washing. 
 
 Insolation. Exposure to sunshine. 
 
 Instantaneous Pictures. Photographs obtained in a frac- 
 tional part of a second of time are said to be instantaneous. There 
 is no particular process more sensitive than any in common use by 
 which instantaneous pictures may be taken. Either the ordinary 
 collodion process in its most efficient working state, or the daguerre- 
 otype process, is employed for this class of pictures. The instan- 
 taneity of the exposure is got by increasing the intensity of light in 
 the image by using a lens with large aperture, which must, of 
 course, be provided with the means of opening and shutting it in- 
 
230 
 
 INV 
 
 IOD 
 
 stantaneously. For this latter purpose many different plans have 
 have been suggested. It is evident that the space to be uncovered 
 and closed should be the smallest through which all the light 
 passes ; hence the absurdity of spring shutters for the dark slide, 
 for the picture is the largest space on which light is incident. The 
 proper place for the instantaneous cap, as it is called, is in front of 
 the front lens. 
 
 According to some recent experiments of Professor Zautedeschi, 
 of Padua, it appears that the presence of the atmosphere in contact 
 with the sensitive collodion film retards the action of light, and that 
 sensitive iodide of silver is darkened much more rapidly “ in vacuo.” 
 It seems probable, therefore, that the chemistry of the process 
 remaining the same a shorter exposure might suffice if the camera 
 could be exhausted of air after the insertion of the plate. This 
 experiment might be tried without any great difficulty or expense, 
 and, if successful, it would be a great step in photography. It 
 seems probable enough that a quantity of air entangled with the 
 other chemicals in the sensitive film might interfere with their mutual 
 reactions under the influence of light, while the atmospheric pressure 
 of 15lbs to every square inch of film might render it more compact 
 and less sensitive. Besides, should any gas be given off during the 
 decomposition of any of the chemicals by light, this would readily 
 escape and diffuse itself in vacuo, instead of remaining to interfere 
 with farther decomposition. Both theory and experiment seem, 
 therefore, to be in favour of exhausting the camera of air in the 
 instantaneous processes. 
 
 Invisible Bays of the Spectrum. The invisible rays beyond 
 the extreme violet are, for a certain distance, actinic, and darken 
 sensitive paper. These rays are the most refrangible, and the 
 undulations are the shortest. They may be rendered visible by 
 being passed into a solution of sulphate of quinine. ( See “ Fluo- 
 rescence.”) The invisible rays of the spectrum beyond the extreme 
 red are, for a certain distance, calorific, and their existence may be 
 proved by the heat which they produce. These rays are the least 
 refrangible, and the undulations have the greatest length. It is 
 probable that both the rays of maximum actinic and calorific power, 
 lie within the limits of the visible spectrum ; but this will depend 
 upon the nature of the substance acted on. 
 
 Interference. See “ Light.” 
 
 Iodates. Salts of iodic acid ; q . v . 
 
IOD 
 
 231 
 
 Iodate or Ammonia. NH 3 , I0 5 , HO. This salt is obtained 
 by saturating iodic acid with ammonia. It forms small crystals 
 sparingly soluble in water. 
 
 Iodate of Potass. KO, i0 5 . This salt is one of the products 
 of the action of iodine on solution of potass. It is insoluble in alco- 
 hol S. G. 810, and requires 14 parts of water at 60° for its solution; 
 but is more soluble in a solution of iodide of potassium. It crys- 
 tallizes in small cubes. When heated it gives off oxygen and is 
 converted into iodide of potassium. It is permanent in the air. 
 
 Iodate of Silver. Ag. 0, I0 5 . This salt is precipitated as a 
 white powder when iodate of potass is added to nitrate of silver. It 
 is very soluble in liquor ammoniae,and is deposited from this solution 
 in small rectangular prisms which retain no ammonia. It is not 
 sensitive to light. 
 
 Iodic Acid. I0 5 . This acid cannot be obtained by the direct 
 action of oxygen upon iodine, but may be produced by the action of 
 sulphuric acid upon iodate of soda. It combines with water and 
 acts powerfully upon the metals, forming salts which are reduced to 
 iodides by the action of he^t. 
 
 Iodides. Salts formed by the combination of iodine with a 
 metal, or other element. 
 
 Iodide of Ammonium. NH 4 1=144. This salt is made thus : — 
 Add iodine to a strong solution of hydrosulphate of ammonia, until 
 it begins to be coloured with iodine. On first adding the iodine a 
 dense deposit of sulphur occurs. The solution is now acid to test- 
 paper, from the presence of a little hydriodic acid. Neutralize this 
 carefully with ammonia, and then filter, evaporate, and crystallize. 
 The crystals are colourless cubes, very deliquescent, and unstable 
 in the air. They should be dried and preserved in glass tubes 
 hermetically sealed. 
 
 Iodide of ammonium is extremely soluble both in water and alco- 
 hol ; but its alcoholic solution becomes discoloured by time, and when 
 exposed to sunshine this change proceeds with great rapidity, the 
 solution becoming at the same time alkaline. 
 
 Iodide of ammonium should not be used as an iodizer for negative 
 collodion, on account of its great instability, by which free iodine is 
 liberated and transferred to the bath, and also because nitrate of ammo- 
 nia is produced in the bath, which is a salt that ought not to exist 
 
232 
 
 IOD 
 
 in a negative bath because of its instability and property of dissolving 
 oxide of silver. Iodide of ammonium may however be used as an 
 iodizer of positive collodion, because in that case the nitrate bath 
 ought to be acidified with free nitric acid, which is produced by the 
 introduction of free iodine into it. 
 
 The common impurities of iodide of ammonium are sulphate of 
 ammonia, which is nearly insoluble in alcohol, and carbonate of 
 ammonia. The latter renders the collodion and nitrate bath alkaline. 
 
 Iodide of Cadmium. Cd. 1=182. This salt is obtained by 
 heating filings of cadmium with iodine, or mixing them in a moist 
 state. It crystallizes in large white six-sided tables, of a pearly 
 lustre, which are fusible, and decomposed at a high temperature. 
 
 Iodide of cadmium is very soluble in water and alcohol. When 
 pure its alcoholic solution is permanent, and becomes discoloured 
 very slowly and slightly by exposure to sunshine. Collodion iodized 
 with iodide of cadmium does not become discoloured or undergo any 
 visible alteration by time, if kept in a cool dark place. This is a 
 great advantage, but, on the other hand, the nitrate of cadmium 
 formed in the N. S. bath is a salt which has an acid reaction and its 
 effects are similar to those produced by free nitric acid. The nitrate 
 bath therefore gets gradually out of order when this iodizer is em- 
 ployed. The permanence of collodion-iodized with iodide of cadmium 
 is probably due to the difficulty with which cadmium is oxidized. 
 
 The common impurity of iodide of cadmium is iodide of zinc. 
 When this is present the collodion becomes gradually discoloured ; 
 zinc being an easily oxidizable metal. 
 
 Iodide of cadmium impairs the fluidity of collodion. 
 
 Iodide of Calcium. Ca. 1=146. This salt is obtained by 
 dissolving carbonate of lime in hydriodic acid, evaporating to dryness, 
 and fusing the residue in a close vessel. When this is dissolved in 
 water and evaporated, it furnishes white deliquescent crystals. 
 
 The nitrate of lime formed in the nitrate bath when this iodizer is 
 employed is a very deliquescent salt, and would therefore tend to 
 preserve the moisture of the excited film. The deliquescent pro- 
 perties of the iodide and nitrate of lime recommend this iodizer for 
 use, although it is not much employed. 
 
 Iodide of calcium is extremely soluble in water, and alcohol, even 
 when absolute. The alcoholic solution is discoloured by light, and 
 collodion iodized with it becomes gradually reddened, as with the 
 potassium salt. It gives excellent negatives, remarkable for their 
 cleanness and density. 
 
IOD 
 
 238 
 
 Iodide of Iron. There are two iodides of iron, viz. a prot- 
 iodide Fe. I, and a periodide, the exact composition of which has 
 not been ascertained. 
 
 Protiodide of iron is formed by digesting iron filings or iron 
 wire in water with iodine, the metal being in excess. A greenish 
 solution is obtained, to which sugar is sometimes added for medicinal 
 purposes. It is extremely unstable, and throws down oxide of iron 
 by exposure to air. On evaporation it yields a grey fusible salt, 
 which is soluble in alcohol, the solution being rapidly decomposed 
 by light. 
 
 When protiodide of iron is used as an iodizer in photography, 
 protonitrate of iron is formed in the nitrate bath, and also in the 
 sensitive film. This acts as a developer, and speedily blackens and 
 spoils the bath ; it also developes the image so long as the plate or 
 paper continues moist ; the picture, therefore, comes out after 
 exposure, by a sort of self-developing process. In consequence of 
 this property of iodide of iron as an iodizer, it has been thought to 
 give extraordinary sensitiveness to collodion, but that opinion is 
 erroneous. 
 
 Iodide of Magnesium. Mg. 1=138. When carbonate of 
 magnesia is added to a saturated alcoholic solution of iodide of 
 potassium, double decomposition occurs; iodide of magnesium is 
 formed and held in solution, and insoluble carbonate of potass 
 precipitated. When the solution is filtered, it may be used as an 
 iodizer for collodion. In its properties it resembles iodide of 
 calcium, the nitrate of magnesia formed in the nitrate bath being 
 deliquescent, and having a slightly acid reaction. This iodizer 
 has been pronounced by some operators a very good one, particularly 
 in hot weather. 
 
 Iodide of Potassium; KI=165. 
 
 Iodine and potassium unite energetically, giving out heat and 
 light ; and potassium burns in the vapour of iodine. The result of 
 this combination is the white anhydrous salt — iodide of potassium. 
 The mode of producing this salt commercially is to add iodine to a 
 solution of potass, until it assumes a brown colour ; then evaporate 
 to dryness, add a little charcoal, and fuse the residue at a red heat, 
 which decomposes any iodate of potass that may be formed ; dissolve, 
 filter, and recrystallize the salt. 
 
 Sometimes iodide of potassium is made by passing sulphuretted 
 hydrogen through a brown mixture of liquor potassm and iodine, 
 till it becomes colourless; expelling any excess of sulphuretted 
 
234 
 
 IOD 
 
 hydrogen by beat, filtering, neutralizing with potass, and crystalliz- 
 ing. Another way is, to decompose either iodide of zinc or iodide 
 of iron by carbonate of potass. 
 
 Iodide of potassium crystallizes both in cubes and prisms. The 
 crystals are anhydrous, and very slightly deliquescent in damp air. 
 They are extremely soluble in water, 100 parts of which dissolve 
 at 65° 143 parts of the salt, with production of cold; but much 
 less soluble in alcohol; absolute alcohol S. Gr. 794, dissolving about 
 8 grains to the ounce; S. Gr. 823, about 25 grains; S. Gr. 835, 
 about 60 grains. 
 
 The impurities contained in commercial iodide of potassium are 
 iodate of potass, carbonate of potass, sulphate of potass, chloride of 
 potassium, and sometimes of zinc, and iron. 
 
 Iodate of potass may be detected by adding a little tartaric acid 
 to the solution, and also a little starch. If iodate be present the 
 tartaric acid decomposes it, and the liberated iodine forms purple 
 iodide of starch. Fusion, at a red heat, decomposes iodate into iodide. 
 
 Carbonate and sulphate of potass are detected by adding chloride 
 of barium to the solution, which throws down white insoluble car- 
 bonate, or sulphate of baryta. Another test for carbonate of potass 
 is to expose the alcoholic solution of the iodide to sunshine ; if it 
 soon becomes slightly discoloured it is pure, if not, it contains car- 
 bonate. The presence of much carbonate of potass renders iodide 
 of potassium highly deliquescent. 
 
 Aqueous solution of iodide of potassium is immediately discoloured 
 by the addition of chlorine water, or nitric acid, but not by the 
 weaker acids in the dark ; in the light the discoloration proceeds 
 more rapidly. 
 
 This salt is much used as the iodizer for the negative processes 
 on collodion or waxed paper. The white spots sometimes seen in 
 negatives, and which are square when viewed in the microscope, are 
 supposed to be due to undecomposed crystals of iodide of potassium. 
 These peculiar spots do not occur with other iodizer s. No iodide 
 produces a more sensitive collodion than iodide of potassium, and it 
 is on the whole a very good one to employ. The alcoholic solution 
 is stable in the dark, but collodion iodized with it becomes gradually 
 reddened by time, and rendered insensitive. 
 
 Iodide of Silver. Ag. I = 234. Iodine and silver are sup- 
 posed to combine directly and produce iodide of silver, for if a silver 
 plate be placed in a bottle containing iodine and left for a short time, 
 it is tarnished with a yellow film supposed to be iodide of silver. If 
 this experiment be made in the dark, and the tarnished plate exposed 
 
IOD 
 
 285 
 
 to light, either under a negative or in a camera obscura, and then 
 submitted to the fumes of mercury, an image is developed, which 
 proves that silver tarnished with iodine is sensitive to light. The 
 yellow substance which forms the tarnish on the metal may not, 
 however, be pure iodide of silver, and there are other reasons for 
 supposing that pure iodide of silver is not sensitive to light. 
 
 Iodide of silver may be obtained by double decomposition, by 
 adding a solution of nitrate of silver to a solution of an alkaline or 
 metallic iodide, such as iodide of potassium or cadmium. The 
 oxygen of the oxide of silver in the nitrate goes to the metal and 
 forms potass or oxide of cadmium ; this is converted into nitrate of 
 potass or cadmium by the nitric acid, which leaves the silver, and 
 the iodine combines with the silver to form iodide of silver, which is 
 a yellow insoluble amorphous substance. When the nitrate of silver 
 is in excess the precipitate is of a strong yellow colour, and is dark- 
 ened very quickly to a pale brown tint by exposure to light ; but 
 when the iodide of potassium is in excess the colour of the precipi- 
 tate is rather paler, and it is then absolutely insensitive to light. 
 These results are the same however thoroughly the precipitate may 
 be washed in water, frequently changed, before exposing it to light. 
 For the same reason it happens that an excited collodion film or 
 waxed paper never entirely loses its sensitiveness by washing, be- 
 cause the iodide of silver was formed from a solution containing 
 nitrate of silver in excess ; but an iodized calotype paper, prepared 
 with double iodide, in which iodide of potassium is in excess, appears 
 to be absolutely insensitive to light, at least so far as the power of 
 developing a picture upon it by the ordinary developers is concerned. 
 Iodide of silver, with nitrate of silver in excess, is the most sensitive 
 to light of all compounds at present known, and is therefore used in 
 the negative processes of photography. It is darkened more rapidly 
 in vacuo than in air, and is also darkened to a coffee colour by being 
 boiled for some minutes with distilled water. When iodide of silver is 
 obtained by double decomposition in the manner described, with 
 nitrate of silver in excess, the nitrate of the alkali or metal also formed 
 in the solution, and from which the precipitated iodide of silver is 
 not thoroughly cleared, affects to some extent the properties of the 
 iodide ; for instance, when nitrate of potass remains, which is a 
 neutral salt, the iodide is more quickly darkened than when nitrate 
 of cadmium, which is an acid salt, remains. 
 
 Iodide of silver is insoluble in water and alcohol, and nearly insol- 
 uble in ammonia. It is soluble in concentrated solutions of the 
 alkaline chlorides, bromides, and iodides, and also in hyposulphite 
 of soda, and cyanide of potassium, forming in every case a soluble 
 
236 
 
 IOD 
 
 double salt of silver and the alkali. Concentrated nitric and sul- 
 phuric acids also decompose it. It maybe reduced by metallic zinc, 
 forming soluble iodide of zinc, and metallic silver as a black powder. 
 When iodide of silver is fused at a red heat it acquires a red colour. 
 It volatilizes before the blow-pipe, leaving a little silver only behind. 
 
 When iodide of silver is dissolved in concentrated solutions of an 
 alkaline iodide, bromide, or chloride, the double salt formed is only 
 soluble in the quantity of water used ; if more be added the solution 
 immediately becomes milky. This effect does not happen when either 
 hypo-sulphite of soda or cyanide of potassium is used as the solvent. 
 
 Iodide of Starch. Iodine combines with starch, and forms a 
 purple compound. Starch is generally used as the test for free iodine. 
 When paper containing starch is immersed in a solution of iodide of 
 potassium it speedily assumes a purple tint, the strength of which 
 depends upon the quantity of starch present in the paper. Also, 
 wherever nuclei of starch exist deep purple spots are produced. Pa- 
 pers which have been bleached with chlorine are also reddened by a 
 solution of iodide of potassium. 
 
 Iodide of Zinc ; Zn. 1=158*6. Zinc and iodine have a very 
 strong mutual affinity. When iodine is added to a mixture of zinc 
 tilings and water, until it becomes slightly brown, and a gentle heat 
 applied, iodide of zinc is produced. On evaporating the solution to 
 dryness in a retort the iodide is fused, and at a higher temperature 
 volatilized ; it condenses in prismatic crystals. 
 
 The aqueous solution of this iodide is very unstable in the air ; 
 oxide of zinc being formed and free iodine liberated. Both the iodide 
 and nitrate of zinc are deliquescent. This iodizer is sometimes used 
 in photography, but its instability is an objection to it. It is not 
 always free from iodide of iron, which discolours and spoils the nitrate 
 bath. 
 
 Iodine, (Greek noting violet coloured,) 1=126. This elementary 
 substance is contained in minute quantity in sea water, and some 
 mineral springs, but enters more largely into the composition of cer- 
 tain sea weeds, chiefly found on the western coasts of Ireland and 
 Scotland ; a minute trace has also been found in cod liver oil, and in 
 the yellow sap of the “ Indus fetedissimus” 
 
 Iodine is chiefly manufactured in Glasgow, from kelp. (See “Kelp.”) 
 This is lixiviated in cold water, and the solution gradually evaporated, 
 when the various salts which it contains, and which are less soluble 
 than the iodides, crystallize and are removed in succession. The dark 
 
IOD 
 
 IEO 
 
 237 
 
 mother liquor which remains contains the iodides. Prom these the 
 iodine is separated by distilling the liquid with sulphuric acid and 
 black oxide of manganese. The iodine passes over as a violet coloured 
 vapour which condenses in the alembic and neck of the retort in long- 
 pointed crystals of a dark colour, opaque, and having a metallic lustre. 
 These are washed out with a little water, and dried with blotting pa- 
 per. The process is minutely described in Dr. Graham 5 s Elements 
 of Chemistry. 
 
 Iodine is sparingly soluble in water, but dissolves readily in alcohol 
 and ether. The commercial sample may be purified by disssolving 
 it in alcohol and precipitating it with water. The impurities, or adul- 
 terations, are principally plumbago, sulphide of antimony, and black 
 oxide of manganese, all of which are insoluble in alcohol. Iodine is 
 dissolved by solutions of the alkaline iodides. 
 
 Iodine is extremely volatile when moist. At 120° it rises rapidly 
 into vapour, at 220° fuses, and at 350° boils and produces dense 
 violet coloured fumes which crystallize in brilliant plates and acute 
 octohedra. The aqueous solution of iodine does not evolve oxygen 
 when exposed to sunshine, nor has it bleaching properties. 
 
 Iodo-nitrate or Silver; Ag.O, N0 5 -|- Ag.I. A boiling satu- 
 rated solution of nitrate of silver will dissolve a small quantity of 
 iodide of silver, and, on cooling, crystals are formed of a double salt 
 which has been called Iodo-nitrate of silver. This salt is also formed 
 when an excited collodion plate or waxed paper is allowed to dry 
 spontaneously, without having been previously washed. In this 
 case the free nitrate on the film or paper becomes concentrated by 
 the evaporation of the water, and is then able to dissolve the iodide 
 of silver, and form a double salt. 
 
 In the latter case, the iodo-nitrate of silver is not decomposed by 
 water and iodide of silver precipitated, nor is it blackened by light, 
 but on the contrary appears to be perfectly insensitive to light. 
 Nevertheless it has been affirmed by Dr. Schnauss that the opposite 
 results are obtained in the former case. An old nitrate bath satu- 
 rated with iodide of silver is certainly rendered turbid by the addition 
 of water. These matters require further investigation. 
 
 Ioduretted Iodides. See “ Hydriodous Acid.” 
 
 Iron. Ee. = 28. This useful metal may be said to occur every- 
 where in combination of some kind. Many of its salts are of use in 
 photography ; these are the chlorides, sulphates, nitrates, iodide, ace- 
 tate, citrate, and oxalate ; q. v. 
 
238 
 
 IKO 
 
 A surface of polished iron is not oxidized in dry air, nor in dis- 
 tilled water which has been freed from air by boiling, but in damp 
 air it is oxidized or rusts very rapidly. This is prevented by the 
 addition of an alkali, as lime, or potass, to the water, and iron will 
 then retain its lustre under water for years ; but acids and neutral 
 salts have an opposite tendency. The reason of this is very obscure. 
 By the contact of zinc, iron is protected from all common sources of 
 oxidizement and corrosion ; hence the value of the process by which 
 iron is “ galvanized,” or coated with zinc by being immersed in melted 
 zinc. 
 
 The tests for iron are prussiate of potass, red or yellow, which 
 give a blue precipitate, and infusion of galls which gives a black 
 colour to the solution, 
 
 Iron, Oxide of. There are four definite oxides of iron viz : — 
 
 The Protoxide . . . Fe. O 
 
 Sesquioxide. . . Fe. 2 0 3 
 
 Black oxide . . Fe. 3 0 4 
 
 Ferric acid . . . Fe^ 0 3 * 
 
 The salts of the sesquioxide of iron are in general converted into 
 salts of the protoxide by the action of light ; and, conversely, the 
 salts of the protoxide act as developers by acquiring oxygen, and 
 becoming converted into per salts. The solutions of these salts are 
 generally unstable in the air, and become oxidized. The most 
 important of them is the proto-sulphate. 
 
 Hydrated protoxide of iron is a white precipitate formed by adding 
 caustic potass to a solution of a pure proto salt. When washed and 
 dried it assumes a greenish colour, and is very readily oxidized, 
 becoming dark-green or brown. 
 
 Sesquioxide, or peroxide, occurs native as haematite, (a hydrate,) 
 and also in beautiful crystals, as specular iron ore, in the island of 
 Elba. It may be produced by adding ammonia to perchloride of iron 
 and washing, drying, and igniting the precipitated hydrate. This 
 oxide is red, and forms the pigment called “ Venetian Bed.” Iron 
 rust is the sesquioxide of the metal. It forms salts which have an 
 acid reaction, and are decomposed by light. They are generally of 
 a brown colour. 
 
 Black oxide of iron, or magnetic oxide, is a compound of the 
 protoxide and sesquioxide. It is a natural product, occurring in 
 regular octohedral crystals, and is incapable of forming proper salts. 
 The black scales of iron picked up in a blacksmith’s forge approach 
 black oxide in composition. The loadstone is black oxide of iron. 
 
1ST 
 
 IYO 
 
 239 
 
 Ferric acid is produced by beating to full redness, for an hour, 
 in a covered crucible, a mixture of one part of sesquioxide of iron, 
 and four parts of dry nitre. This forms an unstable amethystine 
 coloured salt, which is ferrate of potass. When added to a salt of 
 baryta, permanent ferrate of baryta is produced, which is of a 
 crimson colour, and insoluble. 
 
 Isinglass. Fish glue ; a pure form of gelatine. It is soluble 
 in boiling water, and gelatinizes on cooling ; and also soluble in 
 weak acids, but is precipitated by the addition of an alkali. Isinglass 
 is obtained from the air bladders of sturgeons, and principally from 
 the great sturgeon found in the Caspian Sea. It is manufactured 
 chiefly at Astracan, and is bleached by the fumes of burning sulphur. 
 The process of manufacture consists in first steeping the bladders in 
 water, then removing the outer skin, putting them into a hempen 
 bag, squeezing them, softening them between the hands, and twist- 
 ing them into small cylinders, which are afterwards bent into the 
 shape of a lyre. 
 
 Isomerism. (Greek ia og equal, fiepog a part.) There are 
 numerous instances in chemistry in which two bodies are identical 
 in composition, but different in appearance and properties. Such 
 bodies are said to be “ isomeric. 55 
 
 For instance, cyanogen, which is a gas, is represented by the 
 formula C 2 N, while paracyanogen, which is a black solid having 
 different properties, has also the formula C 2 N. 
 
 The difference in properties between isomeric substances is sup- 
 posed to proceed from a difference of arrangement of their atoms. 
 
 Isomorphism. (Greek ktoq equal, jutoptir) form.) There are many 
 instances in chemistry in which similarity of properties is possessed 
 by bodies, or classes of bodies, having the same form. This is 
 remarkably shewn in the case of the chlorides, iodides, and bromides, 
 which are “ isomorphous ; 55 the term being applied to bodies which 
 can replace each other in combination, without producing any 
 essential difference in the crystalline form of the compound. ' Bodies 
 may therefore be classed in isomorphous groups, the individuals of 
 which possess, in many respects, similar properties. 
 
 Ivory. Ivory is the osseous matter of the tusks and teeth of 
 the elephant, hippopotamus, wild boar, several species of seal, and 
 the horn of the sea-unicorn. The latter furnishes the best ivory. 
 
240 
 
 IYO 
 
 The composition of ivory is the same as that of the teeth of 
 animals, viz. 3 parts phosphate of lime, and a little carbonate, with 
 1 part cartilage. 
 
 Photographs may be taken upon plates of polished ivory, and they 
 have remarkable beauty and delicacy. 
 
 Ivory is liable to turn yellow by time. Its whiteness may be 
 restored by moistening and rubbing it with - pumice stone, and 
 exposing it to sunshine while moist, under a glass shade, carefully 
 sealed to the stand at the bottom, in order to retain the moisture 
 and prevent the ivory from cracking. 
 
 Collodion positives may be taken upon dyed ivory. It may be 
 dyed black by boiling it first in a decoction of logwood, and then in 
 a solution of red acetate of iron ; or violet, by first boiling it for a 
 short time with proto-chloride of tin, and then in logwood. The 
 sheets should be dyed first, and polished afterwards. 
 
 Ivory, Artificial. This has been made for photographic pur- 
 poses by mixing sulphate of baryta with albumen, and rolling it 
 into sheets. 
 
 Pinson’s artificial ivory, much used in Prance, is made by immers- 
 ing a sheet of gelatine in alumina dissolved in acetic acid. The two 
 combine, and when the gelatine has imbibed sufficient of the alumina, 
 it is taken out, hung up to dry, and finally polished. 
 
 Artificial ivory may also be made by mixing ivory dust with 
 albumen, and rolling the paste into sheets, then drying and polishing 
 them. 
 
 Ivory, Plexible. Ivory articles may be rendered flexible and 
 semi-transparent by immersing them in a solution of pure phosphoric 
 acid, S. Gr. 1T30, and leaving them till they lose their opacity. 
 They are then washed with water, and dried, when they become as 
 flexible as leather. They are hardened on exposure to dry air, but 
 resume their flexibility on being immersed in hot water. 
 
 Ivory, Vegetable. The milk of the fruit of the Phytelephas 
 mucrocarpa which grows in Central America, hardens and becomes 
 like ivory. It is frequently used as a substitute for ivory in buttons 
 and small turned articles. , ^ 
 
 Ivory Black. Ivory dust carefully calcined in a closed 
 crucible. 
 
JAP 
 
 JUI 
 
 241 
 
 Japan. Black japan for leather is made thus : — 
 
 1st Method. 
 
 Mix Boiled linseed oil . . .1 gallon. 
 
 Burnt umber . . . . * 8 ozs. 
 
 Asphaltum . . . . .3 ozs. 
 
 Boil together, and add sufficient oil of turpentine to give it the 
 proper consistence. 
 
 ,2nd Method. 
 
 Mix Oil of turpentine 
 Shellac 
 Alcohol . 
 
 Lamp black 
 
 2 ozs. 
 
 1 dram. 
 4 ozs. 
 i oz. 
 
 Jew’s Pitch. Bitumen Judaicus : Asphaltum; q. v. 
 
 Jeweller’s Plate Powder. See “Rouge.” 
 
 Jeweller’s Putty. Tin putty, an oxide of tin, made by levi- 
 gating the crusts of oxide that form upon the metal when kept for 
 some time in fusion. It is used for polishing hard bodies. 
 
 Juices oe Plants. Papers stained with the juices of various 
 plants are altered in colour by exposure to sunshine. Sir John 
 Herschel, some years ago, communicated to the Royal Society the 
 results of many interesting experiments made by him on the juices 
 of plants. His method of proceeding was, to crush the petals of the 
 flower to a pulp in a marble mortar, either alone, or with the addi- 
 tion of alcohol, and to express the juice by squeezing the pulp in a 
 linen cloth ; it was then spread upon paper with a flat brush, and 
 allowed to dry spontaneously in the dark. The tint communicated 
 to the paper is not always that of the petal of the flower. The most 
 sensitive colour met with was that of the Corchosus Japonic a , the 
 yellow tint of which is speedily bleached by sunshine. Paper stained 
 with the blue tincture of the double purple groundsel is completely 
 bleached by light. The juice of the Senecio splendidus imparts a 
 rich and deep velvety purple tint to paper, which is very insensitive 
 to light. 
 
 We may add that the juice of the RJius striata , of South America, 
 and of the Rhus venenata , or poison tree, of North America, both 
 communicate indelible black stains to paper. The latter has been 
 used for marking linen. 
 
 R 
 
242 
 
 KAO 
 
 KKE 
 
 Among the juices of plants are many important dyes, such as 
 indigo, madder, saffron, &c., &c., many of which are bleached more 
 or less by light. In the case of indigo, however, the expressed 
 juice from the leaf of the indigo plant is originally colourless, but is 
 darkened to a deep purple tint by exposure to air and light. This 
 dye is very permanent. 
 
 Kaoltn. Porcelain clay, used for decolorizing discolored 
 nitrate baths, and other solutions. The best way of using it is to 
 add a little to the solution, shake well up together several times, 
 and then let it stand to settle ; afterwards decant the clear liquid. , 
 
 Kaolin is a fine pure white clay prepared by levigation from 
 mouldering granite, and used for making vessels of porcelain. It 
 contains nearly equal parts of alumina, or pure clay, and silica, 
 together with about one per cent, of lime and oxide of iron. It is 
 found in veins in primitive mountain districts, chiefly in China and 
 Japan, Saxony, the neighbourhoods of Limoges and Bayonne in 
 Prance, and in England in the counties of Devon and Cornwall. 
 We have also found it in Jersey. 
 
 Kata-Positive. A positive intended to be viewed by reflected 
 light, that is, to be looked at , may for convenience be called a kata- 
 positive, in contradistinction to a u dia -positive, 5 ’ which is intended 
 to be looked through , or viewed by transmitted light. The adop- 
 tion of these terms would, in many cases, prevent confusion. See 
 “ Dia-positive. 55 
 
 Kelp. British barilla: the cinder ash obtained by burning 
 various species of sea-weed, but chiefly the Fucus vesiculosus , or 
 “ bladder-wrack, 55 and the Fucus nodosus , found on the western coasts 
 of Ireland and Scotland. It is the principal source of iodine, and 
 contains chiefly soda salts. Pearlash, or wood-ashes, contains 
 principally salts of potass. Soda was formerly obtained from kelp, 
 and the manufacture of this substance was then extensively carried 
 on by the inhabitants of the west coast of Scotland ; but since the 
 method of obtaining soda from common salt has been introduced 
 the manufacture of kelp has greatly declined. 
 
 Kreosote, Creasote. (Greek, Kpeag flesh, orwfw to save.) 
 This substance is obtained from the distillation of wood tar, by a 
 troublesome process which need not be described in this work. It 
 is a colourless, transparent liquid, of great refractive and dispersive 
 power, having a caustic taste, and a strong odour resembling that 
 
LAC 
 
 243 
 
 of smoked meat. It combines readily with acetic acid and ammonia ; 
 and alcohol, ether, naphtha, and acetic ether dissolve it in all propor- 
 tions. It dissolves many salts, also the resins, camphor, essential 
 oils, and almost all vegetable colouring matters, and coagulates 
 albumen and casein. It mixes sparingly with water. 
 
 The chief use of kreosote is derived from its powerful antiseptic 
 properties. It prevents the putrefaction of animal and vegetable 
 matters, and a drop or two may sometimes be added with advantage 
 to solutions of gelatine, iodizing solutions for waxed paper, ink, 
 &c., &c. A substance similar to kreosote is supposed to have been 
 used by the ancient Egyptians for the purpose of embalming 
 mummies. 
 
 Lac. A species of resin secreted from the puncture made by an 
 insect called the Coccus ficus , in the branches of various plants 
 which grow in the East Indies. 
 
 Lac is a very important article in manufactures, because not only is 
 the resin useful for making varnishes, sealing wax, &c., but the 
 colouring matter combined with it forms a magnificent red dye, 
 which is extensively used as a substitute for cochineah In conse- 
 quence of the puncture of the insect, the twig of the plant becomes 
 encrusted with a red resinous substance, sometimes as much as a 
 quarter of an inch thick. These encrusted twigs are called “ stick- 
 lac. 55 It contains about 7 per cent, of resin, and one-twentieth part 
 of that quantity of colouring matter. This valuable colouring mat- 
 ter is removed by pulverizing the stick-lac, and triturating it with 
 water. What remains is called “ seed-lac. 55 In India this is put 
 into oblong bags of cotton stuff, and a man at each end of the bag 
 holds it over a charcoal fire. The liquefied resin drops through, and 
 is received upon smooth stems of the banyan tree ; in this way it is 
 formed in thin plates, and is called “ shell-lac. 55 
 
 Shell-lac consists of two or more resins, one of which is soluble in 
 all proportions in anhydrous alcohol, ether, and volatile and fat oils. 
 The alcoholic solution is of a reddish colour, and forms an excellent 
 spirit varnish in photography. (See “ Tarnish. 55 ) Shell-lac may 
 also be easily dissolved in dilute hydrochloric and acetic acids, and 
 also in a solution of borax, with the aid of heat. Before dissolving 
 it in alcohol, it should be very finely pulverized and exposed to the 
 air for some months. Shell-lac may be bleached with chlorine, and 
 then makes colourless varnishes. The bleaching is effected by first 
 dissolving it in caustic potass, and then passing chlorine gas through 
 the solution, after which it is pulled and washed in hot water, and 
 twisted into sticks. 
 
 n 2 
 
244 
 
 LAC 
 
 Shell-lac is much used in making sealing wax ; q. v. 
 
 Lac dye is made by evaporating to dryness the liquid in which 
 the pounded crude lac is digested, and making the residue into 
 cakes. This is soluble in dilute sulphuric or hydrochloric acid, and 
 the mordant is protochloride of tin, and bi-tartrate of potass. It 
 gives a fine scarlet colour. 
 
 Lacquer. A varnish composed chiefly of shell-lac dissolved in 
 alcohol, and coloured with gamboge, dragon’s blood, &c. The fol- 
 lowing is a good formula : — 
 
 Seed lac . . . 120 parts 
 
 Gamboge . . . . 120 „ 
 
 Dragon’s blood . . . 120 „ 
 
 Saffron 39 „ 
 
 Kectified alcohol . 
 
 . 1000 „ 
 
 Digest with heat, and strain. 
 
 The following is another formula 
 
 in which turpentine 
 
 solvent : — 
 
 Seed lac 
 
 . 120 parts 
 
 Sandarach 
 
 . 120 „ 
 
 Dragon’s blood . 
 
 • 15 „ 
 
 Turmeric 
 
 • 2 „ 
 
 Gamboge . . ; 
 
 • 2 „ 
 
 Venice turpentine . 
 
 . 60 „ 
 
 Spirits of turpentine 
 
 . 1000 „ 
 
 Digest with heat, and strain. 
 
 Lactate of Silver. A salt of lactic acid and oxide of silver. 
 It forms white acicular crystals, which are very soluble in water, and 
 quickly blackened by light. 
 
 Lactate of Soda. A salt of lactic acid and soda, formed by 
 decomposing acetate of soda by lactic acid. It is very soluble in 
 water. A small quantity added to the nitrate bath produces the 
 effects due to an organic salt of silver, viz., intensity in the blacks of 
 the negative, and want of sensitiveness. The double decomposition 
 of lactate of soda and nitrate of silver produces lactate of silver and 
 nitrate of soda. 
 
 Lactic Acid. C 6 H 5 0 5 + HO. A free acid contained in 
 sour milk ; also in certain animal fluids, and sour rice-meal, wheat- 
 paste, the juice of the beet-root, &c. 
 
LAM 
 
 LAT 
 
 245 
 
 To obtain it, sour whey is evaporated to one-sixth of its volume 
 and filtered. A little lime is added to precipitate phosphoric acid, 
 and the excess of lime removed by oxalic acid. The liquid is then 
 filtered and evaporated to the consistence of syrup, after which the 
 lactic acid is extracted by means of alcohol, in which it is soluble. 
 Pure lactic acid is then obtained from the alcoholic solution by a 
 somewhat complicated process, which need not be described in this 
 place. 
 
 Pure lactic acid is a colourless liquid, of the consistency of syrup, 
 inodorous, extremely sour, deliquescent, miscible in all proportions 
 with water and alcohol, but less soluble in ether. It forms in ge- 
 neral soluble salts. Its S. G. is 1*215. 
 
 Lamp Black. A form of charcoal, obtained by burning the 
 refuse resin left in the distillation of turpentine, and collecting the 
 smoke in chambers hung with old sacking, upon which the soot is 
 deposited. This is swept off from time to time, and is commercial 
 lamp black. 
 
 When lamp black has been heated to redness, it is a very pure 
 form of charcoal, and burns entirely away without leaving an 
 ash. Its principal use is in the manufacture of printer’s ink (q. v.). 
 A black water-colour is made of it by mixing it with honey ; and 
 also Indian ink (g. «.). It will probably, before long, be p.sed exten- 
 sively in photography in the process of printing in carbon. 
 
 Lamp black mixed with French polish is used by opticians for 
 blackening the brass work of optical instruments ; and mixed with 
 size for blackening woodwork. These mixtures leave no gloss when 
 dry. 
 
 Latent Heat. Latent Light. It is stated in the article on 
 Light, that both light and heat are certainly due to the undulations 
 of an elastic ether which pervades space ; and that the rays, both of 
 light and heat are propagated with enormous velocity, and can be 
 refracted, reflected, polarized, &c., by similar means. We have 
 suggested, therefore, that heat may be an undulation of the same 
 universal ether as that in which light is propagated, the difference 
 being that the waves of heat are longer than those of light. There 
 is however this difference between light and heat, that whereas bodies 
 when exposed to radiant heat become heated, that is to say become 
 themselves sources of radiant heat, bodies exposed to light do not in 
 general become self-luminous, or sources of light ; (we say in general 
 because solar phosphori offer a few exceptions to the rule). This 
 may be in consequence of the light received by bodies being converted 
 
246 
 
 LAT 
 
 into heat ; the short waves of light which vibrate in the interstices of 
 a body being lengthened into waves of heat by the mechanical effect 
 of the atoms of the body vibrating amongst them. 
 
 When a body is heated it in general expands, and contracts on 
 cooling, that is, radiates heat while returning to its former dimensions. 
 It is in the highest degree probable that the Sun once extended far 
 beyond the limits of the planetary system, its temperature being then 
 very little above that of space ; that it gradually contracted in dimen- 
 tions, and by so doing became a source- of heat and light to the 
 planets which were from time to time detached from it ; that this 
 contraction of volume may be still going on, while the amount of light 
 and heat propagated by the condensation of the solar materials may 
 be for ages to come continually on the increase, after which a reac- 
 tion may occur similar to what has happened in the case of the earth, 
 which was once in a fluid incandescent state, propagating light and 
 heat into space by its contraction of volume, which volume at last 
 attained a minimum, the surface of the globe cooled into a solid 
 crust, and the earth became as we find it, non-luminous and opaque. 
 
 Matter , then, in motion can act upon ether and communicate 
 vibrations to it ; on the other hand, ether in motion can act upon 
 matter and communicate vibrations to it, altering its form, volume, 
 &c. Among these actions and reactions between gross matter and 
 subtle ether we must endeavour to seek for an explanation of the phe- 
 nomena due to the imponderable agents. 
 
 When a body is heated above the temperature of surrounding ob- 
 jects it gradually cools until an equal temperature is obtained amongst 
 them, in which state the atoms of ether pulsate in unison with those 
 of the bodies. 
 
 Heated bodies do not all cool at the same rate. The time in which 
 a body at a certain temperature cools, when suspended in vacuo, to 
 a oertain lower temperature, is called its “ Specific Heat.” The 
 following table shews the specific heats of different bodies. 
 
 Water . . 
 
 . . 1000 
 
 Zinc . * . 
 
 . 93 
 
 Sulphur. . . 
 
 . . . 188 
 
 Silver. . , 
 
 . 56 
 
 Glass . , . 
 
 . , 117 
 
 Mercury . , 
 
 , 33 
 
 Iron . 
 
 
 Platinum. . 
 
 • 31 
 
 Copper . . , 
 
 , . , 95 
 
 Lead . . . 
 
 . 29 
 
 Experiments seem to indicate that the specific heat of a body varies 
 inversely as its atomic weight. Eor instance, the atomic weight of 
 sulphur is 16, of lead 103 i the specific heats of these substances 
 being as 188 : 20; and the ratio of 29 : 188 being nearly equal to 
 that of 16 : 103. 
 
LAT 
 
 247 
 
 Now we come to tlie particular subject of this article, viz., “ La- 
 tent Heat.” 
 
 Strong brine, i. e. a saturated solution of common salt in water, 
 freezes at 0° of Fahrenheit’s scale ; water at 32°. Suppose then that 
 two vessels, one containing water, the other brine, and each having 
 a thermometer immersed in the liquid marking the temperature, say 
 at 60°, are taken out-of-doors on a winter’s day, when the temperature 
 is 20°. The thermometers at once begin to fall ; that in the brine 
 goes on continually falling until it reaches 20°, while that in the 
 water exhibits a curious phenomenon ; it first sinks gradually until 
 the water begins to freeze, then remains stationary until the whole 
 of the water is congealed or crystallized into ice, (or it may rise a 
 little during the process,) and afterwards goes on falling as before 
 until the temperature of the ice becomes 20°. It remains therefore 
 to explain this curious phenomenon, which accompanies the change of 
 form of a body from the liquid to the solid state. 
 
 It has been supposed, we think erroneously, that water contains 
 “.latent heat,” that is, material atoms of heat in chemical combination 
 with it, and that these are given off during the process of congelation. 
 This supposition of “ latent heat ” is evidently only tenable on a sort of 
 corpuscular theory of heat, for on the undulatory theory of heat “ latent 
 heat,” would be synonymous with latent motion, and the idea woidd 
 involve a contradiction. There cannot be latent heat on the undulatory 
 theory, and it remains for us to offer another explanation which shall 
 be consistent with that theory. 
 
 The facts are simply these : — During the time which is occupied 
 by the water in changing its form from the liquid to the solid state 
 it does not propagate any undulations of heat in the surrounding 
 space, and thereby become colder. The propagation of undulations 
 of heat by the action of the atoms of water upon those of the ether 
 contained in it is for a time arrested. It is only necessary to state 
 the fact in the above form. No further explanation is required. 
 
 Conversely : if a lump of Wenham Lake ice at a temperature of 
 10° be brought into a warm room, it does not begin to melt until 
 the temperature has risen to 32°, and it remains stationary at that 
 temperature until the whole of the ice is melted ; the temperature of 
 the water then begins to rise. The explanation is the same as before. 
 
 “ Latent heat ” maybe a convenient term to employ, but th z facts 
 should not be misunderstood. 
 
 It would be out of place to pursue this subject further, but all 
 the. phenomena of latent heat may be explained in the way we haVe 
 pointed out. 
 
* 
 
 248 LAY 
 
 “ Latent light ” is another expression which has been adopted 
 somewhat hastily in attempting to explain some phenomena of light 
 lately discovered by M. Niepce de St. Victor. Certain substances 
 called “ Solar phosphori ” have the well known property of becoming 
 self luminous by being exposed to light ; just as a piece of slate has 
 the property of becoming hot by being held before the tire. The 
 piece of slate when removed from the tire radiates heat ; — the solar 
 phosphorus, when removed from the sunshine and taken into a dark 
 room, radiates light. The two cases are strictly parallel. This 
 property of solar phosphori has been long known, but M. Niepce 
 has discovered lately that white paper, and other substances, become 
 actinic , after having been exposed to sunshine, and in this state 
 react upon sensitive photographic tablets in the dark room. These 
 substances have been said to absorb light by insolation, and retain 
 it as “ latent light 55 within their pores ; but this explanation involves 
 the same absurdity as the idea of “ latent heat.” The fact probably 
 is, that the insolated substance does not convert the actinic rays 
 into heat rays, as bodies mostly do, but propagates them unchanged, 
 when taken into the dark, and thereby produces the effects due to 
 actinism, when laid upon a sensitive photographic paper or plate. 
 The experiment described by M. Niepce, in which light is supposed 
 to be potted in a sealed cylinder, and liberated after any interval of 
 time by removing the cover, may be explained thus : — 
 
 The inside of a hollow cylinder of tin is lined with paper which 
 has been steeped in a solution of nitrate of uranium. It is then ex- 
 posed to sunshine for some time, and sealed up. On removing the 
 sealed cover, in the dark room, some months afterwards, and apply- 
 ing the open end of the cylinder to a piece of sensitive chloride 
 paper, it is darkened all over ; or if an engraving on india 'paper be 
 laid upon it, so as to act as a negative placed between it and the 
 contents of the tube, an image of the engraving is obtained. The 
 explanation is, that the nitrate of uranium in the paper is deoxi- 
 dized by light, and converted into a proto-nitrate of uranium. This 
 becomes oxidized again in time, and converted into a per-salt, at the 
 expense of the oxygen contained in the aqueous vapour in the tube, 
 thereby liberating hydrogen, which darkens the chloride paper, or 
 prints an image upon it through india paper. 
 
 To seal up light in a pot would be the same thing as to seal 
 up motion , for light is motion. Such an idea is surely absurd. 
 
 Lavender Bays. The faintly luminous rays beyond the violet 
 end of the spectrum are called “ lavender rays.” They are said to 
 
LEA 
 
 249 
 
 be highly actinic, but the maximum of actinic power lies probably 
 within the violet rays ; this, however, may depend upon the nature 
 of the substance acted on. 
 
 Lead. Pb.=104. This useful metal has been known from 
 the earliest times, and was called by the alchemists “ Saturn.” It 
 is found chiefly in the form of sulphide, or “ galena.” Pure lead 
 may be obtained by heating oxalate of lead in a covered crucible. 
 Lead may be rolled into sheets, and drawn into wire ; it melts at 
 612°; and by the joint action of heat and air is easily oxidized, 
 although at ordinary temperatures it undergoes but little change from 
 exposure to air. When in a state of fine division it combines readily 
 with oxygen, and takes fire in the air. When immersed in distilled 
 water it takes oxygen from the air contained in the water, and the 
 oxide formed being soluble to some extent in the water it becomes 
 impregnated with oxide of lead. Hence arises the poisonous cha- 
 racter of water which has been collected in a leaden cistern, or 
 passed through leaden pipes. The most useful salts of lead in 
 photography are the acetate, nitrate, and chloride ; q. v. 
 
 Lead, Oxides of. There are 5 oxides of lead, viz. : the sub- 
 oxide, Pb. 2 O; the protoxide, Pb. 0 ; the red oxide, Pb. 3 0 4 ; the 
 sesquioxide, Pb. 2 0 3 ; and the peroxide, Pb. 0 2 . Of these the prot- 
 oxide only is salifiable. Litharge is protoxide of lead ; and red lead, 
 or “ minium,” the red oxide. Peroxide of lead, or plumbic acid, is 
 . deoxidized by light, and reduced to protoxide. Suboxide of lead is 
 the grey powder formed on the surface of lead after long exposure 
 to air. 
 
 Leaden Pipes, Cisterns, &c. When pure rain water is col- 
 * lected in leaden tanks, or passed through leaden pipes, the lead is 
 oxidized by the air contained in the water, and a small quantity of the 
 oxide formed is dissolved in the water, thereby communicating to it 
 poisonous qualities. The same thing does not, however, happen to 
 the same extent when river or spring waters, which contain in general 
 certain soluble salts, are retained in leaden vessels, because then the 
 surface of the lead soon becomes coated with an insoluble precipi- 
 tate of sulphate, or carbonate, or phosphate of lead, which protects 
 it from being further acted on by oxygen. 
 
 The presence of oxide of lead in the water used for photographic 
 x purposes may in some cases be rather beneficial than otherwise. 
 Por instance, in the pyrogallic developer, when freshly made, it 
 would increase the energy of the development ; but the developer 
 
250 
 
 LEA 
 
 would not keep ; in a few hours, perhaps, it would become discoloured. 
 Similarly, oxide of lead in the nitrate bath would perhaps increase 
 the sensitiveness of the collodion film, but render the bath liable to 
 “ fog ” the picture. 
 
 Lead may be easily detected in water by tests which the photo- 
 grapher generally has at hand. Iodide of potassium would give a 
 yellow tinge, sulphuric acid a white cloudiness, sulphide of ammo- 
 nium a dark discoloration to water containing oxide of lead in 
 quantity sufficient to render its presence sensibly injurious, or other- 
 wise. 
 
 Any brass work in contact with lead would set up electric action, 
 and assist the production of oxide of lead. 
 
 Bottles should not be made of flint glass, as some chemicals act 
 on the lead which it contains. There is no lead in crown glass, 
 and this should always be used for bottles intended to contain 
 chemicals. 
 
 Achromatic lenses should never be exposed to the fumes which 
 sometimes prevail in the laboratory, or dark room. The surface of 
 the flint lens is frequently injured from this cause, and becomes 
 iridescent. 
 
 Leather. An insoluble compound of tannin and gelatine. 
 Tannin is a very delicate test of gelatine, and if added to water 
 containing the one five-thousandth part of gelatine it produces a 
 cloudiness in it. It combines so energetically with gelatine, that 
 if a piece of skin or bladder of sufficient size be suspended in a solu- 
 tion of tannin the whole of it is in time abstracted from the water, 
 and the skin becomes converted into leather. 
 
 In the manufacture of leather the skins of animals are first steeped 
 in lime water, in order that the hair and cuticle may be more easily 
 separated, and then softened by allowing them to enter into a certain 
 state of putrefaction, or by the action of acids. They are then 
 immersed in an infusion of oak bark, which contains tannin, and in 
 time converts them into leather. The process is continued sometimes 
 for several months, the infusion being renewed from time to time, 
 and is not arrested until the entire substance of the skin has become 
 brown throughout. Sometimes infusions of other astringent veget- 
 ables containing tannin, such as valonia, sumach, catechu, and the 
 barks of the willow, birch, elm, and Spanish chesnut, are substituted 
 for oak bark. 
 
 This is ordinary tanning : but there are other kinds of leather, 
 as follow : 
 
 Tawed leather. An insoluble compound of gelatine and chloride 
 
LEA 
 
 LEN 
 
 251 
 
 of aluminium. The skin is first steeped in an alkaline liquor, then 
 in a solution of alum and common salt, which react on each other 
 and produce chloride of aluminium and sulphate of soda. White 
 glove leather is prepared in this way. 
 
 Wash leather. The skin is prepared and softened in the usual 
 way, then steeped in oil, and afterwards in an alkaline solution. 
 
 Curried leather. Common tanned leather while still moist is 
 besmeared with oil, which as the water evaporates, takes its place 
 in the pores of the leather and renders it waterproof. This is also 
 called “ upper-leather.” 
 
 Leather Varnish. If a sheet of paper, or a positive on 
 paper, be first immersed in a solution of gelatine, dried, and then 
 steeped in a solution of tannic acid, the gelatine will be rendered 
 insoluble, and converted into a species of leather varnish ; and by 
 repeating the operation several times a considerable glaze may be 
 communicated to the paper. This varnish, by filling the pores of 
 the paper and rending it better able to resist the action of moisture 
 and external agents, would no doubt tend to the preservation of the 
 proof. Gallic acid has no such action upon gelatine, and must not 
 be substituted for tannic by mistake. 
 
 Lemon Juice. This is added to the nitrate bath in the process 
 of printing by development upon plain paper ; q. v. It contains about 
 6 per cent, of citric acid, some mucilage, and possibly a little oil of 
 lemons ; also a white, tasteless substance termed “ hesperidin,” 
 which is soluble in 60 parts of hot water, and also in alcohol, and ether. 
 
 Lens. A lens is a transparent body constructed for the purpose 
 either of concentrating or scattering rays of light transmitted through 
 it. 
 
 Lenses are in general made of glass or rock crystal, and ground 
 with spherical surfaces ; the axis of the lens being the line joining 
 the centres of the spheres, aud therefore a line with respect to which 
 the lens is symmetrical. 
 
252 
 
 LEN 
 
 In the above figure six different forms of lenses are exhibited ; 
 and light is supposed to pass through them in the direction shewn 
 by the arrow.. 
 
 No. 1 is called “ bi-convex ; No. 2, plano-convex ; Nos. 3 and 4, 
 concavo-convex, or more frequently “ meniscus No. 5, concavo- 
 plane; No. 6, double concave. The nature of the surface upon 
 which light is incident determines the first word of the compound 
 appellative. 
 
 The first three lenses are thicker in the middle than at the edge, 
 and are called “ converging lenses, 55 because they cause pencils that 
 are refracted through them to converge more than they did before. 
 Nos. 4, 5, 6, are thicker at the edge than in the middle, and are 
 called “diverging lenses, 55 because they cause pencils that are 
 refracted through them to diverge more than before. 
 
 It has been shewn in the article on the prism that when a ray of 
 light is refracted through a prism it is turned, or caused to deviate, 
 from the edge of the prism. Now, we may suppose that the small 
 portion of the spherical surface of a lens, on which a ray of light is 
 incident, coincides with the tangent plane to the sphere at that 
 point; and similarly with respect to that small portion of the 
 opposite spherical surface from which the ray emerges. 
 
 
* LEN 
 
 253 
 
 small parts of the lens at E and S may be supposed to coincide with 
 the tangent planes at those points, and these planes will in general 
 intersect, if produced, in a straight line, thereby forming a prism 
 whose refracting angle is SAE. Now, if* we draw Sd parallel to 
 QE, the emergent ray ST will have its angle of deviation, dST, 
 from the edge of the prism, and will follow the course indicated in 
 the figures. The mode in which a convex lens causes a pencil to 
 converge, and a concave lens to diverge, will therefore be easily 
 understood from the following figures, bearing in mind that the 
 greater the angle of a prism may be, the greater is the deviation of 
 a ray refracted through it. 
 
 No single lens is free either from spherical or chromatic aberra- 
 tion (q. v.). These errors are corrected by combining two or more 
 lenses together. In a single convex lens, the surfaces of which have 
 radii of different lengths, there is the least spherical aberration when 
 the most convex side is presented to the incident pencil. When the 
 refractive index of the glass is 1*5, the radii of the surfaces should 
 be as 1 : 6, and the most convex side presented to the origin of light. 
 This lens has the least spherical aberration of all single lenses, and 
 is called the “ crossed lens.” If we call the aberration of the 
 crossed lens unity, the following table, calculated by Sir John 
 Herschel, shows the aberration of some other forms of lens : — 
 
 Crossed lens .... 
 
 . 1- 
 
 Double convex I 
 
 > equal radii 
 „ concave J 1 
 
 . 1*567 
 
 Plano-convex 1 
 
 . 1*081 
 
 „ concave J 
 
 Convex side turned to parallel rays. 
 
 
 Plano-convex *1 
 
 . 4*2 
 
 „ concave J 
 
 Plane side turned to parallel rays. 
 
 
254 
 
 LEN 
 
 If two plano-convex lenses are placed with their convex sides in 
 contact, and the focal length of the first he to that of the second as 
 1 : 2*3, the aberration will be only one fourth that of the single 
 crossed lens. 
 
 Aberration may be entirely destroyed by placing a meniscus and 
 convex lens with their centres in contact, according to the following 
 table, — the convex lens being turned to parallel rays. 
 
 Eocal length of convex lens . 
 
 . -10*000 
 
 Radius of first surface 
 
 . . - 5-833 
 
 „ second surface 
 
 . +35-000 
 
 Eocal length of meniscus 
 
 . . - 5-497 
 
 Radius of first surface . 
 
 . - 2-054 
 
 „ second surface 
 
 . . - 8-128 
 
 Eocal length of compound lens 
 
 . - 3-474 
 
 In correcting for spherical aberration the same kind of glass may 
 be used for both lenses, but when chromatic aberration is cured by 
 combining lenses in contact, they must be made of different kinds 
 of glass. The principle on which lenses are achromatized is ex- 
 plained in the article “ Prism.” It is only necessary to remark in 
 this place that the formula by which the central pencil refracted 
 through a compound lens is rendered achromatic, does not also 
 include the case of oblique and excentrical pencils, otherwise than as 
 an approximation to the desired result. 
 
 Having thus briefly introduced the subject of lenses, we will pro- 
 ceed to discuss more minutely the construction of those with which 
 the photographer is principally concerned. They are the achro- 
 matized meniscus of Dr. Wollaston, intended principally for views, 
 and the portrait and view combinations of Professor Petzval, the 
 latter of which has been called by M. Voigtlander the “ Orthoscopic 
 lens.” 
 
 1. The Achromatized Meniscus, or Common View Lens. 
 
 This lens is composed of two, which are cemented together with 
 Canada balsam. The first is of flint glass, and double concave; 
 the second of crown glass and double convex. The concavity of the 
 front surface of the flint lens is very slight ; this surface may even 
 be quite plane. The radius of the inner surface is the same for 
 both lenses, as they are cemented together. When this lens is used 
 a stop is placed at a suitable distance in front of it. 
 
 But a lens of the meniscus form may be achromatized in two different 
 ways. When the flint glass receives the incident rays the construe- 
 
LEN 
 
 255 
 
 tion is that described above ; but the crown glass may be placed 
 so as to receive the incident rays. In this case the crown lens is a 
 meniscus, and the flint lens concavo-convex. The two forms are 
 exhibited in the following figs.: — the letters C,E denoting crown and 
 flint. 
 
 When the focal length of the achromatized lens, the radius of the 
 front surface, and the materials used are the same, the focal length 
 of the crown lenses are the same in both cases, and also the focal lengths 
 of the flint lenses ; the radius of the posterior surface is also the same 
 in both cases. The first form has been in use for a number of years, 
 but the second has been lately introduced and is said to give less 
 spherical aberration. 
 
 The object of this view lens is to give a good image on a flat sur- 
 face, pretty equally illuminated, and well defined, when an angular 
 field of view of from 35° to 40° is included ; the intensity of light 
 in the image being a matter of secondary importance, and the 
 spherical aberration being sufficiently reduced by the use of a small 
 diaphragm. In other words, the object of this lens is to give a 
 uniformly good picture, including a wide angular field, without 
 particular reference being made to the amount of light admitted, or 
 the time of exposure of the photograph. 
 
 The principle of this form of lens will be best understood by 
 discussing, in the first place, the case of a single plano-convex lens, 
 with a stop in front. 
 
 Let AB be a plano-convex lens presented to objects at such a 
 distance that pencils from them may be considered cylindrical ; and 
 suppose an oblique cylinder of rays MANB from one of these dis- 
 tant objects not situated upon the axis of the lens, to be incident 
 upon the plane side of it. Every ray of this large cylindrical pencil 
 then suffers the same amount of refraction on entering the lens, and 
 the pencil within the glass is a cylinder having its rays parallel to 
 mk. or #B. We have now done with the plane surface, and the 
 
256 
 
 LEN 
 
 case is simply that of a cylindrical pencil within a sphere about to 
 emerge. It remains to be seen what becomes of it. 
 
 One of its raysDE, if produced backwards, passes through the 
 centre C of the spherical surface. This ray does not suffer deviation 
 at emergence, but proceeds in the same straight line, DE, produced 
 to E. Eays emerging at an equal distance from E cut the line DE 
 in the same point and have equal aberration, and the caustic surface 
 is symmetrical with respect to EE, which is therefore its axis; and 
 EE is equal to the principal focal length of the lens for a direct 
 pencil. 
 
 It appears therefore that EE is a constant quantity, not dependent 
 on the obliquity of the pencil ; CE is also a constant quantity, being 
 the radius of the spherical surface AB ; therefore CE is a constant 
 quantity. 
 
 Hence it follows that when a plano-convex lens is directed to ex- 
 tremely distant objects, the image lies upon a spherical surface which 
 has the same centre as the posterior surface of the lens. 
 
 This will perhaps be understood better by reference to the follow- 
 ing figure : — 
 
 AB is the lens ; C, the centre of the sphere AB ; P, Q, R, fixed 
 stars emitting cylindrical pencils of light; QC^, the axis of the 
 lens ; andj9, q, r , the images of the stars, which lie on the spherical 
 surface shown by the dotted line, and the centre of which is C. 
 
 The stop is placed, as shown in the figure, for the purpose of 
 cutting off the outside rays of the pencils, and thereby reducing the 
 amount of spherical aberration in each pencil. It will easily be seen 
 
LEN 
 
 257 
 
 that the position of the stop for any particular pencil depends upon 
 its obliquity ; and that the greater the obliquity the nearer the stop 
 must be placed to the lens. In practice, the stop should be placed 
 so as best to suit the most oblique pencils ; but the inaccuracy thus 
 introduced with respect to the less oblique pencils is hardly appre- 
 ciable. 
 
 It will be seen by the foregoing figure that the amount of light in 
 the image depends not only on the size of the stop, but also on the 
 obliquity of the pencil, being greatest in the case of the direct pencil, 
 and diminishing as the obliquity increases. For this reason there 
 is not in the common view lens absolute equality of illumination, 
 the centre of the field having the most light and the edges the 
 least. 
 
 From the case of the single plano-convex lens, the transition is 
 easy to that of the same lens achromatized, and thence to the achro- 
 matic meniscus. In these two latter cases it may be considered as 
 very approximately true that the field for distant objects is a sphere 
 concentric with the posterior convex surface of the lens. 
 
 If the stop be placed immediately in contact with the lens the 
 radius of the field is shortened and the centre of the posterior surface 
 itself, and not of the sphere of which it is a portion, becomes the 
 centre of the field. This is explained in the article on the “ Optical 
 centre q. v. There would, however, be no distortion in this case, 
 while there is considerable distortion when the stop is put at a distance 
 from the lens, in order to get a flatter field. See “ Distortion.” 
 
 s 
 
258 
 
 LEN 
 
 The general form and principle of the common view lens having 
 now been described, it remains to give the exact formula for its con- 
 struction. • 
 
 The data are : — 1st, the focal length of the lens ; 2nd, the indices 
 of refraction for the crown and flint glasses ; 3rd, the dispersive powers 
 of the glasses ; and 4th, the radius of the front surface. Let these 
 quantities be expressed by the following symbols : — 
 
 E= focal length of compound lens. 
 
 /x x = refractive index of flint glass. 
 
 „ „ crown glass. 
 
 D = ratio of dispersive power of front glass to back glass. 
 E= radius of anterior surface of lens. 
 
 The unknown quantities are : — 
 
 s radius of inner surface of lens. 
 t= ,, posterior surface of back lens. 
 f v =z focal length of front lens. 
 
 / 2 = „ back lens. 
 
 Then, the equations which connect these quantities are — 
 
 111 A 
 
 — +-= . .(1) -=-D . . . (2) 
 
 fl f l F f i 
 
 -( 3 ) -= 0 * 2 - 1 ) ..( 4 ) 
 
 E 8/ f 2 \s tj 
 
 By which four equations, the four unknown quantities may be de- 
 termined. 
 
 It is immaterial in the above formula whether the flint or crown 
 lens be placed in front, so as to receive the incident rays. If the 
 flint lens be placed in front, the formula gives the common view lens 
 which has been in use for a number of years. If the crown lens be 
 placed in front, the formula gives the lens shown by the second of 
 the figures at page 255. 
 
 We have said that the radius of the front lens is arbitrary, but it 
 depends upon the size of the lens and the work it is intended to do. 
 As the lens increases in size the focal length remaining the same, the 
 stop is placed further in front of it, the field becomes flatter, and there 
 is more distortion ; at the same time the front surface must become 
 flatter. When these conditions are reversed, i. e. as the Lens becomes 
 smaller, the stop is placed nearer to it, the field becomes more concave, 
 there is less distortion, and the front surface should then become more 
 
LEN 
 
 259 
 
 concave. Some little latitude is therefore allowable in the radius of 
 the front surface, and it may be considered as arbitrary, within certain 
 limits ; the mean being about 3 feet radius for a three-inch lens. 
 
 The second equation must be fulfilled in order that the lens may 
 be achromatic. No attempt is made to cure spherical aberration 
 except by the stop. The central pencil from Q, would have a finer 
 focus, q, if the small central portion of the lens through which it 
 passes could be turned with its convex side to the incident rays. See 
 what has been said before respecting the aberration of a plano-convex 
 lens. The remark is equally applicable to the compound lens which 
 is nearly plano-convex. The large lens of an opera glass, which is 
 compound and convexo -plane, is placed with the convex side to the 
 objects ; and for the same reason the front lens of the portrait com- 
 bination, which is nothing but a view lens reversed, has its convex 
 side to the objects. 
 
 It now only remains to discuss the best mode of mounting the 
 view lens ; which is shewn in the following figure : — 
 
 The lens is placed in the tube with its concave side to the stop. 
 A ring is then laid against the convex side, and it is screwed tightly 
 in its place. The tube works in a jacket, which is screwed into a 
 flange in the front of the camera ; the open end of it is closed by 
 a cap ; and for better security to the lens a cap is also screwed on 
 at the other end, when the instrument is packed for travelling. 
 Within the tube, a little in front of the lens, is a fixed diaphragm ; 
 and in front of that a moveable tube which carries the front dia- 
 phragm, which can be so adjusted as to revolve within a projecting 
 
 s 2 
 
260 
 
 LEN 
 
 box, in order that its full aperture may be presented perpendicularly 
 to the darkest part of the picture, and its edge, so to speak, towards 
 the sky, or lightest objects ; and at the same time any one of a set 
 of stops of different sizes may be introduced as circumstances require. 
 This contrivance is sufficiently explained by the figure. As the tube 
 can be turned in the jacket, the plane of the stop, when placed 
 obliquely to the axis of the lens, can be brought into any required 
 position. The front part of the revolving disc which carries the 
 stop, has a groove at the bottom and on both sides, and the stop, 
 which is an oblong disc with a round hole in the middle, is slid into 
 this groove like the lid of a box. 
 
 It will be seen that an annulus of the circumference of the lens is 
 sacrificed, the lens must therefore be made so much larger as to allow 
 for this. The object of this sacrifice of glass is to prevent the re- 
 flection of light from the broad edge of the lens. In the common 
 mode of mounting the view lens this edge forms a broad luminous 
 ring, which is seen on looking into the camera at the open end, and 
 of course the light which enters the eye from this ring would fall 
 upon a sensitive plate if put in the same position. 
 
 The diaphragms within the lens-tube entirely prevent the reflec- 
 tion of light from the inside of the tube. A strip of black cotton 
 velvet should be wound round the edge of the lens before putting it 
 into its cell ; and both the inside of the tube and diaphragms should 
 be blackened with a mixture of lamp black and spirit varnish. There 
 are times when it is convenient to be able to push the diaphragm 
 nearer to the lens, and no mounting is complete which does not ad- 
 mit of this being done when necessary. 
 
 When a very small stop is used the diameter of the circular space 
 covered by the lens with good detail is nearly equal to its focal 
 length. 
 
 Distant objects have a shorter focus than near ones. If / be the 
 principal focal length of the lens, u the distance of an object from 
 it, and v the focal length of the lens for the distance u, then v is 
 determined by the following equation : — 
 
 111 
 
 v f u 
 
 Since the field for objects equidistant from the lens is concave and 
 not flat, it follows that the objects at the sides of the picture should, 
 if possible, be nearer to the lens than those in the middle. When 
 this cannot be managed recourse must be had to a small stop, which, 
 
LE1ST 
 
 261 
 
 by cutting off the outer rays of pencils, renders them very long and 
 attenuated in the neighbourhood of their focus, and then the picture 
 may be placed a good deal out of true focus in places without ap- 
 preciable loss of detail. By means of the small stop, objects at un- 
 suitable and greatly varying distances from the lens, can therefore 
 be brought into tolerably good focus upon a flat focussing screen. 
 
 2. M. Petzval’s Portrait Combination. 
 
 The object of this instrument is to obtain an image well defined 
 in its principal parts, when a large volume of light is admitted. In 
 taking a portrait it is evident that the time of exposure should 
 be reduced as much as possible, because after remaining in a con- 
 strained position for a long time the features of the sitter betray 
 an expression of the discomfort felt. A lens of large aperture must 
 therefore be employed in portraiture, so long as photographic pro- 
 cesses remain in their,, present state of insensitiveness. As soon 
 however as the chemist shall discover the means of rendering these 
 processes more sensitive, the optician will be released in a corres- 
 ponding degree from the necessity of constructing lenses of large 
 aperture, and the defects to which such lenses are liable, and which 
 admit of no remedy, will be avoided by using smaller apertures. 
 The nature of the defects of large lenses have, however, been mis- 
 understood and exaggerated by persons ignorant of mathematics , 
 The portrait combination of Professor Petzval is that which has 
 been generally adopted by opticians, and the arrangement of the 
 lenses is exhibited in the following figure. 
 
 The front lens A B is a compound lens, exactly like the common 
 view-lens, but placed with its convex side to the objects. It is 
 achromatic, but not entirely aplanatic ; this defect being remedied 
 by the posterior lens. 
 
 The posterior lens is composed of two lenses separated by a small 
 space ; that next the front lens is of flint glass, convexo-concave, 
 and divergent , being thinner in the middle than at the edges ; the 
 other is biconvex, and of crown glass, being placed with its most 
 convex side next to the concavity of the flint, as shewn in the figure. 
 
262 
 
 LEN 
 
 The posterior compound lens is achromatic, and the object of 
 separating the lenses and giving them curves so different from that 
 of the front lens is to cure spherical aberration in the entire com- 
 bination. 
 
 Both lenses are therefore convergent, and the effect of the posterior 
 lens is to shorten the focus of the front lens. It may be well to 
 observe in this place that when a convergent lens is rendered 
 achromatic by combining two lenses of different kinds of glass, the / 
 concave lens must be of flint, and the convex lens of crown glass. 
 On the other hand, when a divergent lens is achromatized, the con- 
 cave lens must be of crown, and the convex lens of flint glass. 
 
 The dimensions of this form of lens, given by measurement of 
 one which we constantly use and which is now before us, are as 
 follow : 
 
 Aperture of front lens .... 
 
 4 inches 
 
 Eocal length of ditto . . . % . 
 
 36 „ 
 
 Aperture of back Jens .... 
 
 4 „ 
 
 Eocal length of ditto . . . . . 
 
 24 „ 
 
 Distance between lenses . . 
 
 6 „ 
 
 Eocal length of combination measured 
 from back lens . . . . . 
 
 16| )> 
 
 Eocal length of combination, reversed 
 
 12 „ 
 
 The curves depend entirely on the refractive and dispersive 
 powers of the glass used, and as these vary with every “ pot of 
 metal,” it might only mislead the reader were we to state the exact 
 curves of the lenses of the instrument before us. 
 
 To return to the figure. 
 
 Let PQB be an object placed before the lens, P being a bright 
 point upon its axis. A direct pencil from P covers the entire sur- 
 face, AB, of the front lens, and is refracted by it towards a point /. 
 This converging pencil A/B is incident upon the portion hk of 
 the posterior lens, and is by it refracted to a point p, at which the 
 image of P is formed. It will be observed that hpk is a cone of 
 light having a very large vertical angle hpk ; both spherical and 
 chromatic aberration must therefore be accurately corrected in such 
 a pencil, and the focussing screen placed accurately at p, or the 
 indistinctness of the image would be considerable. The optician 
 has therefore greater difficulties to encounter in the construction of 
 the portrait lens than in that for views. 
 
 We have next to consider the case of an oblique pencil. 
 
 The central pencil does not cover the whole of the back lens, 
 (which is the same size as the front lens,) but merely the central 
 
LEN 
 
 263 
 
 portion, h k. The angle D A.h is about equal to 3°. If therefore 
 an oblique pencil, as QAB, makes an angle Q,CP of about 3° 
 with the axis of the lens, the upper ray of the refracted pencil will 
 run along the tube A D, and the whole pencil will come to a focus 
 at q. The image therefore between p and q will be equally illumi- 
 nated. But if a more oblique pencil, as that from B, be incident on 
 the lens, only a portion of the refracted pencil can pass through the 
 back lens, the remainder of it being cut off by the upper part of the 
 tube, which reflects it in the manner shown in Eig. 3, page 59 ? 
 This is one of the great defects of the portrait lens, and the mode 
 of mounting it. Sometimes the back lens is made a little larger 
 than the front lens, and then the equality of illumination extends 
 over a greater space, but the outer portion of a large back lens does 
 not perfectly cure spherical and chromatic aberration, and from this 
 cause indistinctness is introduced. The fact is, the portrait lens 
 with full aperture has many serious defects, and the sooner pho- 
 tographers are enabled to give up using it the better. It should 
 never be used from choice, but only from necessity. 
 
 The field of a portrait lens is not nearly so flat as that of a view 
 lens of equivalent focus ; and the angle of view properly included 
 by a portrait lens is much narrower. 
 
 When \he light is sufficiently good, a stop may be used with a 
 portrait lens ; and the question has arisen as to which is the proper 
 place for it — in front of the front lens, or between the lenses. This 
 question we will discuss with the help of the following figure : — 
 
 The lenses are represented by the lines AB, BE, as before. B 
 is the origin of an oblique pencil, the axis of which, BC, makes the 
 same angle with the axis of the lens, as if it were the extreme 
 oblique pencil of a view lens, i.e., PB is about equal to one-third 
 of PC. Now CA is equal to one-third of AD, and the line BC 
 suffers little or no refraction on passing through the front lens, 
 therefore the axis of the refracted pencil between the lenses passes 
 very nearly through D, as shown in the figure. 
 
 Now we may suppose ‘that a whole pencil, BAB, comes accu- 
 rately to a focus at r, for if it does not, the lens is worthless, inas- 
 much as it ought to cure spherical aberration in the oblique, as well 
 
264 
 
 LEN 
 
 as central pencils, when the focus is merely examined by unassisted 
 vision. We may assume therefore that every ray of the pencil 
 RAB which really passes through the back lens, passes also through 
 r. This being understood, it follows that by placing a stop midway 
 between -the lenses, at m, a small excentrical portion, R^, of the 
 large pencil R A B is incident at n, , passes through the stop at m , 
 the back lens at 0, and then comes to a focus at r ; while if a small 
 stop is placed at 0, the central portion of the large pencil RAB 
 passes through it ; then through the back lens at D, and thence, as 
 before, to r. 
 
 When, therefore, a very small stop is used, it scarcely matters 
 whether it be placed at c or m ; but if a larger stop be used, it would 
 obviously be better placed at m, because if placed at c , a portion of 
 the rays of the refracted pencil would be cut off by the tube, and 
 there would not be equality of illumination in the image. It must 
 be remembered, however, that the focus at r is not absolutely per- 
 fect, and that, theoretically , the rays incident at C go to r with less 
 spherical aberration than those incident at n. 
 
 But a portrait lens is not intended to be used with a small stop. 
 When a small quantity of light will do the work required, it is far 
 better to use the orthoscopic lens. Nevertheless, stereoscopic 
 pictures are sometimes required to be taken instantaneously, and 
 sometimes with the ordinary exposure for views ; in this case a pair 
 of small portrait lenses, furnished with stops, will answer both pur- 
 poses, inasmuch as stereoscopic pictures need not include so wide a 
 field as ordinary views. 
 
 The best arrangement for taking instantaneous pictures, including 
 a tolerably wide field, with a lens of large aperture, is as follows : — 
 
 Reverse the portrait lens, that is, turn it with the posterior lens 
 to the objects ; and let the other lens, which is a view lens, be made 
 larger than usual. Then place a stop immediately in contact with the 
 lens that is presented to the objects. The arrangement will then be 
 as represented in the figures at pages 60, 61, and 67 ; and this will 
 give a much flatter field than the portrait lens used in the ordinary 
 way, and the picture will be more uniformly covered with good 
 definition, and more equally illuminated, although possibly it may 
 not be quite so good as before ^n the centre. The focal length of 
 the combination is a trifle shorter when used in this way. 
 
 A portrait lens, therefore, is only intended to be used for sub- 
 jects which must be taken quickly, or instantaneously. Tt does not 
 give so flat a field as a view-lens ; nor does it give equality of illumi- 
 nation ; nor can objects at different distances be all brought into 
 good focus ; nor does it include a wide angular field ; nor does it 
 give images free from distortion. In short, everything is sacrificed 
 
LEN 
 
 265 
 
 to the centre of the picture, and the lens should only be used when 
 no other will answer the purpose. As for using a stopped portrait- 
 lens, instead of a view-lens, for ordinary out-of-door pictures, the 
 principle is incorrect for the reasons which have been stated, and 
 the plan only answers when a narrow field of view is included. 
 
 The next lens to be described is the best of the three, and every 
 photographer should possess one. It has been called the “ Ortho - 
 scopic lens,” or “ Compound View-lens,” and the following is a 
 description of it, given by us in the “ Photographic Notes,” No. 
 49, April 15, 1858, shortly after its introduction into this country. 
 
 8. The Orthoscopic Lens. — The term “ orthoscopic ” is 
 derived from the Greek words opdog right, and ctkotteu) I see. The 
 meaning is that the lens gives images more free from distortion than 
 other lenses. 
 
 The orthoscopic lens is an arrangement consisting of two achromatic 
 compound lenses separated by an interval, as shown in the figure. 
 
 The front lens is the larger, and is the same as the front lens of 
 the present combination for portraits ; that is to say it is composed 
 of a double convex lens of crown glass, cemented with Canada balsam 
 to a concave lens of flint, the entire lens having negative focal length 
 — that is, causing parallel rays to converge to a focus on the opposite 
 side of the lens to the origin of light. 
 
 The posterior lens is an achromatic compound lens formed of two, 
 not cemented together but merely touching at the edges, and having 
 a space between them in the middle, as shewn in the figure. The 
 inner lens is of crown glass, and double concave, the flatter side 
 being next to the front lens. The outer lens is a meniscus of flint 
 glass, having its convex side outwards and next to the picture. The 
 deepest concavity of the crown lens is therefore opposite to the concav- 
 ity of the meniscus. As these lenses are not cemented together but 
 have a cavity between them, their inner surfaces may sometimes 
 require wiping ; they are therefore merely deposited in the cell which 
 receives them, and are fixed in their place by a countercell which is 
 screwed until it touches a brass ring laid in contact with the outer 
 lens. When a stop is used it is placed between the ring and the 
 
266 
 
 LEN 
 
 countercell, as shewn in the figure. The diameter of the posterior 
 lens is about two-thirds that of the front lens, and the distance be- 
 tween them is about half the diameter of the front lens. 
 
 The posterior compound lens has positive focal length, and would 
 cause parallel rays to diverge from a point on the same side of the 
 lens as the origin of light. The front lens therefore tends to bring 
 parallel rays to a focus, the back lens to s.catter them wider apart. 
 In popular language the front lens is convex and magnifies, the back 
 lens is concave and diminishes. 
 
 We would take this opportunity of observing that in optics the 
 signs plus and minus are introduced into formulae from their property 
 of being able to represent not merely the operations of addition and 
 subtraction, but also contrariety of position or direction. By calling 
 lines measured on one side of a lens positive, and on the other side 
 negative, and affixing the signs plus or minus to the magnitude of a 
 line, according to its position, it is possible to make one fornula in- 
 clude a great variety of different cases. The convention adopted in 
 optics is, to call lines measured from the lens towards the origin of 
 light positive, and in the opposite direction negative. In other branches 
 of mathematics it is found very convenient to adopt similar conven- 
 tions in which the signs plus and minus indicate opposite qualities , 
 as well as mere addition and subtraction. 
 
 The exact particulars of the compound lenses are as follow : — 
 
 Front lens — Diameter 8 centimetres 
 
 Focal length 40 do. (Negative) 
 
 Back lens — Diameter 5 centimetres. 
 
 Focal length 90 do. (Positive) 
 
 Distance between the lenses 4 centimetres. 
 
 Focal length of entire combination . . .63 do. (Negative) 
 
 Diameter of the field 54 do. 
 
 [A centimetre is about two-fifths of an inch.] 
 
 These dimensions apply to the 3 -inch lens, but by dividing or mul- 
 tiplying them all by any given quantity, the corresponding dimensions 
 of any other size of lens may be obtained. 
 
 The front lens is placed with its convex side to the view, and the 
 back lens with its convex side to the picture. 
 
 Since the front lens of this new instrument is the same in every 
 respect as the front lens of the portrait combination, the latter may 
 be converted into the former by removing its posterior lens and sub- 
 stituting for it the posterior lens of the orthoscopic combination, 
 mounted, of course, at its proper distance from the front lens. 
 
LEN 
 
 267 
 
 In order to cut off reflected light from the inside of the tube, a 
 stop is placed midway between the front and back lens ; but not so 
 as to intercept any of the legitimate rays of light 
 
 Such is the construction of the orthoscopic lens. It is essentially a 
 View-Lens, and is not intended for portraiture. We have therefore 
 to discuss its merits as a view-lens when compared with the ordinary 
 form, and in doing so must direct our attention more particularly to 
 the following points : — 
 
 1st, — Elatness of field, and the included angle of view. 
 
 2nd, — Ereedom from distortion. 
 
 3rd, — Equality of illumination. 
 
 4th, — Perfection of focus, and freedom from spherical aberration. 
 
 5th, — Coincidence of the visual and actinic foci. 
 
 6th, — General convenience, freedom from diffused light, copying 
 powers when the focus is elongated, power of rendering aerial per- 
 spective, and other good qualities. 
 
 We shall discuss these matters in the order in which they stand. 
 
 1st, — Elatness of field, and the included angle of view. 
 
 In determining the flatness of field of any lens, we have to compare 
 the course of the most oblique with that of a direct pencil ; and the 
 simplest plan is to suppose the pencils cylindrical, or that the lens is 
 pointed at extremely distant objects ; should it be found to answer 
 well in this case it will be equally good for all ordinary purposes. 
 
In the above figure the lenses are represented by straight lines 
 strong and black, the front lens passing through A and the back lens 
 through C ; AC^ being their common axis. A stop is placed behind 
 the back lens, and in contact with it. In order to fix the ideas, and 
 render what we have to say more intelligible, we shall suppose the 
 lens to be a No. 1, having a combined focus of rather more than 11 
 inches, and covering a picture 10x8, with a half-inch stop. 
 
 It will be seen, from the above figure, that the oblique pencil passes./ 
 eccentrically through the front lens and centrically through the back 
 lens. If, then, q be the focus of the direct pencil from a distant 
 point Q, and p the focus of an oblique pencil from a distant point P, 
 we have to compare the length Cp with C q, in order to discover the 
 flatness of field, and how far it deviates from a sphere whose centre 
 is C ; and we would observe that unless it does deviate, and that 
 pretty considerably, from such a sphere, the lens would be next to 
 worthless for the purpose intended. We have to show, then, that 
 the focal length, C p, of the oblique pencil, is greater than the focal 
 length, C q, of the direct pencil, and to calculate the difference be- 
 tween them. 
 
 Let us consider first the case of the direct pencil incident at A. 
 After refraction through the front lens it converges towards m, the 
 the principal focus of that lens ; the distance A m being 8 inches (in 
 round numbers). This converging pencil is then refracted by the 
 posterior lens, the positive focal length of which is 18 inches (in round 
 numbers). The effect of this is to diminish the convergency of the 
 rays and bring them to a focus at q, which is further than m from C. 
 The distance AC being one inch, Cm is 7 inches, and Qq is then 
 found in the following way : — 
 
 Multiply 7 by 18, and divide the product by their difference ; — 
 that is, divide 126 by 11. This gives C^=ll -fi inches. 
 
 Next, let us consider the oblique pencil which proceeds from a 
 distant point P, is incident eccentrically on the front lens at B, and 
 passes centrically through the back lens at C. 
 
 Through A, the centre of the front lens, draw a dotted line AO, 
 parallel to BP, and with A as centre, and A m as radius, strike an 
 arc of a circle cutting AO at 0. Then, AO equals 8 inches; and 
 the oblique pencil at P will, after passing through the front lens, 
 converge towards the point O (as shown by the dotted lines). 
 
 Now we come to the pith of the matter. What happens at the 
 second lens ? 
 
 We have at the second lens an oblique pencil, incident centrically, 
 and converging towards 0. Join therefore CO, and produce it to^. 
 
LEN 
 
 269 
 
 Also, with C as centre and Cq as radius, strike a circle cutting Cp 
 at n. C n is therefore equal to C q. 
 
 Now, adopting the same formula as in the former case in order to 
 find Qp, we must multiply CO by 18 and divide the product by their 
 difference. What then is the length of CO ? 
 
 In the reply to this query will be seen the great ingenuity of M. 
 Petzval’s arrangement ; for it appears that CO is greater than Cm. 
 
 The proof of this is easy enough. Any two sides of a triangle 
 are, together, greater than the third, therefore OC and CA are to- 
 gether greater than AO, and therefore than Am. Take away the 
 common part AC, and CO is proved to be greater than Cm. 
 
 The actual difference between CO and Cm in the No. 1 lens, with 
 the extreme oblique pencil, is about the one tenth of an inch. If 
 then we multiply 7 to by 18 and divide the product by their difference, 
 we get Cjt? = llT^g ins. The difference between Co and Cm, (np,) 
 is therefore nearly one-third of an inch. This is of course in favour 
 of flatness of field, because it brings p nearer to the plane through q. 
 
 In the common view lens, presented to extremely distant objects, 
 the field is very approximately a sphere the centre of which is the 
 centre of the convex surface of the lens. On taking the exact dimen- 
 sions of the orthoscopic lens, and working the problem out completely, 
 we find that, as regards flatness of field, the common view lens has a 
 little the advantage ; and this result of theory is also borne out by 
 the experiments we have made with both forms of lens differing but 
 little in their focal length. In what follows, however, we shall show 
 that in other respects the orthoscopic lens has many and great advan- 
 tages over the common view lens. 
 
 So much for flatness of field. Next, with respect to the angular 
 extent of the field of view. It might be supposed that the common 
 view lens having the advantage in flatness of field, it ought to include 
 a wider angular field ; but that is not the case, because the distortion 
 produced by the common view lens is so great, and' becomes so un- 
 bearable when a certain small angle of field is exceeded, that it is 
 necessary from this cause to restrict that angle to about 35°. In 
 the case of the orthoscopic lens, the distortion is not only different in 
 character , but much less in amount , and therefore, although the field 
 is not absolutely so flat, still an angle as great as 47° may be included. 
 This is a great merit of the orthoscopic lens, and one which should 
 recommend it particularly to the notice of the landscape photographer ; 
 because the bad effects of curvature of the image may be remedied by 
 using a small stop, but for distortion there is no such remedy. A 
 small stop gives a finer point to the pencils, but does not affect the 
 curvature of the lines of the picture. 
 
270 
 
 LEN 
 
 We now come to the 2nd topic, viz : Freedom from distortion. 
 
 In the common view lens with the stop in front, the oblique pencils 
 do not pass straight through the margin of the lens, but are bent 
 out of their course, inwards, towards the centre of the picture. This 
 produces distortion, in a way in which we will endeavour to explain. 
 Suppose the stop extremely small, and the lens removed ; a perspec- 
 tive view would then be formed on the focussing screen, but larger 
 than the picture produced by the lens, and quite free from distortion. 
 Suppose a plain irregular polygon to be the figure represented, the 
 angles of which on the focussing screen are points A, B, C, D, 
 &c., and let O be the point where the axis of the camera cuts it. 
 Draw radial lines O A, O B, O C, 0 D, &c. Now introduce the lens. 
 Inconsequence of the deflection of the axes of the pencils which pass 
 through the margin of it, towards the point 0, the image of the poly- 
 gon will be smaller than before, and its angular points a , b, c , d, &c., 
 will lie on the lines O A,0 K, 0 C, &c., nearer to O. Now if the decre- 
 ments a A , £B, cC, &c., were exactly proportional to the radial lines 
 OA, OB, OC, &c., the small polygon abed would be exactly 
 similar to the large one ABCD — and there would be no distortion ; 
 but no such law is observed in the production of these decrements 
 as that of direct proportionality to the radial line, and the greater 
 the radial line may be, a fortiori greater the decrement becomes. 
 This produces distortion, and causes all straight lines which do not 
 pass through the centre of the picture, to be bent inwards at their 
 extremities. See “ Distortion. 55 
 
 Now let us turn to the orthoscopic lens. Here we see that the 
 axes of the oblique pencils are bent outwards out of their course, and 
 a larger picture produced than if a small hole, without lenses, were 
 put at C. Instead of Cerements we have now mcrements of the 
 radial lines — and the production of these increments depends upon 
 a different law from that of the decrements in the former case, and 
 produces less distortion. What little distortion there is, has the effect 
 of rendering straight lines convex to the centre of the picture, by 
 bending their extremities outwards, but this defect is so inconsiderable 
 as to be scarcely appreciable, and so far as it exists it has the good 
 effect of increasing the comparative size of the side objects, and 
 therefore throwing the central objects apparently further back ; while 
 the common view lens has the opposite effect to such an extent 
 as to interfere greatly with the aerial perspective, by diminishing the 
 objects at the sides of the picture, which are generally the nearest, 
 and thereby bringing the central and generally most distant objects 
 apparently too much forward. 
 
 3rd, — Equality of illumination. Through m imagine a line mk 
 
LEN 
 
 271 
 
 drawn at right angles to mC, and cutting Cp in 7c. Now let a 
 straight line passing always through this imaginary point 7c sweep 
 round the circumference of the stop at C and mark out with its end 
 a circle on the front lens, in the neighbourhood of B, which would 
 be its centre. Similarly, let a line passing through m sweep round 
 the circumference of the stop and trace another circle on the front 
 lens, the centre of which would be A. These two circles viz., that 
 at A and that at B will be equal to another. But if instead of the point 
 k we take the point 0, nearer to C, the circle at B will then become 
 larger than before and consequently larger than that at A. Hence 
 it follows that the area on the front lens covered by an oblique pencil 
 is greater than that covered by the direct pencil. On the other hand 
 the obliquity of the pencil occasions loss of light, as shown by the 
 dotted lines across it. The gain from one cause does not entirely 
 compensate for the loss from the other, and there is not perfect 
 equality of illumination ; but in the common view lens there is loss 
 from obliquity of incidence without any set off to counterbalance it. 
 The orthoscopic lens has therefore the advantage as regards equality 
 of illumination. 
 
 4th, — Perfection of focus, and freedom from spherical aberration. 
 In the common view lens no attempt is made to cure spherical aber- 
 ration, or improve the focus, except by means of the stop. Every- 
 thing is sacrificed to flatness of field. There would be much less 
 spherical aberration in the central pencil if the lens could be turned 
 with its convex side to the view. But in the orthoscopic combina- 
 tion spherical aberration may be, and is, completely remedied, and 
 the quality of the focus, both of the direct and oblique pencils, greatly 
 improved. 
 
 5th, — Coincidence of the visual and actinic foci. With four glasses 
 instead of two, more lines of the spectrum may be united, and there- 
 fore the chemical focus improved, so that the orthoscopic lens has 
 the advantage in that respect. • 
 
 6th, — In lightness and convenience, and freedom from diffused 
 light, the orthoscopic lens has greatly the advantage over the other. 
 In copying, the common view lens cannot be used when the copy is 
 to exceed one half the size of the original ; but when the focus of the 
 orthoscopic lens is lengthened by bringing the object near it, it gives 
 a sharp and good image up to full size. It is in fact the best copying 
 lens that has yet been produced. 
 
 We have now discussed minutely the three forms of lens mostly 
 used by photographers. It only remains to describe the mechanical 
 means by which lenses are ground and polished. , 
 

 272 
 
 LEH 
 
 The Grinding and Polishing of Lenses. 
 
 The piece of glass of which a lens is to be made is cut out of a 
 plate of sufficient thickness, and this is brought roughly into shape 
 by means of pincers, a grind-stone, and an old mould. A pair of 
 guages and moulds are required for each surface of the lens. The 
 pair of guages are made by striking the required curve on a brass 
 plate, and cutting through the line very accurately; one of the 
 guages is therefore .convex, the other concave. 
 
 Two moulds are then made of brass or iron, and turned in a 
 lathe, to fit the guages. One of these moulds is therefore hollow, 
 the other convex. To ensure accuracy they are then ground one 
 against the other, with some fine emery between them. 
 
 The piece of glass to be ground is now attached to a holdfast by 
 a few drops of melted pitch. The holdfast is a round brass plate a 
 little smaller than the lens. The mould is then applied to the upper 
 surface, together with some coarse emery and water, and the grinding 
 is accomplished by hand, by turning the mould continuously round 
 upon the glass, giving a cross rub now and then, using gentle pres- 
 sure, and taking care that the edge of the mould never goes beyond 
 the centre of the glass. When the glass has assumed the shape of 
 the mould, and touches it in every part, the coarse emery is washed 
 off and finer emery substituted for it. The grinding is then con- 
 tinued until the surface of the lens assumes a uniform dead appear- 
 ance, and all scratches are removed. During this process the mould 
 which is in use is frequently ground against the other, in order to 
 prevent any change of form that might occur in it. Having pro- 
 ceeded thus far, finely powdered pumice stone is substituted for the 
 fine emery, and after a little more grinding the surface is ready for 
 polishing. The other surface of the lens is then treated in the 
 same way. 
 
 It now remains to polish the surfaces. 
 
 This is done with moulds of pitch ; and fine colcothar is used 
 instead of emery or pumice powder. (Colcothar is the red peroxide 
 of iron, called “Bouge,” or jeweller’s polishing powder ; q. v.) The 
 polishing moulds are made thus : equal parts of pitch and resin are 
 melted together and poured into the metal mould (which is previously 
 heated), to the depth of about a quarter of an inch ; the cold mould 
 is then laid upon the melted pitch, and when it has set the whole 
 is thrown into water and suddenly cooled. The cold mould is then 
 detached froip the pitch. 
 
 As the polishing proceeds the pitch becomes hot with- the friction, 
 
LIG 
 
 273 
 
 and sticks to the glass, so that the labour of polishing increases 
 towards the end of the operation, which must not be interrupted 
 until finished. A drop or two of water is added to the colcothar at 
 first, but no fresh colcothar or water are afterwards added. If the 
 pitch becomes too adhesive it must be breathed upom 
 
 Lenses are sometimes ground by machinery, but hand grinding is 
 thought the best for fine work. 
 
 An account of the kind of glass used for photographic lenses, 
 and the defects to which it is subject, is given in the article “ Op- 
 tical Glass ;” q . v. * 
 
 Light. What is light? Why does it render objects visible, 
 exhibit different colours, produce chemical changes in bodies, and 
 so forth ? These are questions of the highest importance in science, 
 and of especial interest to the photographer. We shall endeavour 
 briefly to explain in the present article what light is, and why it 
 produces many curious phenomena of which no photographer should 
 be ignorant. 
 
 Two theories have been advanced with respect to the physical 
 nature of light, one called the “ Corpuscular,” the other the 
 “ Undulatory” theory. On the former hypothesis it was supposed 
 that a luminous body emits particles of light, just as a fowling 
 piece discharges a volley of small shot, and that these minute 
 particles, after travelling through space with immense velocity, at 
 length impinge upon bodies. On the undulatory theory it is sup- 
 posed that a luminous body communicates undulations to an elastic 
 ether which pervades space, that these undulations are transmitted 
 with prodigious velocity, and that they constitute light. 
 
 We speak of the corpuscular theory in the past tense because it 
 has now been abandoned by men of science as incapable of explaining 
 some of the most important phenomena of light ; and we speak of the 
 undulatory theory in the present tense, because it is now generally 
 adopted, aud considered to rest on quite as satisfactory evidence as 
 the Law of Universal Gravitation. In the preface to Professor 
 Airy’s “ Tract on the Undulatory Theory of Light,” he expresses 
 himself thus strongly on the subject : — 
 
 “The undulatory theory of optics is presented to the reader as 
 having the same claims to his attention as the Theory of Gravita- 
 tion, — namely, that it is certainly true, and that by mathematical 
 operations of general elegance it leads to results of great interest. 
 With regard to the evidence for this theory ; if the simplicity of a 
 hypothesis which explains with accuracy a vast variety of phenomena 
 of the most complicated kind, can be considered a proof of its 
 
 T 
 
274 
 
 LIGr 
 
 
 correctness, I believe there is no physical theory so firmly established 
 as the theory in question. This can be felt completely, perhaps, 
 only by the person who has both observed the phenomena and made 
 the calculations ; as to my own pretensions to the former qualifica- 
 tion, I shall merely state that I have repeated nearly every experi- 
 ment alluded to in the following tract. This character of certainty, 
 I conceive to belong only to what may be called the geometrical part 
 of the theory ; the hypothesis, namely, that light consists of undula- 
 tions depending on transversal vibrations, and that these travel with 
 certain velocities in different media, according to the laws here 
 explained. The mechanical part of the theory, as the suppositions 
 relative to the constitution of the ether, the computation of the 
 intensity of reflected and refracted rays, &c., though generally pro- 
 bable, I conceive to be far from certain.” • 
 
 Such are the opinions of the Astronomer Eoyal, expressed in the 
 year 1831, with respect to the certainty of the undulatory theory of 
 light ; but since that time some points of difficulty in the hypothesis 
 have been cleared up, and it now rests on a firmer basis than ever. 
 In fact the Corpuscular Theory is now completely exploded, and 
 we allude to it as a matter of history, as we might to any exploded 
 system of Astronomy. Should the reader, therefore, find in any 
 popular treatise on Optics the Corpuscular Theory treated with any 
 sort of gravity or respect, or in any way than as a delusion, he may 
 infallibly conclude that the author is but imperfectly acquainted 
 with his subject. 
 
 Light, then, is the undulation of an ether which pervades space, 
 just as Sound is the undulation of the air ; but both the media and 
 the species of undulation are different in the two cases. Air is a 
 material ponderable substance, — lumeniferous ether, an imponderable 
 substance, and therefore not strictly material according to the defini- 
 tion of matter (see “ Imponderable Agents”) ; still it mag be, and 
 no doubt is material, although it has not actually been proved to 
 possess weight. Should space be filled with a material ether, how- 
 ever subtle, it would act as a resisting medium to the motions of the 
 heavenly bodies, and in the case of the comets which have but little 
 mass, and move with enormous velocity, its effect as a direct force 
 continually opposing their motion would be to cause them to 
 describe smaller orbits in an appreciably shorter period of time. 
 Now it is found that the periods of the comets of Encke and Biela 
 are actually diminished by a few hours in every revolution round 
 the Sun ; but this may happen in consequence of their passing 
 through nebulous matter which appears to surround the Sun for a 
 considerable distance, so that it does not afford absolute proof of 
 
LIG 
 
 275 
 
 the materiality of the lumeniferous ether, although it increases the 
 probability of its being subject to the common laws of grosser matter. 
 
 The species of undulations are also different in the cases of Light 
 and Sound. In common light the vibrations of the molecules of 
 ether take place in a line at right angles to the direction of propaga- 
 tion of the ray ; in sound the particles of air vibrate in the direction 
 of propagation. The undulations of light may be compared to those 
 produced by throwing a stone into a smooth and deep pond of water ; 
 those of sound to the waves produced in a field of corn when the 
 wind sweeps over it. In the former case, that of the water set in 
 motion by the stone, each particle of water moves up and down in a 
 vertical line ; in the case of the field of corn, each ear of corn moves 
 backwards and forwards in a horizontal direction, or something ap- 
 proaching to it. The illustration is certainly far from being perfect, 
 but it may assist the reader in forming a true conception of the dif- 
 ference between light and sound. There is also a great difference 
 between the velocity of propagation, and length of the wave in the 
 cases of light and sound. Light travels in vacuo at the enormous 
 rate of 192,000 miles in a second, and the average length of a wave 
 of light is about the one fifty-thousandth part of an inch ; while 
 sound only travels at the rate of about 1100 feet in a second, and 
 the length of a wave of sound capable of affecting the auditory 
 nerves of man, lies between a few inches and several feet , — the short 
 wave most frequently repeated giving the high note, the long wave 
 less frequently repeated, the low note. 
 
 But before carrying these analogies any further, we will endeavour 
 to explain more clearly the precise nature of an undulation of 
 light. 
 
 A luminous body is supposed to be a material substance the 
 particles of which are in a state of intense agitation. These set in 
 motion the molecules of ether next to them, and this motion is com- 
 municated from molecule to molecule, along a line of molecules, 
 with amazing velocity, and in the following manner : 
 
 Suppose AB to be the direction of a ray of light, and bed atoms 
 of ether \ then, the atom b oscillates or vibrates up and down along 
 
 T 2 
 
276 
 
 LIG 
 
 the line el, at right angles to AB; the atom c along the line fm ; 
 d along the line gn ; and so on. 
 
 The velocity of the atom at b gradually decreases as it moves 
 towards e, and on arriving at e altogether vanishes ; the molecule 
 then returns towards b, acquiring fresh velocity as it approaches b , 
 attaining its maximum of velocity at b , then gradually losing it 
 again as it approaches l, and losing it entirely at l ; after which it 
 returns to e as before ; and so on. Similarly with respect to every 
 atom along the line AB ; and it must be clearly understood that no 
 atom actually travels from A towards B, but merely continues to 
 vibrate through an exceedingly small space in a line at right angles 
 to AB. 
 
 This being understood, we have to explain how an undulation is 
 produced and propagated. It will be remembered that although 
 light travels with amazing velocity, yet that velocity is measurable 
 and finite, so that a molecule of ether, b , at one part of the line AB 
 may be vibrating, while another molecule, d , is at rest. Suppose 
 then we consider the state of the vibrating atoms which constitute 
 a ray of light, at a particular epoch of time. It is evident that one 
 atom may be at e , while another is at jp, another at o, another at q , 
 another at /, and so on. If then we draw a curved line through 
 the instantaneous positions of the atoms, epoqf \ that curve will 
 represent an undulation, or wave of light ; and the particles e and /, 
 q and s, are said to be in the same “ phase” of undulation ; the 
 length of the wave being the distance ef, or qs. 
 
 . A ray of common light is composed of undulations which are 
 propagated in the manner described in all possible planes passing 
 through AB. A ray of “ plane polarized light” is one in Avhich 
 the undulations are propagated in only one plane which passes 
 through AB. A ray of “ circularly polarized light” is one in which 
 the curved outline of the undulation, instead of lying on a plane, 
 forms a spiral round AB like the thread of a corkscrew, and called 
 a “helix.” A ray of “ elliptically polarized light” is one in which 
 the spiral, instead of being coiled, so to speak, round a circular 
 cylinder, as in the former case, is coiled round an elliptical cylinder. 
 The subject of polarized light will be discussed presently. 
 
 The effect produced by a ray of light is due to the blow of the 
 last vibrating atom against the material substance upon which it is 
 incident. As the undulations are propagated by the luminous body 
 continuously, these blows follow one another in rapid succession, 
 and a vast number of very small blows thus administered produce 
 an appreciable effect in a finite time. This effect is moreover con- 
 siderably increased when a number of rays are brought to a focus, 
 and act upon the same point. 
 
LIG 
 
 277 
 
 Light, therefore, is motion; or shall we say that light is the 
 means by which a blow is transmitted from the luminous body to 
 the body upon which light is incident. 
 
 Light travels in vacuo with uniforip velocity ; but there are 
 different kinds of light, that is, light which exhibits different colours, 
 viz. red, orange, yellow, green, blue, indigo, violet. These different 
 colours are produced by the different lengths of the waves of light, 
 as exhibited in the following table : 
 
 Extreme red *00075 millimetres 
 
 line A in Spectrum . . . *00074 „ 
 
 •0006879 
 0006559 
 0005888 
 0005265 
 0004856 
 0004296 
 0003963 
 00037 
 00036 
 
 Hence it appears that the waves of red light being the longest, 
 the number of undulations in a given time are the fewest ; and the 
 waves of violet light being the shortest, its undulations are the 
 quickest. 
 
 When light passes from vacuum into a transparent medium, or 
 from a rare medium into a denser, the velocity of the waves is 
 diminished, and vice-versa. The index of refraction, “p,” in 
 geometrical optics, expresses in physical optics the ratio which the 
 velocity of a wave of light in vacuo bears to its velocity in the 
 medium into which it passes. This quantity “ p” is greater for 
 violet than red light ; it would appear, therefore, that their velocities 
 being equal at incidence, the red ray travels faster through a refract- 
 ing medium than the violet ray. There would consequently appear 
 to be a connection existing between the length of a wave and the 
 velocity of its propagation. This circumstance is stated as a difficulty 
 at page 285 of Professor Airy’s Tract on the Undulatory Theory of 
 Light. The difficulty has, however, been since removed by Professor 
 Powell, of Oxford, who has demonstrated that within a refracting 
 medium there is actually a difference between the velocities of red 
 and violet light, the condition being that the intervals between the 
 vibrating molecules of ether should bear a sensible ratio to the 
 length of an undulation, which condition is fulfilled within the 
 
 B 
 
 C 
 
 D 
 
 E 
 
 E 
 
 G 
 
 H 
 
 I 
 
 Extreme violet 
 

 278 LIG 
 
 refracting medium, although apparently not in space, where the 
 velocity of light of all colours is the same. 
 
 All material bodies are supposed to be more or less elastic, their 
 particles not being in actual contact, and the interstices between 
 them filled with lumeniferous ether. It is easy to conceive therefore 
 that the chemical phenomena of light, and we may add of heat, and 
 probably electricity, are produced by motion among the particles of 
 the ether within the interstices of bodies, which communicates 
 motion to the material atoms of the body itself, and alters their 
 mutual arrangement. On this supposition, there can be no such 
 thing as latent heat, latent light, or latent electricity, any more than 
 there can be latent motion, which is a contradiction in terms. 
 If we suppose light, heat, actinism, and the various forms of elec- 
 tricity, when developed in any body to be nothing more than the 
 motion of an ether pervading all space, and filling the interstices of 
 every substance, but varying in the length, velocity, and species of 
 its undulations, we may explain by one general hypothesis a vast 
 variety of astonishing phenomena due to agents between which many 
 strong analogies are found to exist. Heat, for instance, is proved 
 to be the undulation of an elastic medium, and its rays can be 
 reflected, refracted, polarized, and made to exhibit interference just 
 in the same way as rays of light. In short, there is a high degree 
 of probability that the actinic, calorific, and luminous properties of 
 the sunbeam are due simply to the different lengths of the undu- 
 lations which are transmitted — a long wave (comparatively speaking) 
 like the red exhibiting in a marked degree the effects due to heat , 
 a short wave like the violet those due to actinism , and a wave of 
 medium length, those due to light. 
 
 The laws of the reflexion and refraction of light can be easily 
 explained on the undulatory theory, but not without having recourse 
 to a mathematical demonstration which is not sufficiently elementary 
 for the present work. The reader is referred for this demonstration 
 to Professor Airy’s Tract, pages 277 to 296, and also to Herschel’s 
 “Treatise on Light.” 
 
 We shall now consider some of the phenomena of Polarized 
 Light. 
 
 We have said that a ray of common light is composed of undu- 
 lations which take place in all possible planes passing through the 
 direction of the ray. Now the internal structure of certain sub- 
 stances is such, that when a ray of common light is incident upon 
 them, only the undulations which take place in a certain plane or 
 planes can be propagated through the substance, and the others are 
 arrested. An instance of this occurs in the case of Iceland spar, 
 
the crystals of which are rhombs, and are said to be “ doubly re- 
 fracting.” 
 
 Er is a ray of common light incident at r upon a crystal of 
 Iceland Spar. On entering the crystal, the ray is divided into two, 
 and suffers what is called “ double refraction.” One part of it, ro 
 is refracted nearly according to the usual law, and emerges in a direc- 
 tion oo' , parallel to E r. This is called the “ ordinary” ray. The other 
 part of it, re , suffers refraction according to a new law, (which is 
 somewhat complicated) and need not be enunciated in this place,) 
 and takes the direction re, after which it emerges in the direction 
 ee', parallel to Er, the direction of the incident ray. This is called 
 the “ extraordinary” ray a It is evident that after emergence the 
 ordinary and extraordinary rays are parallel to one another. On 
 examining their properties by methods which will be described 
 presently, it is found that both the rays oo' and ee' are what is 
 called “polarized,” that is to say, the undulations take place in 
 one plane only , the plane of the undulations of the ray oo ' being 
 perpendicular to that of the ray ee ' . These planes are called the 
 “planes of polarization,” and the rays are called “polarized rays,” 
 the term being derived from the idea entertained by Newton, that a 
 ray of light has sides or poles. 
 
 In the case of the Iceland spar, both the polarized rays are trans- 
 mitted ; but in that of a thin plate of tourmaline, cut parallel to the 
 axis of the crystal, only one of the polarized rays is transmitted ; 
 and if this polarized ray be received upon another plate of tourma- 
 line placed parallel to the first as regards its plane, but crossways 
 to the other in that plane, the ray will be altogether stopped. This 
 effect may be popularly explained in the following way : — 
 

 280 LIG 
 
 A ray of common light, consisting of undulations in alt possible 
 planes, is incident upon a plate of tourmaline, which is to all appear- 
 ance a transparent substance, but its internal structure is such as to 
 resemble the parallel bars of a grating, or wires of a cage. Tf then 
 we consider the undulations of the ray of common light as taking- 
 place upon a number of cards, all passing lengthways through the 
 direction of the ray, it will be only one of these cards that can be 
 pushed between the bars of tourmaline, and the other cards will be 
 stopped. Again, if we receive »this one card, which is the polarized 
 ray from the first tourmaline, upon a second plate of tourmaline, 
 with its bars placed crossways to the first, it will be completely 
 stopped ; but if the bars of the second tourmaline be placed parallel 
 to those of the first, it will pass through readily enough. This 
 explanation will perhaps convey a sort of popular idea of what is 
 meant by polarized light. A ray of common light may be con- 
 sidered as round, like a ruler ; a ray of polarized light as flat, like a 
 riband. 
 
 Light is polarized by reflexion as well as by refraction, and all 
 reflecting surfaces have the property of polarizing light more or less, 
 according to the angle at which it is incident upon them. Sir David 
 Brewster discovered that when the tangent of the angle of incidence 
 is equal to the refractive index of the medium upon the surface of 
 which light is incident, the reflected ray is completely polarized, and 
 therefore its undulations take place in one plane only. Dor instance, 
 the surface of plate glass is a reflecting surface, and the refractive 
 index of plate glass is about D54, which is the tangent of an angle 
 of 5 7°. If, then, a ray of common light is incident upon the surface 
 of plate glass at an angle of 57°, it will be completely polarized by 
 reflexion, and the reflected ray will not pass through tourmaline 
 placed in a particular position ; nor will it be reflected by another 
 plate of glass placed in a particular position with respect to the 
 first. 
 
 Polarized light consists of the same colours as common light, and 
 the waves interfere in the same way. Photographic pictures may be 
 taken by it, although the time of exposure would require to be in- 
 creased. Photographic experiments with polarized light have not 
 yet, however, received much attention. In the process of copying 
 negatives by light transmitted through them after having suffered 
 reflexion at the surface of a plane reflector, the whole of the light 
 which falls upon the sensitive plate would be polarized if the re- 
 flector were placed at a particular angle with the incident rays. 
 
 By means of the law of the tangent, discovered by Sir David 
 Brewster, the refractive index of opaque bodies may be ascertained 
 
LIG 
 
 281 
 
 by finding the angle of incidence at which complete polarization by 
 reflexion takes place. 
 
 Although the subject of polarized light is one of the greatest in- 
 terest, yet the scope and object of this work do not permit us to say 
 more about it. The phenomena next to be described are those of 
 Interference. 
 
 Beturning to the figure at page 275. 
 
 If we suppose another undulation to be propagated along the line 
 AB, in such a way as to combine with the first, the elevations and 
 depressions of the first would be increased and the effect at the ex- 
 tremity B would be doubled ; but if the second undulation were 
 such that its highest point came exactly over the greatest depression 
 of the first, the undulations would exactly counteract each other, 
 and no effect would be produced atB. Between these two extremes 
 there would be of course an infinite number of mean effects, accord- 
 ing to the way in which the waves were superposed. In the same 
 way, by letting a stone fall into a pond of still water, undulations 
 are produced ; and if a second stone be dropped in the same place, 
 the undulations occasioned by the first may be either increased or 
 diminished, or even altogether destroyed, and smooth water pro- 
 duced, according to the state of the first undulations at the instant 
 of time when the second series were propagated. It would appear, 
 therefore, that two rays of light falling upon the same spot might 
 either produce increased or diminished brightness, or even absolute 
 darkness ; and this is found experimentally to be the case. For, let 
 two rays of homogeneous light, red light suppose, emitted from dif- 
 ferent sources of light A, B, be transmitted through two pin holes 
 in a darkened box, and received upon the same spot, C, of a white 
 screen. If the length of the beam A C be equal to that of the beam 
 BC, or if the difference between AC and BC be any multiple of the 
 length of a ray of red light, i. e., any multiple of *000025 8ths of an 
 inch, the undulations will exactly combine at C, and the intensity 
 of the red spot produced by either ray singly will be doubled. But 
 if the difference between AC and BC be any odd multiple of half 
 the length of a wave of red light, so that the crest of the undulation 
 of one ray may be superposed on the depression of the undulation of 
 the other ray, darkness will be produced at C. Two rays of light 
 incident upon the same spot may therefore produce darkness, and in 
 the same way two rays of heat may produce cold, and two rays of 
 sound silence. This remarkable effect cannot possibly be ex- 
 plained on the corpuscular theory, for on the theory of the emis- 
 sion of particles, more particles ought to produce more light instead 
 of darkness. 
 
282 
 
 LIG- 
 
 The phenomena produced by the interference of waves of light 
 are among the most beautiful in optics. Nothing can exceed the 
 splendour of the coloured images exhibited in many of the experi- 
 ments which are illustrative of interference, but in this place we 
 can only briefly allude to the fact that the colours produced by thin 
 films are caused by the interference of the rays reflected from the 
 inner surface of the film, with those reflected from the outer surface, 
 while the iridescence of mother of pearl, and the varied hues of 
 iridescent ornaments, are occasioned by fine lines existing in the sur- 
 faces of these bodies, which cause the interference of waves of light. 
 Newton’s rings, for instance, and the varied colours of soap-bubbles 
 are produced by interference. 
 
 We now pass on to the Diffraction or Inflexion of light. 
 
 It is assumed in geometrical optics that light can only proceed in 
 a straight line, and therefore that a body which intercepts it must 
 necessarily cast a shadow of definite form, sharp outline, and uniform 
 intensity of blackness. This assumption may have its uses in geo- 
 metrical optics, but in physical optics it is found to be not strictly 
 correct, for it appears that a ray of light, or line of undulations, is 
 actually bent round the corner, so to speak, in passing close to the 
 edge of an opaque body ; so that when an opaque body which inter- 
 cepts the light proceeding through a small orifice into a darkened 
 chamber is sufficiently narrow, and, at a proper distance from the 
 opening, the rays which are bent round the opposite sides of it in- 
 terfere and produce alternate bands of light and darkness across its 
 shadow ; and in every case, whatever may be the shape or size of 
 the intercepting body, it is found that interfering waves of light 
 produce a series of dark lines and coloured spaces round the edge of 
 its shadow. To explain this phenomenon it is supposed that when 
 light is admitted through a small hole into a darkened chamber, the 
 central rays of the pencil pass straight on and produce a light spot 
 upon the opposite wall, while the undulations which immediately touch 
 the sides of the opening have the property, like those of sound, of 
 communicating undulations obliquely to the ether within the box, 
 and thereby of producing refracted rays which travel witli diminished 
 velocities, and, by interfering at the edge of the shadow, produce 
 the dark lines and coloured spaces in question. 
 
 It has been thought by some persons that the inflexion of light 
 round the edges of dark objects which intercept it might tend to 
 produce indistinct positives, when these are taken in a copying 
 camera by light which is transmitted through a transparent negative. 
 But this idea is erroneous. We have shown in the article on the 
 “ Condenser” that the light parts of a transparent negative which 
 
LIG 
 
 283 
 
 has either the sky, or a luminous background, or a light and con- 
 denser, behind it may be considered as made up of a system of 
 bright points, each of which is the origin of a divergent pencil of 
 light so that the bright point immediately adjacent to the edge of 
 any dark part of the negative is the origin of a divergent pencil of 
 light which is refracted by the lens to a focus. Now it is evident 
 that any rays of light which may be bent by inflexion round the 
 edge of the dark part of the negative would only add so many more 
 rays to this divergent pencil, and that the lens would refract them 
 to the same focus as the other rays of that pencil. They could not, 
 therefore, produce indistinctness in the picture. It is important 
 that this should be clearly understood. The indistinctness produced 
 by copying-lenses is occasioned by spherical aberration not being 
 properly corrected in them. 
 
 We must now draw to a close our remarks on the interesting 
 subject of the physical nature of light with a few observations on 
 the colour of natural objects, and the theory of the decomposition of 
 light by absorption. 
 
 The colour of a natural object (when its surface is not iridescent 
 and the colours produced by interference), is due to its absorbing 
 all the rays of light which fall upon it, and reflecting or radiating 
 only those of its particular colour. The colour of a substance is 
 therefore due to some peculiarity of its structure. A black substance 
 absorbs'all the rays of light and reflects none, becoming at the same 
 time heated, or, to speak more correctly, radiating heat rays, which 
 seems to indicate that the absorbed light becomes heat. A white 
 substance, on the contrary, reflects all the rays of light, and absorbs 
 none, and does not become heated (comparatively speaking). This 
 again bears out the idea of the conversion of absorbed light into 
 heat. 
 
 The reader may enquire what, in general, becomes of absorbed 
 light, or lost light. We cannot do better than quote the following 
 reply of Sir John Herschel to this question : 
 
 “ The question, £ What becomes of light ?’ merges in the more 
 general one, £ What becomes of motion ?’ and the answer on dyna- 
 mical principles is, that it continues for ever. No motion is, strictly 
 speaking, annihilated ; but it may be divided, and the divided parts 
 made to oppose, and in effect destroy one another. A body struck, 
 however perfectly elastic, vibrates for a time, and then appears to 
 sink into its original repose. But this apparent rest is nothing else 
 than a state of subdivided and mutually destroying motion, in which 
 every molecule continues to be agitated by an indefinite multitude 
 of internally reflected waves, propagated through it in every possible 
 
284 
 
 LIG 
 
 direction from every point in its surface on which they successively 
 impinge. The superposition of such waves will, it is easily seen, at 
 length operate their mutual destruction, which will be the more 
 complete the more irregular the figure of the body, and the greater 
 the number of internal reflections. 5 ’ 
 
 The theory of the decomposition of light by absorption, brought 
 forward some years ago by Sir David Brewster and since advocated 
 by numerous popular writers, has not been adopted by men of 
 science, but, on the contrary, condemned by Professors Airy, 
 Helmholtz, and others. The idea that the seven colours of the 
 spectrum can be decomposed by absorption into three, viz., red, 
 blue, and yellow, has been shown to be an illogical inference from 
 experiments imperfectly conducted. There must be considered to 
 exist as many different kinds of light as there are different lengths 
 of waves within the limits of the visible spectrum ; and when the 
 undulating ether ceases to produce the phenomena of light and 
 colour, its undulations may be called heat, actinism, or as the case 
 may be, according to the effects they produce on grosser matter. 
 
 With respect to the chemical action of light, this must be consi- 
 dered as due, in all probability, to the vibratory motions of the ether 
 within the interstices of a body establishing a mechanical disturbance 
 amongst its particles, which either enables or compels them in 
 certain cases to form a new arrangement, or enter into new com- 
 binations with each other. By adopting the notion that there is no 
 absolute contact between atoms, and that all bodies are more or less 
 elastic, we can readily imagine that the undulations of the universal 
 ether may act as a mechanical force in promoting chemical combina- 
 tion, or decomposition, determining crystallization, and so on. We 
 need not in this place again enumerate any of the principal phe- 
 nomena of actinism, these having been discussed at some length in 
 the article on the “ Chemistry of Photography,” but it may be well 
 to define clearly what the term actinism means. Assuming, then, 
 as highly probable, that the phenomena of heat, light, actinism, and 
 the various kinds of electricity are produced by undulations of the 
 same universal ether, which differ only in the lengths of the waves 
 and the velocities of their propagation, we should define actinism to 
 include all such chemical changes as are effected by waves of ether 
 varying in length between that of a ray of yellow light and of an 
 invisible ray of the greatest ascertained refrangibility. According 
 to this definition, whatever the chemical effects of the red and yellow 
 rays may be, they would be attributable either to heat or light, and 
 not to actinism, for both heat and light are known to produce 
 important chemical changes. But, after all, a definition of this 
 
LIG 
 
 285 
 
 kind is only of use until by some fortunate generalization of causes 
 we are enabled to substitute a better one for it. 
 
 The question of “Latent Light” is discussed with that of 
 <e Latent Heat q. v. 
 
 Lignin ; C 36 H 22 0 22 * This substance may be considered as a 
 compound of carbon and water. It is of great importance in 
 photography, first because pvroxyline is made from it, and secondly 
 because paper is a nearly pure form of it. 
 
 Lignin forms the solid framework of plants. It is obtained in a 
 pure form by removing from saw dust, or any other kind of finely 
 divided woody fibre, all soluble matter, by steeping it in hot and 
 cold water, boiling it in alcohol, water, solution of potass, weak 
 hydrochloric acid, and lastly in distilled water, and then drying the 
 residue at 212°. Or in addition to the above treatment, it may be 
 bleached by chlorine, and rinsed in acetic acid. The cleansed and 
 bleached fibres of linen or cotton, are tolerably pure lignin. 
 
 Pure lignin is white, tasteless, and insoluble in water, alcohol, 
 ether, and the oils, or hydro-carbons. Its S. G. is 1*5. When 
 acted on by a cold concentrated solution of sulphuric acid, it is 
 converted into dextrine and grape sugar; cold concentrated nitric 
 acid converts it into xyloidin, having nearly the same properties as 
 that obtained from starch ; the continued action of hot nitric acid 
 on it produces oxalic acid; hydrochloric acid blackens, but does 
 not dissolve it, and the acid becomes red or brown ; a hot and 
 strong aqueous solution of potass produces oxalate and acetate of 
 potass. It is evident, therefore, that in the process of making 
 pyroxyline by acting on lignin with nitro-sulphuric acid, other com- 
 pounds may be formed which would in general be injurious in 
 collodion. 
 
 Lignin combines energetically with various salts and metallic oxides, 
 and this property is very important in the arts of dyeing and calico 
 printing, in which colouring matters are made to combine with textile 
 fabrics ; and also in the preservation of timber from dry rot, and 
 of canvas from mildew, &c. This property lies also at the founda- 
 tion of the photographic processes on collodion and paper, for had 
 lignin, in its natural form as paper, or in its altered form as pyroxy- 
 line, no power of combining chemically with metallic oxides, the 
 photograph would merely lie upon the surface of the film, and could 
 be blown by a breath or removed by a touch from it. It may how- 
 ever be the presence of the organic matter that is necessarily asso- 
 ciated with the material of photographs which either causes or assists 
 them to fade. 
 
286 
 
 LIM 
 
 LIN 
 
 Wood may be preserved from dry rot by Kyan’s patented process 
 of steeping it in a solution of bichloride of mercury ; or in one of 
 sulphate of iron, sulphate of copper, or chloride of zinc. The latter 
 salt is especially useful in protecting sail cloth from mildew. Alu- 
 mina combines energetically with calico and linen, and is much used 
 as a mordant in dyeing. 
 
 Woody fibre appears to be permanent in dry air, or completely 
 under water, but not when exposed alternately to the action of air 
 and damp ; the ultimate effect of the gradual process of decay being 
 the removal of all the elements but a portion of the carbon from the 
 lignin. Hence it is that some forms of coal, as anthracite, are nearly 
 pure carbon. One of the products of the decomposition of lignin is 
 “ fire damp ;” another, carbonic acid. The beds of coal in different 
 parts of the world are supposed to have been formed by the decom- 
 position of the forests of monster ferns which at one time covered a 
 large portion of the earth’s surface, in all latitudes, before it cooled 
 down to its present temperature, and when its atmosphere was too 
 much impregnated with carbonic acid to be fit for the support of 
 animal life. 
 
 Lime-light. When a jet of mixed oxygen and hydrogen gases 
 is ignited, the flame is scarcely visible, although intensely hot ; but 
 by introducing solid matter into it, by causing it to play upon a ball 
 of lime a most intense white light is produced. {See “ Flame.”) 
 This is called the “Drummond light.” It is highly actinic, but less 
 so than the light produced by the charcoal points of a voltaic battery. 
 The Drummond light is employed at public institutions for the exhi- 
 bition of dissolving views, and microscopic specimens. Another form 
 of lime light, invented by Messrs. Horne and Thornth waite, of 
 Newgate Street, consists in urging a jet of oxygen through the flame 
 of a spirit lamp, and causing it to play upon a ball of lime. The 
 light is very white and intense, although not equal to that of the 
 Drummond light. It is called the “ Oxy calcium light ;” (q.v.) and is 
 very convenient for exhibiting the magic lantern to a small assemblage 
 of spectators as well as in certain photographic operations when 
 conducted by artificial light. 
 
 The incandescent lime is gradually dissipated, or sublimed, and 
 the lime ball must be renewed from time to time. 
 
 Like. The one-twelfth part of an inch. 
 
 Linseed Oil. A drying oil, obtained by expressing the seeds of 
 common flax, which yield from 20 to 25 per cent, of their weight. 
 
LIT 
 
 287 
 
 LIQ 
 
 Its S. Gr. is -9395 at 52°. It may be cooled down to 40° without 
 solidifying. It is soluble in 40 parts of cold, and 5 parts of boiling 
 alcohol, and in 1*6 parts of ether. A small quantity of the alcoholic 
 solution is sometimes added to spirit varnishes to diminish their 
 brittleness. 
 
 The drying quality of linseed oil is increased by boiling it from 
 three to six hours, and then stirring into it from 7 to 8 hundredths 
 of its weight of litharge ; (< q . v.) In this operation the lead is partially 
 reduced, and a little oleate and stearate of lead are formed, which the 
 oil holds in solution. 
 
 A mixture of boiled linseed oil and mastic varnish forms a gela- 
 tinous substance much used by artists, and called “Magilp.” 
 
 Linseed oil is extensively used in paints and varnishes, and also in 
 printer’s ink. 
 
 Liquor Ammonite. Aqueous solution of Ammonia ; q. v. 
 
 Liquor Potass^e. Aqueous solution of Potass ; q . v. 
 
 Liquorice Sugar. See “ Glycyrrhizine.” 
 
 Litharge; Massicot. Protoxide of lead, Pb. 0=112. The 
 “ galena,” or native sulphide of lead, from which lead is commonly 
 obtained, sometimes contains silver. When this is the case a blast of 
 hot air is passed over the fused mixed metals. This oxidizes the 
 lead, but has no effect upon the silver. The oxide of lead, or litharge, 
 is therefore blown off, and collected in a suitable chamber. 
 
 Litharge is a heavy yellow powder, slightly soluble in water, to 
 which it gives an alkaline reaction. It melts at a red heat and tends 
 to crystallize on cooling, but on reaching 212° falls into a powder. 
 When melted it combines energetically with siliceous matter, and 
 would destroy an earthen crucible. 
 
 Litharge is much used by painters as a dryer. See “ Linseed oil.” 
 Dark red litharge is called “ litharge of gold;” the pale variety, 
 “ litharge of silver.” 
 
 Water which has been filtered through litharge increases the energy 
 of the developer which is dissolved in it, but this should be used im- 
 mediately, for it will not keep. There is also danger of its producing 
 fog upon the plate. 
 
 Litmus; Tournesol; Lacmus. A violet coloured paste, sold 
 in the form of blocks or pyramids. It is made, like archil, by treat- 
 
288 
 
 LOG 
 
 LUT 
 
 ing certain lichens which grow on rocks by the sea side, in the 
 following manner : — 
 
 They are cleaned and ground into a pulp with water ; then, am- 
 moniacal liquors from the gas works are added, and the mass frequent- 
 ly stirred and exposed to the air as much as possible. In this way a 
 peculiar colouring matter is produced, which when perfect is pressed 
 out, and mixed with chalk or plaster of paris so as to form a paste. 
 This is the “ Archil,” of commerce, much used as a purple dye. 
 Another variety, made in the same way in Holland, from the lichens 
 called Roccella tartarea , and Lecanora tartarea , is called “Lit- 
 mus.” It has a violet-blue colour, is easy to pulverize, and is partially 
 soluble in water, and dilute alcohol, leaving a residuum composed of 
 carbonate of lime, clay, silica, gypsum, and oxide of iron combined 
 with the dye. 
 
 The colour of litmus is reddened by acids, and afterwards restored 
 by alkalies. Litmus is much used for making Test-papers ; q. v. 
 
 Logwood. The heartwood of the Hmmatoxylon Campechianum , 
 of the West Indies ; brought to Europe in logs about 3 feet in length. 
 The infusion of this wood is of a dark red or purple colour, and is 
 used in dyeing and staining. It gives purples and blues, and also 
 blacks of various intensities by means of iron and alum bases. Its 
 colouring matter is called “ Hematin.” When chipped logwood is 
 for some time exposed to the air it loses a portion of its dyeing pro- 
 perty. 
 
 Decoction of logwood absorbs oxygen from the atmosphere, and 
 will then precipitate gelatine, a property which it did not at first 
 possess. 
 
 Lunar Caustic. Nitrate of silver fused and moulded into sticks. 
 Commercial lunar caustic is sometimes adulterated with the nitrates 
 of potass, zinc, lead, and copper, and should not be used in photo- 
 graphy. 
 
 Lutes. Used for securing the junctions of vessels, and prevent- 
 ing the escape of their contents when submitted to various chemical 
 operations, as distillation, &c. The lutes used for ordinary purposes 
 are slips of bladder, linseed meal made into paste with gum water, 
 albumen and quick lime, putty, and a fat lute composed of pipe clay 
 and drying oil. Windsor loam is used as a lute to withstand a high 
 temperature ; this is made by mixing clay and sand into a stiff paste 
 with water. If intended to vitrify, borax or red-lead should be mixed 
 with it f 
 
MAG 
 
 289 
 
 Magic Lantern. An instrument for exhibiting magnified 
 images of transparent pictures upon a screen. The arrangement 
 of the different parts of the apparatus will be understood from the 
 following figure. 
 
 A is the source of light ; which may be either an argand lamp 
 with a reflector behind it, or better still, a lime-ball rendered incam 
 descent by passing a jet of oxygen through the flame of a spirit 
 lamp, and causing the flame to act upon it. ( See “ Oxy calcium 
 light.”) BC are the lenses of the condenser, (See “ Condenser,”) 
 which collect all the rays of light which proceed from the lamp, 
 within the angle BAB, and cause them to converge in such a way 
 as to pass through the transparent picture ED. The front lens BB 
 is plano-convex, the back lens C a “ Crossed lens ;” q. v. ED is 
 the transparent picture, which must be placed in an inverted 
 position. L is a combination of lenses having short negative focal 
 length ; and de the screen on which the magnified image of ED is 
 received, and rendered visible. 
 
 The distance of the slide ED from the lens at L is rather greater 
 than the principal focal length of the combination, and then the 
 screen, which is in the other conjugate focus of the lens, is at a 
 much greater distance from it. A pencil diverging from E is 
 brought to a focus at e, and a pencil from D at d ; and so on. The 
 combination of lenses at L which the photographer wdll find by far 
 the best is the common portrait combination ; and w r hen this is 
 used, the posterior lens of the combination should be placed next to 
 the picture, and a stop should be placed between the lenses, as 
 occasion may require. The focus of the condenser should fall pretty 
 nearly on the lens at L which is nearest to the screen. 
 
 In order to obtain a perfectly flat field upon the screen, the 
 picture ED should be painted, or photographed, upon a curved 
 surface like a watch glass. ( See “ Micro -photography.”) 
 
 Magic lantern slides are painted in transparent colours ground in 
 Canada balsam. 
 
 u 
 
290 
 
 MAG 
 
 MAN 
 
 Photographic slides for the magic lantern may either be printed 
 upon dry collodionized or albumenized glass plates, by superposition 
 of the negative, or they may be printed by the wet collodion process, 
 by means of a Copying 'Camera ; q. v. In this case a little chloride 
 of magnesium added to the iodizing solution is an improvement, for 
 it gives greater vigour and a better colour to the blacks. The glass 
 should be perfectly transparent in the light parts ; but stereoscopic 
 slides backed with ground glass may be exhibited in the magic 
 lantern, and produce a tolerably good etfect. 
 
 Sometimes the screen, or sheet, on which the image is thrown 
 is wetted in order to render it semi-transparent, and the spectators 
 are then placed behind it, and consequently see nothing of the 
 apparatus. 
 
 Dissolving views are produced by means of two lanterns, the axes 
 of which are directed towards the same part of the screen, and 
 each of which contains a slide. When a view is to be changed, 
 the cap which closes one lantern is gradually opened, while the 
 other lantern is being shut off. This is done by turning a handle 
 which puts in motion a piece of mechanism contrived for the purpose. 
 
 Magilp. A gelatinous compound made by mixing boiled oil 
 and mastic varnish, in about equal proportions, It is much used 
 by artists for thinning oil colours, and “ glazing” on delicate tints 
 in the finishing of the picture. 
 
 Magma. When a mixture of substances forms a thick pasty 
 mass it is called a “magma.” 
 
 Magnesium. Mg. = 12. A metal, the base of magnesia. It 
 resembles silver in appearance, is ' hard and malleable, and quickly 
 converted into magnesia by the action of damp air. 
 
 Magnesium, Oxide of; Magnesia. Mg. 0 = 20. A white, 
 heavy, insipid powder, nearly insoluble in water, and having a very 
 feeble alkaline reaction on vegetable colours. It absorbs carbonic 
 acid and water from the air, but these may be driven off again at a 
 red heat. Its salts have in general a bitter taste, and many of 
 them, in particular the chloride of magnesium, and nitrate of 
 magnesia, are deliquescent. The affinities of magnesia for the acids 
 are in general about equal to those of ammonia. 
 
 Manganese, Black Oxide of ; Mn. 0 2 . This is a mineral 
 which occurs native in Devonshire, Somersetshire, and Aberdeen- 
 shire. Its chief, and perhaps only use to the photographer, is as a 
 
MAN 
 
 MAE 
 
 291 
 
 source of oxygen gas for the oxycalcium light. When heated in a 
 retort to a full red heat it gives olf oxygen, and becomes converted 
 into a sesquioxide, Mn. 2 0 3 ; it should be well dried before being 
 put into the retort. When added, in the proportion of about one 
 part of black oxide of manganese, to three parts of chlorate of potass, 
 and moderately heated in a retort, over a common fire, oxygen is 
 abundantly given off. The presence of the manganese greatly 
 assists the evolution of oxygen from the latter salt, without appearing 
 itself to undergo decomposition. 
 
 Per Manganate of Potass : K O, Mn. 2 0 7 , This salt is easily 
 prepared thus : — 
 
 Equal parts of finely powdered black oxide of manganese and 
 chlorate of potass are mixed with one part of caustic potass dissolved 
 in a little water, then evaporated to dryness, and exposed to a tem- 
 perature just short of ignition. The mass is then treated with hot 
 water, the insoluble oxide separated by decantation, and the deep 
 purple liquid concentrated by heat until crystals form upon its surface. 
 It is then left to cool. The crystals are a deep purple colour and 
 not very soluble in water. 
 
 This salt is a powerful oxidizer, and its solution becomes decom- 
 posed and decolorized by contact with impurities existing in the 
 atmosphere of towns. (See “ Sepometer.”) It has also been used 
 as a means of testing the comparative stability of positive prints pro- 
 duced by various processes. 
 
 Marine Cement ; Marine Glue. Digest from 2 to 4 parts 
 of india rubber, cut into small pieces, in 34 parts of benzole, and 
 promote solution by heat and agitation. The solution should have 
 the consistence of thick cream. Add to it 62 parts of powdered 
 shellac, and melt the mixture over the fire, stirring it well. Then 
 pour it upon plates of metal, so that it may cool in sheets. 
 
 To use it, heat it in an iron vessel to about 250°, and apply it 
 with a brush to the surfaces to be joined. 
 
 Markin g Ink. Eirst apply the following mordant to the linen : — 
 
 Carbonate of soda . . . .2 ozs. 
 
 Distilled water . . . . . 1 pint. 
 
 Then write upon it, when dry, with the following ink 
 Nitrate of silver . . .1 drachm. 
 
 Powdered gum arabic . . . 2 drachms. 
 
 Sap green .... 1 scruple. 
 
 Distilled water . . . . 1 ounce. 
 
 u 2 
 
292 
 
 MAS 
 
 MEL 
 
 Or, the following ink may be applied without a mordant : — 
 
 Nitrate of silver . . .1 ounce. 
 
 Carbonate of soda . . . li ounce. 
 
 Tartaric acid . . . .2 drachms. 
 
 Ammonia . * . . . 2 ounces. 
 
 Archil . , . . . oz. 
 
 White sugar . . . . 6 drachms. 
 
 Powdered gum arabic . .10 drachms. 
 
 Distilled water . . . . quantum suff. 
 
 Dissolve the nitrate of silver and carbonate of soda separately, then 
 mix, wash the precipitate, put it into a mortar and add the tartaric acid 
 until effervescence ceases. Add the ammonia to dissolve the tartrate 
 of silver, then mix in the other ingredients with the distilled water. 
 
 Mastic. A species of resin much used in varnishes. It comes 
 from the Levant, and occurs in small drops or tears of a pale yellow 
 colour, which are the produce of the Pistacia lentiscus. It contains 
 about 90 per cent, of a resin easily soluble in alcohol (masticic 
 acid), and a viscid and difficultly soluble resin. 
 
 Matches, Congreve, or Lucifer. Make the following ingredients 
 into a paste with water, and dip the matches into it : — 
 
 Gum arabic . . . . . 16 parts 
 
 Phosphorus, powdered « . . 9 „ 
 
 Nitrate of potass . . . . 14 „ 
 
 Black oxide of Manganese . . 18 „ 
 
 Measures. See Tables at the end. 
 
 Melting Point of Metals. The following table is extracted 
 from “ Turner’s Elements of Chemistry”: — 
 
 
 / Mercury 
 
 Fahrenheit. 
 
 . . -39° 
 
 
 Potassium 
 
 . . 136° 
 
 Fusible below a red 
 
 Sodium 
 
 . 190° 
 
 heat, or 980°, the 
 
 Tin . 
 
 , . 442° 
 
 heat of a common 
 
 Cadmium 
 
 . 450° 
 
 fire being about 
 
 Bismuth . 
 
 . . 497° 
 
 1200°. 
 
 Lead 
 
 . 612° 
 
 
 Zinc 
 
 . . 773° 
 
 
 x Antimony 
 
 . . — 
 
 
 Silver 
 
 , . 1873° 
 
 
 Copper . 
 
 . 1996° 
 
 
 Gold 
 
 . . 2016° 
 
 
 Cast Iron 
 
 . 2786° 
 
MEH 
 
 MET 
 
 293 
 
 
 Cobalt and nickel are rather less fusible than iron. 
 
 Platinum and chromium are only fusible before the oxyhydrogen 
 blow-pipe. The same- may be said of most of the very scarce 
 metals. 
 
 Mercury; Hg. = 100. A white metal, fluid at ordinary tem- 
 peratures, and solid at —40°. It boils and becomes vapour at 660°, 
 and emits vapour at all temperatures above 40°. The principal 
 ore of this metal is the sulphide, or native cinnabar. 
 
 Perfectly pure mercury may be agitated in contact with air, 
 oxygen, hydrogen, nitrogen, carbonic acid, and alcohol ; but when 
 impure it becomes coated with a grey powder, which is a mixture of 
 the oxide of the foreign metal and finely divided mercury. On the 
 other hand, when mercury is agitated with water, ether, or oil of 
 turpentine, it is reduced to grey powder, which is composed of 
 minute globules of mercury blended with the foreign body. 
 
 Mercury combines with several of the metals, and forms “ amal- 
 gams.” 
 
 Mercury, Oxide of. There are two oxides of mercury, viz., 
 the black, PIg. 3 0, and the red, Hg. 0. Both are salifiable. The 
 black oxide is reduced by light into Hg. and Hg. O. The red 
 oxide becomes black when heated, but red again on cooling. It is 
 superficially reduced by light, and becomes black. These oxides 
 form a great number of curious and important salts. 
 
 Mercury Bath. The box in which daguerreotypes are deve- 
 loped by the fumes of mercury. It should be made of iron, in the 
 form of an inverted pyramid, provided with a thermometer, and 
 supported upon an iron stand. The plate should be placed horizon- 
 tally over the mercury. This form of apparatus was first used in 
 America, and may now be seen at most photographic depots in this 
 country. It is very simple and inexpensive, 
 
 Metagallic Acid. An organic substance formed by the oxid- 
 ation of gallic acid. It combines with oxide of silver, and other 
 metallic oxides, and displaces carbonic acid from the alkaline car- 
 bonates. It is produced by rapidly heating gallic acid up to 480°, 
 when carbonic acid and water are evolved, and metagallic acid 
 remains as a black shining substance, insoluble in water, alcohol, 
 and ether. Its equivalent is supposed to be C 12 H 3 0 3 , HO. 
 
 Metagelatine. When a strong solution of gelatine has been 
 
boiled and cooled several times it ceases to gelatinize on cooling, 
 and remains fluid. In this state it is called metagelatine, and may 
 be used successfully as a preservative substance in the collodion 
 process. It is a much less powerful reducing agent than honey, and 
 therefore less likely to fog the collodion plate. 
 
 The mode of preparing metagelatine, originally described by 
 Mr. Maxwell Lyte, is as follows : — 
 
 Dissolve 14 ounce of pure white gelatine in 10 ozs. of boiling 
 water. Add 60 minims of strong sulphuric acid diluted with 
 24 ozs. of distilled water. Boil for five minutes, and then let the liquid 
 cool. Then boil it again for five minutes and let it cool. Should 
 it still gelatinize on cooling, repeat the operation. When it remains 
 fluid on cooling, neutralize the acid with powdered chalk, and 
 remove the insoluble sulphate of lime by squeezing the mixture 
 through a cloth. 
 
 Metagelatine dries to a hard transparent film upon the collodion. 
 When intended to be used as a moist preservative, a little golden 
 treacle, (not honey nor glucose,) should be added to it. 
 
 Methyl. Me. (Greek, /jieOv wine, and v \ r ) wood.) A hypo- 
 thetical substance, the composition of which is assumed to be C 2 H 3 , 
 and which forms the supposed base of a series of methyl compounds 
 analogous to those of ethyl. 
 
 Metrylic Ether. Me. 0 = C 2 H 8 O. When equal parts of 
 “ wood alcohol, 55 (or “pyroligneous spirit, 5 ’ as it is termed,) and 
 sulphuric acid are distilled together, a gas is evolved, which, when 
 collected over mercury and purified by potass, is found to consist of 
 oxide of methyl, or metliylic ether. It has an etherial odour, with 
 a taint resembling peppermint, and may be taken up abundantly 
 by alcohol, or wood-alcohol, or ether. 
 
 Methyltc Alcohol. Me. O + H 0 = C 2 H 3 0 + H 0. This 
 substance, called “wood alcohol, 55 or “ pyroxy lie spirit,” is a hy- 
 drated oxide of methyl, and analogous to the alcohol of the 
 ethyl series. It is one of the products obtained from the distillation 
 of wood. In its properties it greatly resembles alcohol, and as 
 there is no duty on it, it is much used as a solvent for varnishes, 
 &c. Its taste is hot and pungent; S. G. *800; it boils at 150°, at 
 the mean pressure of the atmosphere ; mixes in all proportions with 
 water, alcohol, and ether ; and is neutral to test paper. 
 
 According to an excise regulation, when ordinary alcohol is 
 
MIC 
 
 295 
 
 mixed with 10 per cent, of wood alcohol, it is sold free of duty, 
 under the name of “ methylated spirit. 5 ’ 
 
 Methylated spirits are extensively used in the manufacture of 
 collodion, on account of the heavy duty on ether and alcohol. It is 
 probably due to the variable quality of these spirits that so much 
 bad collodion finds its way into the market* 
 
 Mica. A transparent mineral, of a pearly lustre, mostly brought 
 from Siberia. It may be divided into sheets, which are hard and 
 flexible. It is an ingredient of granite and gneiss. 
 
 Micro-Photogkaphy. Under this head may be included two 
 different processes. One is of little or no practical utility, and con- 
 sists in copying objects on an exceedingly small scale, the photograph 
 being intended to be viewed through a magnifier, or microscope. The 
 other, which is a branch of photography of the highest interest and 
 importance, consists in producing enlarged photographs of minute 
 objects — that is, in fixing the images obtained in the microscope. 
 
 With respect to the former of these two processes, a few words 
 will suffice. In order to obtain an extremely minute image of an 
 object, it must either be placed at a much greater distance than 
 usual from the lens, or a lens of very short focus must be used. 
 Any of the ordinary forms of photographic lens may be made of 
 one inch focal length, and then by placing the object or group to 
 be copied at a suitable distance, a photograph may be obtained in 
 a drop of collodion. The only difficulty in the process is that of 
 focussing. This may be done on the sensitive film itself, a yellow 
 glass being interposed between it and the lens while focussing, and 
 a powerful magnifier employed to examine the focus. Negatives 
 may, of course, be copied either by transmitted natural or artificial 
 light. Enough has now been said about a process which must 
 strike any reasonable person as somewhat trifling and childish, when 
 he considers how many valuable applications of photography remain 
 yet to be worked out. 
 
 The copying of magnified images in the microscope, in such a 
 way as to satisfy sound criticism, is a matter of much greater 
 difficulty. The principle of the thing is simple enough, and will 
 be understood by refeience to Eig. 6, page 63, in which P Qmay 
 be considered a microscopic object, and p q its magnified image ; 
 but the difficulty consists in obtaining a flat field when the ratio 
 between the size of the image and the object is considerable. In 
 the low-power lens of an ordinary microscope this ratio is as 8:1; 
 and the size of the field about 1*4 inches. By using a lens of double 
 
296 
 
 MIL 
 
 the focal length, and removing the object to double the distance from 
 it, a picture of the same size is obtained, the lateral pencils have 
 less obliquity, and the field is much flatter, but the amount of light 
 is reduced to one fourth. It is evident, therefore, that the optical 
 difficulties are diminished by increasing the intensity of the light. 
 It would be well, therefore, to perform micro-photographic operations 
 by sunshine transmitted through the object by means of a reflector 
 and condenser ; or else with the Bude light. A great amount of 
 solar light is, however, accompanied with great heat, and a difficulty 
 is introduced from this cause. The photographer has, however, two 
 other resources. He may either work upon plates which are ground 
 spherical, like a watch glass ; or the object itself may be inclosed 
 between curved glasses. The former plan would probably be found 
 the best. 
 
 With respect to the best form of lens. There is an erronous im- 
 pression afloat among amateur microscopists who know but little of 
 optics, that the obliquity of the pencils when high powers are used 
 is something considerable, and that modern science has brought under 
 its control the various vagaries of oblique pencils. This is not the 
 case. The vagaries of oblique pencils are incurable, and like the 
 quadrature of the circle they set at defiance all the resources of the 
 mathematician. The truth is, the obliquity of the lateral pencils in 
 the microscope is inconsiderable compared with that of the lateral 
 pencils in an ordinary camera obscura. The best form of lens for 
 obtaining micro -photographs is therefore an achromatic single lens, 
 as represented in Fig, 6, and constructed on a similar principle, to 
 the object glass of the telescope. By increasing the number of lenses, 
 light is lost, and distortion introduced. 
 
 The principles of this branch of photography having been now, it 
 is hoped, clearly explained, the various modes of applying them may 
 be left to the ingenuity of the reader. 
 
 Milk. The milk of different animals does not appear to vary 
 much in composition. It is composed of three principles, viz., butter, 
 cheese, (or casein ,) and whey, (or serum). 
 
 When examined under the microscope milk appears as a transpa- 
 rent fluid in which small white globules are diffused. These rise to 
 the surface when the milk is left at rest, and form cream, which may 
 be removed by skimming. The globules are supposed to be the 
 butter contained in small membranous bags, which are broken by 
 churning. Ether has at first no effect upon these globules, but the 
 membrane is soluble in acetic acid, and when dissolved the butter is 
 liberated and ether readily takes it up. 
 
MIN 
 
 MOS 
 
 297 
 
 Casein is a substance resembling albumen in its properties. It is 
 soluble in an alkali, and may be coagulated by an acid. See “ Ca- 
 sein.” 
 
 Serum or Whey is the watery matter of milk. It contains a sugar 
 called Sugar of Milk, q. v., and also various salts. 
 
 Milk contains about 14 per cent, of butter, 14 per cent, of casein, 
 and the remainder whey. It is either neutral or slightly alkaline, 
 but quickly becomes acid by exposure to air, from the formation of 
 lactic acid. 
 
 Both casein and whey are useful in photography, but milk should 
 not be used in any process until the cream has been completely re- 
 moved by skimming. Curd or casein is produced by adding an acid, 
 or better still a piece of rennet, to fresh milk. See “ Bennet.” 
 
 Minium. Bed Lead ; q. v. 
 
 Mordant. A class of substances used in dyeing, their effect 
 being to cause the dyeing material to combine with the fabric to which 
 it is applied, so that it cannot be removed by simple washing, or any 
 ordinary treatment. Mordants are in general metallic oxides which 
 have an affinity for the organic matter of the stuff, and by combining 
 with it, cause the particles of the dye to combine also, and form a 
 species of triple compound. The mordant most likely to be useful in 
 photography is bichromate of potass when reduced by light. This 
 enters into powerful combination with lignin, and takes down with, 
 it any particles of carbon; or pigment, or colouring matter that are 
 mixed with the bichromate, In this way textile fabrics may be printed 
 by photography. 
 
 Moser’s Images. M. Ludwig Moser has described at different 
 times in “ Poggendorff’s Annalen,” a variety of curious experiments 
 in which images were obtained by contact and developed by vapours 
 in a similar way to the images in the process of Daguerre. Prom 
 these experiments he has drawn certain conclusions, which are 
 generally supposed to be erroneous. Por an account of them the 
 reader is referred to “ Hunt’s Besearches on Light,” page 248. Mr. 
 Grove, and Professor Volpicelli have also obtained latent images 
 capable of being developed by vapours, by means of contact aided by 
 electricity. The account of these experiments will be found in the 
 2nd volume of “ Photographic Notes,” edited by the author. This 
 subject is one of great interest, and no doubt intimately connected with 
 photography ; it is however at present involved in so much obscurity, 
 that we think it better to refer the reader to other sources of infor- 
 
298 
 
 MOU 
 
 NAP 
 
 mation than to occupy space in this work with more than a brief 
 notice of it. 
 
 Mounting Prints. This consists in attaching the print, either 
 entirely, or by its edges, to a sheet of cardboard, by means of an 
 adhesive cement. 
 
 Of all adhesive substances, starch appears to be the most suitable. 
 It should not be made too thick, and after being boiled should be 
 strained through a cloth, and used quite fresh, as sour starch would 
 be certain to cause the fading of the print . 
 
 The face of the cardboard is first damped with a moist sponge, to 
 cause it to expand in the same degree as the print when the starch 
 is applied to it. If this be not done, the print contracts on drying, 
 and draws the cardboard out of shape. The print is then laid with 
 its face upon a slab of glass, and the starch spread thinly and evenly 
 upon the back with a stout hog-hair paint-brush. The print is then 
 applied carefully to the damp cardboard, and lightly pressed into 
 contact, and air bubbles pressed out, with a linen rag. A sheet of 
 cardboard is then laid upon it, and the rag rubbed over this pretty 
 forcibly in order to ensure the perfect adhesion of the print to the 
 mount in every part. The cardboard upon which the print is mounted 
 is then pinned up by one corner to dry, and afterwards placed under 
 a book -binder’s press, or a board loaded with weights. 
 
 Trench cardboard is mechanically the best for the purpose, and 
 the thicker it is the easier the operation becomes ; but the bluish tint 
 of Trench cardboard is very objectionable, as it contrasts disagreeably 
 with the tint of most photographs. Cream colour is far better ; and 
 there is no doubt that cardboards might be manufactured of a great 
 variety of suitable tints, and that the general effect of photogra- 
 phic prints would be greatly improved by being mounted upon such. 
 Stereoscopic subjects should be mounted upon cardboards nearly if not 
 absolutely black ; and collodion positives on paper either upon gilt 
 or deep purple cardboard. As a general rule a photograph should 
 never be contrasted with anything absolutely white, or what is 
 even worse, of a bluish white like Trench paper. 
 
 Muriatic Acid. See “ Hydrochloric acid.” 
 
 Naphtha ; Bock Oil. C 6 H 5 . A combustible and volatile 
 liquid, resembling oil of turpentine. It occurs naturally, and may 
 also be made artificially. As a natural product it exists in the soil 
 at Baku, on the north east shore of the Caspian, at Amiano in the 
 duchy of Parma, at Zibio in the duchy of Modena, at Neufchatel 
 
NAT 
 
 NEG 
 
 299 
 
 
 in Switzerland, at Clermont in France, at Val di Noto in Sicily, 
 at Trinidad, Barbadoes, Bangoon, &c. The naphtha of commerce 
 comes mostly from Trinidad. As an artificial product it is obtained 
 from the distillation of petroleum, or the coal-oil of the gas works. 
 
 The S. G. of Persian Naphtha is about *775, of coal naphtha about 
 *820. 
 
 Naphtha does not congeal at 0° ; it is not soluble in water, but 
 communicates a smell and taste to it ; it dissolves in absolute 
 alcohol, ether, and oils, and is a solvent of the resins, as well as of 
 phosphorus and sulphur in small quantities. It softens and gela- 
 tinizes india rubber, and this glairy varnish is spread upon textile 
 fabrics to render them waterproof. 
 
 The boiling point of naphtha is about 320°. It is not acted on 
 by potassium and sodium, and is used for preserving those highly 
 oxidable metals from the action of the air. 
 
 The naphtha above described is sometimes called mineral naphtha 
 to distinguish it from wood alcohol or “ wood naphtha,” which is a 
 totally different substance. 
 
 Naphtha may be economically used for burning in spirit lamps. 
 
 Natural Colours. To obtain photographs in the natural 
 colours is, of course, a grand problem ; but the probability of its 
 ever receiving a solution is, in our opinion, very small ; and, so far 
 as we can see, nothing has yet been done to inspire a hope that it 
 may ever be accomplished. The different colours produced upon 
 various sensitive tablets by coloured images seem, in every case, to 
 arise from the different state of decomposition of the sensitive 
 material by different degrees of actinic power in the light which do 
 not correspond with the different tints of the natural colours. 
 
 In order to produce a photograph in the natural colours, the 
 coloured image formed in the camera must be received upon a 
 tablet so constituted as th t every colour may, where it acts, produce 
 such an effect as that when the tablet is afterwards exposed to white 
 light the same coloured rays may be emitted at the particular spot 
 impressed. To discover the means of producing such a tablet really 
 seems to be hopeless, although certainly not impossible. 
 
 We refer the reader to a pamphlet published by Mr. Thomson, 
 (of Messrs. Boss and Thomson, Edinburgh) for particulars of various 
 experiments which have been made by different persons with the 
 view to the discovery of photography in the natural colours. 
 
 Negative. A photograph in which the lights and shades are 
 reversed, and the opposite of those in nature, is called a “ negative.” 
 
300 
 
 NIT 
 
 The value of a negative consists in the means it affords of multiply- 
 ing positive prints in which the lights and shades are true to nature. 
 The best negatives are those which are taken upon glass coated 
 with a uniformly transparent and structureless him of collodion, 
 albumen, &c. ; but as glass is a costly, heavy, and fragile substance 
 negatives are sometimes transferred from glass to a him of gutta 
 percha ; and are frequently taken upon paper, waxed in order to 
 render it more evenly transparent. Tor a certain class of bold 
 artistic subjects paper may be considered suitable. 
 
 The various negative processes upon collodion, albumen, paper, 
 &c., are described under their respective heads. 
 
 Nitrate or Ammonia. NH 4 O, N0 5 , This salt is obtained 
 by neutralizing ammonia, or carbonate of ammonia, with dilute 
 nitric acid, evaporating and crystallizing. At 228° it fuses ; at 
 356° boils, without decomposition; and at 400° is decomposed into 
 nitrous oxide, (laughing gas,) and water. It is slightly deliquescent, 
 and soluble in rather more than its own weight of water, and in 
 about six times its weight of alcohol. It is much used in making 
 freezing mixtures. 
 
 Nitrate of ammonia is not a stable salt, for it parts with ammonia 
 to the air, and becomes acid with free nitric acid. When the haloid 
 salts of ammonia are used in photography, nitrate of ammonia is 
 formed in the nitrate bath, and hence arises the danger of the 
 bath becoming acid with free nitric acid, from the escape of the 
 ammonia. Nitrate of ammonia is a solvent of oxide of silver ; and 
 this constitutes another objection to the use of the ammonia salts in 
 the negative processes. 
 
 Nitrate of Baryta. Ba. O, N0 5 . This salt is formed by 
 dissolving the native carbonate of baryta in dilute nitric acid, 
 evaporating to dryness, redissolving and recrystallizing. It is per- 
 manent, and anhydrous ; soluble in 12 parts of cold, and 4 parts of 
 boiling water, and insoluble in alcohol. It is decomposed at a red 
 heat, and yields pure baryta. 
 
 Nitrate of baryta is used in making a good iron developer for 
 collodion positives. If dissolved in hot water the solution must be 
 allowed to get quite cold before being added to the iron salt. 
 
 Nitrate of Cadmium. Cd. 0, N0 5 + 4 HO. This salt is 
 deliquescent, and soluble in alcohol. It has an acid reaction, and 
 its accumulation in the nitrate bath, when iodide and bromide of 
 cadmium are used in the iodizing solution, is highly injurious in the 
 
NIT 
 
 301 
 
 negative process, by destroying tlie sensitiveness of the film, and 
 rendering the image thin and metallic, while in the positive bath it 
 is liable to fog the plate. 
 
 Nitrate of Iron. There are two nitrates of iron, the proto- 
 nitrate, and the pernitrate. The protonitrate is a deoxidizing sub- 
 stance, obtained in solution by mixing solutions of protosulphate of 
 iron and nitrate of baryta, according to their equivalent proportions. 
 This solution is decomposed at a boiling temperature, and becomes 
 converted into pernitrate of iron by long exposure to air. 
 
 Protonitrate of iron is used either alone, or in conjunction with 
 protosulphate of iron, as a developer of collodion positives. It is 
 not an energetic developer, and when used alone renders the whites 
 of the image too metallic. 
 
 Pernitrate of iron is a reddish brown deliquescent substance, 
 soluble in water and alcohol, and decomposable at a red heat. 
 
 Nitrate of Lead. Pb. O, N0 5 . A white, anhydrous salt, 
 obtained by acting on litharge with hot nitric acid diluted with 2 
 parts of water, (the litharge not being in excess,) evaporating and 
 crystallizing. It is soluble in 8 parts of water, and insoluble in 
 alcohol. 
 
 When nitrate of lead and litharge, in equal weights, are boiled 
 together in water, and the solution filtered and crystallized, white 
 crystals are formed of subnitrate of lead, which are nearly insoluble 
 in cold water. 
 
 Nitrate, or subnitrate of lead, added to the negative nitrate bath 
 has been supposed to render the process more sensitive. In the 
 positive collodion process a tolerably good developer may be made 
 by substituting nitrate of lead for nitrate of baryta. 
 
 Nitrate of Magnesia. This salt is deliquescent, soluble in 
 an equal weight of water, but nearly insoluble in alcohol. It has 
 an acid reaction, and therefore renders the nitrate bath slightly acid 
 when iodide or chloride of magnesium are used in the iodizing 
 solution. Prom its deliquescent properties, solution of nitrate of 
 magnesia has been used as a means of preserving the moisture of 
 the sensitive collodion film, but not with much success. 
 
 Nitrate of Potass ; Nitre ; Saltpetre. KO, N0 5 . This 
 important salt is obtained in India in" large quantities, but in an 
 impure form, by lixiviating certain soils ; and in Prance, and other 
 countries, it is obtained from old mortar rubbish, in which organic 
 
302 
 
 NIT 
 
 matter has supplied the nitrogen wherewith to form nitrate of lime, 
 which is then decomposed by carbonate of potass. 
 
 Pure nitre is not rendered turbid, when in solution, either by 
 nitrate of silver, or nitrate of baryta. It crystallizes in long six 
 sided prisms, and is anhydrous, the moisture of the crystals being 
 merely interstitial. They are permanent in the air. Nitre is soluble 
 in about 4 parts of water at 60 and in equal parts of boiling water, 
 but is nearly insoluble in alcohol ; it fuses at 600 without decom- 
 position, and at a red heat is slowly decomposed and yields oxygen, 
 lib. of nitre giving off about 12,000 cubic inches of gas. 
 
 Pulminating powder is made by mixing 3 parts of nitre, 2 of dry 
 carbonate of potass, and 1 of sulphur. If a small quantity of this 
 mixture be placed upon a shovel over the fire, it first melts and 
 blackens, and then explodes with violence. 
 
 Gunpowder is made by mixing 1 part of charcoal, 1 of sulphur, 
 and 6 of nitre, all in fine powder. 
 
 The principal impurities in nitre are the chlorides of sodium and 
 potassium, sulphate of potass, and nitrate of soda. 
 
 Nitrate of potass added to protosulphate of iron forms a tolerably 
 good developer for collodion positives, but nitrate of baryta is better, 
 because sulphate of baryta is insoluble, and may be separated from 
 the solution, which sulphate of potass cannot. 
 
 Pyroxyline is sometimes made by adding nitrate of potass to 
 sulphuric acid, instead of mixing the sulphuric and nitric acids. 
 The plan is not good, because it is extremely difficult to remove the 
 whole of the bisulphate of potass which is formed from between 
 the fibres of the pyroxyline with which it is entangled. 
 
 The nitrate of potass formed in the nitrate bath when iodide of 
 potassium is used as an iodizer, is neutral, and appears to be inert 
 in the process. 
 
 Nitrate of Silver. Ag. O, NO s ■= 170. This is one of 
 the most important salts at present used in photography. It is 
 obtained in its purest form by dissolving pure silver in pure nitric 
 acid, S. G. 1*25, evaporating and crystallizing, and then redissolv- 
 ing and recrystallizing. The salt is then neutral to test paper. 
 Commercial nitrate of silver is made by dissolving silver coins, or 
 silver plate, in nitric acid, evaporating and crystallizing, washing 
 the crystals with nitric acid, and then redissolving and recrystalliz- 
 ing. The alloy of copper and other metals is not completely got 
 rid of in this wav. Commercial nitrate of silver is sometimes 
 adulterated with the .nitrates of potass, zinc, lead, &c. Impure 
 nitrate of silver is one of the greatest evils with which the photo- 
 
NIT 
 
 303 
 
 grapher has to contend, and his best remedy is to precipitate the 
 chloride by adding salt to the solution of impure nitrate, then to 
 reduce the chloride, by fusion in a crucible with twice of its weight 
 of carbonate of soda, to a button of pure metallic silver, and re- 
 dissolve this in pure nitric acid. 
 
 Nitrate of silver crystallizes in large, flat, nearly transparent, four 
 or six sided tables. It is soluble in about an equal weight of cold 
 water, and in four times its weight of boiling alcohol, but sparingly 
 soluble in cold alcohol. It is anhydrous, and may be fused and 
 run into moulds. It is slightly deliquescent in moist air. Its 
 solution is perfectly colourless ; and neither the crystals nor the 
 solution are affected by light, unless organic matter be present. It 
 is a powerful caustic, in consequence of its ready decomposition by 
 organic matter; and therefore highly poisonous, salt being the 
 best antidote. 
 
 It has been recommended to use fused nitrate of silver in photo- 
 graphy, because all the free nitric acid which it may contain is 
 driven off by the heat in the process of fusion ; but this is a bad 
 plan when the nitrate of silver is adulterated with nitrate of potass, 
 because nitrite of potass is first formed by heat, and this becomes 
 oxidized at the expense of the nitrate of silver, and forms nitrite of 
 silver, a most injurious substance to introduce into a nitrate bath. 
 Fused nitrate of silver is generally alkaline, probably because some 
 oxide is formed which is combined with the nitrate ; the alkalinity 
 should be neutralized with acetic acid for a negative bath. Fresh 
 distilled water should always be used for dissolving nitrate of 
 silver, as pump or river water contain salts which produce a cloudy 
 precipitate with it. Rain water collected in leaden tanks should 
 on no account be used, as it contains oxide of lead in solution, 
 which is very likely to fog the negative. (See “ Leaden Tanks.”) 
 Nitrate of silver may easily be fused in a porcelain capsule over a 
 spirit lamp. 
 
 If a piece of copper be suspended in a -solution of nitrate of silver, 
 the silver is precipitated in beautiful crystals; but a piece of 
 polished iron or steel is not acted on. A little mercury poured into 
 the solution occasions the precipitation of crystals in the form of a 
 shrub, called the Arbor Diance. 
 
 Nitrate or Uranium. There are two nitrates of this metal, 
 the protonitrate, UO, N0 5 , and the pernitrate, or nitrate of the 
 sesquioxide, U 2 0 3 , N0 5 -f6 HO. The latter is the common 
 crystallized salt, obtained by dissolving any ofj,he oxides of uranium 
 in nitric acid. It forms yellow prisms, which are efflorescent, and fuse 
 
304 
 
 NIT 
 
 in their water of crystallization. It is very soluble in water, alcohol, 
 and ether. By exposure to light it is reduced to the protonitrate. 
 
 Nitrate of Zinc. A deliquescent salt, having an acid reaction. 
 It has been used as a means of preserving the moisture of sensitive 
 collodion plates, but the plan does not seem to answer. 
 
 Nitrate Bath. Many of the failures in photographic opera- 
 tions, particularly in the negative collodion process, may be traced 
 to the nitrate bath being out of order. 
 
 1st. It may not be of the right strength. Test it with the silver 
 meter ( q . tf.),*and add either water or nitrate of silver, as the case 
 may require. 
 
 2nd. It may be alkaline to test paper. To a negative bath add 
 acetic acid, a drop or two at a time, until acidity is established. 
 To a positive bath add nitric acid. The bath becomes alkaline in 
 consequence of the introduction of carbonate of potass, an impurity 
 contained in iodide of potassium. An alkaline bath fogs the plate, 
 and gives a very dense picture. 
 
 3rd. It may be too acid. This diminishes the sensitiveness of 
 the film, and in the case of acidity from nitric acid produces grey 
 metallic negatives devoid of density, and brings about by great 
 over-exposure the reverse action of light* In the case of a great 
 excess of acetic acid, the negatives are all black and white, and 
 devoid of half-tone. Acidity keeps the lights clean and transparent. 
 It is brought about either by the fermentation of the ether and 
 alcohol which are continually added to the bath, or by the accu- 
 mulation of an acid salt, such as nitrate of cadmium. The remedy 
 for excess of nitric or acetic acid is carbonate of soda. Add a 
 solution of this, a few drops at a time, until the bath is rendered 
 alkaline, then acidify slightly with acetic acid, and filter. The 
 addition of carbonate of soda produces a yellow turbidity, due to 
 carbonate of silver; when acetic acid is added acetate of silver is 
 formed, and carbonic acid liberated ; the acetate of silver is, for the 
 most part, dissolved in the nitrate bath, the remainder is removed 
 by filtering. The shaking of the bath in travelling frequently 
 renders it acid by causing the fermentation of the organic matter. 
 
 4th. The picture may be fogged all over, and yet be thin and 
 grey, and the bath acid. This state of things is not so easily 
 explained, nor is it easy to suggest a certain remedy. A plan which 
 frequently answers is to add nitric acid, and let the bath stand for a 
 day or two, then to neutralize with carbonate of soda, and acidify 
 slightly with acetic acid. 
 
NIT 
 
 305 
 
 Sometimes a bath in this state' is cured by exposing it for a day 
 or two to sunshine, and thus throwing down a deposit of silver and 
 organic matter. 
 
 Another plan is to add a solution of carbonate of soda to the 
 nitrate bath until the whole of the silver is thrown down as yellow 
 carbonate of silver. This should be allowed to settle and the clear 
 liquid poured off. The carbonate should then be well washed in 
 several waters, dried, and roasted in a crucible ; after which it may 
 be dissolved in nitric acid, which liberates the carbonic acid and 
 produces nitrate of silver ; — or if the presence of lead be suspected, 
 acetic acid may be added, which forms insoluble acetate of silver and 
 soluble acetate of lead ; the acetate of silver must then be well washed 
 and lastly dissolved in nitric acid, and the nitrate of silver solution 
 evaporated and crystallized. 
 
 Or, instead of throwing down the silver as a carbonate , it 
 may be precipitated as an oxide, by adding solution of caustic 
 potass. The oxide should then be treated in the same way as the 
 carbonate. 
 
 The advantage of either of the above methods is that it separates 
 the silver from the excess of nitrates which may have accumulated 
 in the bath. 
 
 Sometimes a bath may .be cured by simply evaporating, crystal- 
 lizing, and redissolving the nitrate of silver in pure distilled water. 
 
 Nitric acid is an infallible remedy for nitrite of silver in a bath. 
 
 A nitrate bath is frequently put out of order by dirty plates being 
 immersed in it. 
 
 Ammonia should never be added to the nitrate bath for negatives. 
 A fixed alkali is far better. 
 
 Nitric Acid. N0 5 =54 ; or in its hydrated state, N0 5 HO. 
 
 Nitric acid may be obtained by passing electric sparks through a 
 mixture of 7 volumes of oxygen and 3 of nitrogen. Hence the 
 explanation of the occurrence of nitric acid in rain water, during 
 thunder storms. The commercial red fuming nitric acid is manu- 
 factured by distilling two parts of nitrate of potass with one of 
 sulphuric acid; 1121bs. of nitre and 561bs. of sulphuric acid yield- 
 ing about 5 01bs. of nitric acid ; the proportions admit, however, of 
 variation. The red colour and fumes are due to peroxide of 
 nitrogen. 
 
 Nitric acid has been obtained in an anhydrous state by a compli- 
 cated process. It is a white solid. The liquid acid, in its more"' 
 concentrated form, has a specific gravity ]*$, and consists of 1 
 equivalent of anhydrous nitric acid and 1^ of water. 
 
 x 
 
306 
 
 NIT 
 
 Concentrated nitric acid does not act on copper, tin, or silver, 
 without the addition of water, which is decomposed and oxygen 
 furnished to the metal. When the metal is oxidized the nitric acid 
 converts the oxide into a nitrate. 
 
 Nitric acid is a powerful oxidizing substance, and acts energetic- 
 ally upon organic matter ; also upon phosphorus, sulphur, and carbon 
 with the aid of heat. 
 
 All the salts of nitric acid are soluble in water, and most of them 
 are neutral. They are, in general, decomposed by sulphuric acid 
 aided by a gentle heat. There are no double nitrates, nor any 
 nitrates with excess of acid. The principal impurity in nitric acid 
 is sulphuric acid. 
 
 Nitrite of Silver. Ag.O, N0 3 =154. This salt is obtained 
 by fusing together equal parts of nitrate of potass and nitrate of 
 silver. Oxygen is driven off from the former, which is converted 
 into nitrite, and this again acquires oxygen by converting the nitrate 
 of silver into nitrite. By dissolving the mixture in a small quantity 
 of boiling water the nitrite crystallizes out on cooling. The crystals 
 are long slender needles, soluble in 120 parts of water at 60°. This 
 salt is easily decomposed by a strong acid. When present in a 
 nitrate bath it fogs the picture. 
 
 Nitro-Hydrochloric Acid ; Aqua-Begia. A mixture of two 
 parts of hydrochloric acid, and one part of nitric acid. This mixture 
 dissolves gold, hence its name. Common salt added to nitric acid, 
 or nitre to hydrochloric acid, have the same property. The rationale 
 of the process appears to be, that nitric acid plus hydrochloric acid 
 produces chlorine, nitrous acid, and water. The chlorine dissolves 
 the gold. 
 
 Nitrous Acid. N0 3 . This acid is by some chemists called 
 hyponitrous acid. It is not easily isolated. It forms salts called 
 nitrites, none of which have any interest in photography except the 
 nitrite of silver ; q.v. 
 
 Nitrogen. N=14. An elementary gas. It is obtained by 
 burning phosphorus in a closed vessel containing air, and passing 
 the gas through lime water ; or by agitating a liquid amalgam of 
 lead and mercury with air in a closed vessel for two or three hours, 
 when the lead abstracts the oxygen. It has neither smell nor taste, 
 nor any action upoi%vegetable colours ; nor is it a supporter of 
 combustion. Atmospheric air contains principally 1 atom of oxygen 
 
NIT 
 
 NON 
 
 307 
 
 to 2 of nitrogen, in a state of mixture, not combination. Nitrogen 
 is a little lighter than air. 
 
 Nitrogen, Oxides op. There are 5 oxides of nitrogen, viz : — 
 Nitrous oxide, (laughing gas) . . * NO 
 
 Nitric oxide, (binoxide of nitrogen) . . N0 2 
 
 Nitrous acid . . * . , . NO s 
 
 Peroxide of nitrogen , . . . . N0 4 
 
 Nitric acid N0 5 
 
 Nitrogen, Peroxide of. N0 4 . When two volumes of binoxide 
 of nitrogen (N0 2 ) are mixed with one of oxygen, in an exhausted 
 vessel, they combine with the evolution of heat, and form vapour of 
 peroxide of nitrogen. This condenses into a liquid at 0°, and crys- 
 tallizes at a lower temperature. The liquid is pale yellow at 32°, 
 and deep orange at 60°; it boils at 82°; and when exposed to the 
 air at common temperatures evaporates in yellow fumes. It is 
 produced in nitric acid by exposure to light. 
 
 The vapour of peroxide of nitrogen is composed of 1 volume 
 of nitrogen and 2 volumes of oxygen, condensed into one volume. 
 
 Nitro-GtLUCOSE. This substance is made by acting on finely 
 powdered cane sugar with nitrosulphuric acid; the proportions 
 being 2 ounces sulphuric acid, 1 ounce nitric acid, 1 ounce cane 
 sugar. The pasty mass at first formed is stirred for some minutes, 
 and it then separates from the liquid in lumps. When these are 
 kneaded in warm water until the acidity is removed, they acquire a 
 white and silky lustre. 
 
 This organic substance, when added in very small quantity to 
 collodion, increases the density of the negative, and renders the film 
 less sensitive to light. 
 
 Noble Metals. Gold, platinum, silver, and a few other metals, 
 are called “ noble metals,” on account of their feeble affinity for 
 oxygen, for they may remain in fusion for many hours in contact 
 with air without becoming oxidized. 
 
 Non-reversing Slide. This is a camera slide, so contrived 
 that collodionized glass plates may be exposed in it with the back 
 of the plate next to the lens, and the film next to the back shutter. 
 The best plan is to make an ordinary slide deep enough to hold 
 two glass plates, with a space at least equal *to the thickness of a 
 plate between them ; a glass plate is then to have a small triangular 
 
 x 2 
 
308 
 
 NOE 
 
 OIL 
 
 piece of glass cemented to each corner ; the collodionized plate is 
 laid, film upwards, in the slide, and the other plate laid with the 
 corners upon it ; the back shutter is then closed with its spring 
 pressing against the upper plate. « 
 
 A non-reversing slide should always be employed for taking 
 collodion positives, and also for negatives when the prints are in- 
 tended to be viewed in the reflecting stereoscope. 
 
 Normal. The normal to a. surface at any point, is the straight 
 line perpendicular to the tangent plane at that point. 
 
 Object Glass. In a telescope, or microscope, the glass placed 
 next to the object to be viewed is called the “ Object glass.” The 
 arrangement of lenses at the opposite end of the instrument, through 
 which the spectator looks, is called the “ Eye Piece.” The glass 
 next to the eye is called the “ Eye Glass ;” and that next the object 
 glass the “ Field Glass.” 
 
 In an astronomical telescope, or compound microscope, there are 
 only these three lenses. 
 
 Oils. Oils are divided into two great classes, viz., Fixed, and 
 Volatile, (or essential.) 
 
 The Fixed oils are distributed largely through the animal and 
 vegetable kingdoms. In the former the fatty matter is enclosed in 
 membraneous cells existing in various parts of the body of the ani- 
 mal ; in the latter they are obtained by expression from the seed, 
 kernel, root, bark, and other parts of plants. Fatty substances may 
 be classified under the heads of Stearine and Oleine, the former solid 
 and resembling suet, the latter liquid at ordinary temperatures. They 
 may be again classified according to their property either of drying , or 
 becoming rancid by exposure to air and light. Most oils, whether 
 fixed or volatile, absorb large quantities of oxygen by exposure to 
 air and light ; — in the case of drying oils the effect produced is the 
 formation of a skin or resinous varnish ; — in the other case the oil is 
 decomposed and becomes rancid and acid. Drying oils are much 
 used in paints and varnishes. See “ Drying Oils.” 
 
 Volatile oils are contained principally in various parts of odori- 
 ferous flowers, and shrubs. They are obtained in general by distil- 
 lation of the dried leaves, &c. with water, and sometimes with salt 
 and w r ater, which raises the boiling point. The volatile oil and steam 
 go over together, and when condensed in the receiver, the oil in most 
 cases separates and floats upon the surface of the water. A small 
 quantity is also dissolved in the water, to which it communicates its 
 
peculiar smell. Eose water is an instance of this. A drop of fixed 
 oil leaves a permanent stain upon paper, a drop of volatile oil does 
 not. Volatile oils are mostly soluble in alcohol, fixed oils are not ; 
 both kinds are, however, freely soluble in ether. When water is added 
 to a mixture of a volatile oil and alcohol it renders it turbid. The 
 odoriferous spirits called “ Lavender water,” “ Eau de Cologne,” 
 “ Eau de jasmine,” &c. are solutions of a volatile oil in alcohol. 
 Volatile oils combine with acetic and oxalic acid ; but with the excep- 
 tion of oil of cloves, they do not combine with alkalies to form soaps. 
 They dissolve all the fat oils and the resins. 
 
 Oils are supposed to be compounds of an organic acid with gly- 
 cerine as a base. When an alkali is added to the fixed oils, at a 
 boiling temperature, the glycerine is displaced and the new compound 
 formed is soap. Oils contain a large quantity of hydrogen. Eixed 
 oils are bland and mild to the taste, volatile oils acrid and nauseous. 
 
 Olefiant Gas. C 3 H 3 . This gaseous hydrocarbon is obtained 
 by distilling 2 parts of sulphuric acid and one of alcohol. It may 
 be collected over water. It is difficult to purify it from all traces of 
 ether, though when pure it is inodorous. Water absorbs one eighth 
 of its volume of olefiant gas, and ether and alcohol twice their volume. 
 Sulphuric ether frequently contains a trace of olefiant gas. It is 
 combustible, and consumes three volumes of oxygen, producing 
 carbonic acid and water. 
 
 Optical Centre. Every single lens has a certain point called 
 its optical centre ; no such point, however, exists in the case of an 
 achromatic lens, or combination of lenses. This should be distinctly 
 understood, because ignorant persons frequently commit the blunder 
 of speaking of the optical centre of a combination of lenses. 
 
 Confining our remarks, therefore, to the case of the single lens. 
 
 If a ray of light, incident at any degree of obliquity upon a single 
 lens, strikes it at such a spot as that the direction of the refracted 
 ray within the glass, produced if necessary, passes through a certain 
 point in the axis of the lens called the optical centre, the direction 
 of the ray after emergence will be parallel to that at incidence. 
 
 This effect is brought about by the following circumstance : — 
 
 A ray of light after refraction through a plate proceeds in a direc- 
 tion parallel to that which it had before. Now if the course of the 
 ray within the glass when produced passes through the point 
 called the optical centre, and we draw a tangent to the anterior 
 surface of the lens at the point of incidence of the ray, and another 
 tangent to the posterior surface of the lens at the point of emergence 
 
310 
 
 OPT 
 
 of the ray, we shall find that these two tangents are parallel, so that 
 the lens for that particular ray may be considered as a plate, and 
 the ray does not suffer deviation by being refracted through the lens, 
 but merely displacement. 
 
 The position of the optical centre is constant, and independent of 
 the obliquity of the incident ray ; so that in any whole pencil, no 
 matter what its obliquity may be, which is incident upon the front 
 surface of a lens, there is, provided that surface be large enough , a 
 particular ray, and only one , the direction of which after refraction, 
 passes through the optical centre. 
 
 The optical centre of a single lens is found thus : — - 
 
 Tf r be the radius of the front surface of a lens, s the radius of the 
 back surface, and t the thickness of the lens, then the distance of 
 the optical centre, measured along the axis of the lens from the centre 
 
 r t 
 
 of the face of the front surface, is equal to 
 
 The optical centre of a double convex lens is within the glass ; — of 
 a plano-convex lens it is at the centre of the face of the back surface ; — 
 and of a meniscus lens it is without the glass and behind it. By 
 giving to r and s the proper algebraical sign, and a given magnitude, 
 the position of the optical centre of any single lens may be readily 
 found. 
 
 The use of the optical centre will be understood by referring 
 to the figure on page 1. The focus, or circle of least confusion, of 
 the pencil QAB is somewhere in the neighbourhood of c. Now the 
 optical centre of the lens being within the glass the ray QCc passes 
 through it, and may be considered as very approximately a straight 
 line. If then we draw this line, and set off C c equal to the focal 
 length of the lens, we find the point c very approximately, and without 
 going through the laborious investigation of the bent pencil QBE. 
 
 If, in this figure, AB were an achromatic lens, the point c would 
 be found approximately by considering the lens as single, or homo- 
 geneous, and of the same external form. 
 
 In the case of a combination of lenses, this mode ctf proceeding 
 evidently breaks down. A combination can neither have an optical 
 centre, nor any point at all analogous to it. 
 
 Optical Glass. In making flint glass for optical purposes the 
 great point to be attended to is, to prevent the lead from distributing 
 itself unequally, and thus forming strive or waves, from the lead 
 naturally gravitating to the bottom of the pot. Various plans 
 have been adopted which need not be described in this place. The 
 
% 
 
 OEG 
 
 OXA 
 
 311 
 
 most celebrated optical glass is that manufactured by M. Guinand, 
 of Brennets, near Geneva. His process is a secret. The analysis of 
 Guinand’s dense flint optical glass, S. G. 3*616, yields oxide of lead 
 43*05 per cent., silica 44*3 per cent., and potass 11*75. The best 
 German optical glass is made with 7 parts pure red lead, 3 parts 
 finely ground quartz, and 2 parts calcined borax. 
 
 Organic Matter. The effects of organic matter in the various 
 photographic processes are described in the article on the chemistry 
 of photography, and also in the particular accounts of these pro- 
 cesses. 
 
 The general effect of organic matter in combination with the re- 
 duced silver of the photographic image is to increase the density of 
 the image when viewed by transmitted light, to redden the colour, 
 to add surface vigour to positive prints, to diminish the chances of 
 permanence of the photograph, and to reduce the sensitiveness of the 
 excited plate or paper to light. 
 
 Orpiment. As. S 3 . Yellow sulphide of arsenic. Ammoniacal 
 solution of orpiment is sometimes used as a dye. Orpiment is the 
 basis of the pigment called “ King’s Yellow” 
 
 Orthographic Projection. This is a mode of representing an 
 object in perspective, when the eye is supposed to be at an infinite 
 distance, so that the visual rays from the different points of the ob- 
 ject are parallel instead of converging to a point at a finite distance, 
 as in common perspective. See “ Perspective.” 
 
 Orthoscopic Lens. See “ Lens.” 
 
 Oxalate of Iron. The peroxalate of iron is an important salt 
 in photography, as its solution affords the means of measuring the 
 intensity of actinic power, by observing how much of it is decomposed 
 in a given time, the effect of light being to reduce the salt to a prot- 
 oxalate, which is deposited, (with the evolution of carbonic acid,) in 
 quantities which are found to be proportional to the intensity of the 
 light and the time of its action. 
 
 Peroxalate of iron is made by decomposing the perchloride of iron 
 with oxalate of potass. It is thrown down as a difficultly soluble 
 yellow powder, which is taken up by excess of oxalic acid. 
 
 Oxalate of Potass. K O, C 2 0 3 + H O. This salt is formed 
 by accurately saturating a solution of carbonate of potass by oxalic 
 
312 
 
 OXA 
 
 OXY 
 
 acid. It is soluble in rather less than 3 parts of water at 60° ; and 
 is slightly deliquescent. 
 
 Oxalate of Silver. Ag. 0, C 2 0 3 . Oxalic acid is sometimes 
 formed by the decomposition of collodion, and in this way oxalate of 
 silver is produced in the nitrate bath. It is a white powder, insolu- 
 ble in water but soluble in nitric acid ; it blackens by exposure to 
 light ; and forms a double salt with oxalate of potassa. 
 
 Oxalic Acid. C 2 0 3 + H 0. This poisonous acid is found in the 
 different varieties of sorrel, in combination with lime or potass. It is 
 readily obtained by acting on sugar with nitric acid. The crystals are 
 intensely sour, and so much resemble those of Epsom salts that persons 
 have been poisoned by mistaking the former for the latter. They 
 are soluble in 15 parts of water at 50°, in 9 at 60°, and fuse in their 
 own water of crystallization at 212° ; in alcohol they are less soluble 
 than in water, and in ether still less than in alcohol. 
 
 Oxalic acid, boiled with chloride of gold, throws down metallic 
 gold, and carbonic acid passes off. 
 
 The affinity of oxalic acid for bases is about equal to that of sulph- 
 uric acid. 
 
 Oxygen. 0=8. (Greek o£ve sharp, yevveiv to produce.) An 
 elementary gas, the name of which is derived from its property of pro- 
 ducing acids. The atmosphere contains about one-fifth part, by 
 volume , of oxygen gas, in a state of mixture ; and water eight-ninths, 
 by weight , in combination with hydrogen. 
 
 Oxygen is the great supporter of life and combustion. Animals, 
 by breathing, withdraw it from the air and return carbonic acid in 
 exchange. Vegetables, during the action of light in the day time, 
 absorb carbonic acid and return oxygen ; hence the balance is main- 
 tained. 
 
 Oxygen is rather heavier than air, and when pure is tasteless, 
 colourless, and inodorous. Its chief use to the photographer is for 
 affording a brilliant light for exhibiting photographic magic lantern 
 slides, or when a powerfully actinic artificial light is required in any 
 of his operations. (See the following article.) The readiest mode 
 of obtaining oxygen for this purpose is as follows : — 
 
 A copper retort, containing a mixture of about 3 parts of chlorate 
 of potass and 1 part of black oxide of manganese, (thoroughly well 
 mixed together,) is placed upon a common fire. In a short time 
 oxygen gas is evolved. This is conveyed by an india-rubber tube 
 attached to the nozzle of the retort into a bottle of water, called the 
 
OXY 
 
 OZO 
 
 313 
 
 purifier, which may stand upon the ground. After passing through 
 the water, which cools and purifies it, the gas passes along another 
 india-rubber tube into a large wedge shaped india-rubber bag, which 
 is capable of containing sufficient gas for an evening’s entertainment. 
 This bag may be filled in about half an hour, from half a pound, or 
 less, of the mixed manganese and chlorate. When required for use 
 the gas bag, filled with oxygen, is placed upon the ground between 
 two boards hinged together, and upon the upper one weights are laid 
 sufficient to force the gas out at the required rate, along a tube, either 
 to the spirit lamp of the oxycalcium light apparatus, or to the point 
 where it unites with the hydrogen jet of the Bude light, 
 
 Perfectly pure oxygen is obtained from chlorate of potass alone, 
 and collected over mercury after having been passed through a tube 
 containing fused chloride of calcium. 
 
 Oxycalcium Light. This brilliant white light is produced by 
 passing a jet of oxygen into the flame of a spirit lamp, and direct- 
 ing the flame upon a small ball or cylinder of lime. ( See Pig. 6, 
 page 63.) 
 
 Oxymel. A syrup made by mixing honey and vinegar in the 
 proportions of 1 part by weight of distilled vinegar to 2 parts by 
 weight of purified honey. Evaporate the mixture at 170°, to the 
 consistence of syrup, and strain. Keep it in a cool place. It should 
 be clear, and of a yellowish brown colour. 
 
 Oxymel-Process. See “ Preservative Processes.” 
 
 Ozone. Supposed to be an allotropic form of oxygen, or 
 oxygen in a peculiarly active state. Some chemists suppose it to 
 be a peroxide of hydrogen, which seems more probable. 
 
 “ Ozone is formed when electricity is discharged into the air, 
 when water is decomposed, and when phosphorus is allowed to act 
 on moist air at ordinary temperatures. It is best prepared by 
 putting a short stick of phosphorus, well scraped, into a two-quart 
 bottle, with as much water at the bottom as will half cover the 
 phosphorus. The mouth of the bottle should then be slightly closed 
 and the bottle set aside. A column of thin smoke will almost 
 instantly begin to arise from the phosphorus, which is luminous in 
 the dark. Ozone may almost immediately be detected if a piece of 
 paper steeped with iodide of potassium and starch be put into the 
 bottle, the starch becoming blue. When air much charged with 
 ozone is inspired, it becomes highly poisonous. Small animals put 
 
 \ 
 
314 
 
 PAN 
 
 into it die ; and it acts as a powerful irritant to the air passages. 
 As ozone, a colourless gas, is chiefly recognized by this property, 
 it has led to some curious speculations. By many, its presence, 
 in excess or diminution, is supposed to be capable of explaining 
 many conditions of the air which produce disease. Its smell is 
 precisely similar to what is produced when a powerful electro- 
 pneumatic machine has been long worked, and it is said that the 
 operators in such cases experience symptoms not dissimilar to those 
 of influenza.” — Glover's Chemistry. 
 
 Panoramic Camera. This is a form of Camera invented by 
 the author, in which pictures may be taken upon one flat plate, 
 including an angle of 90°, or more if required, without introducing 
 the defects due to oblique pencils, such as distortion, indistinct- 
 ness, &c. 
 
 The following figure will, it is hoped, be intelligible with a few 
 words of explanation. 
 
 The 'lens is mounted in a long narrow tube, or box, the same height 
 as the camera. This revolves about an axis placed immediately 
 over the stop. Inside the camera are placed two hoops, very 
 
PAN 
 
 315 
 
 strong and stout, one at the top, the other at the bottom of it. The 
 dark slide always presses against these hoops. They are circular 
 arcs, the centres of which are in the axis of rotation of the tube. 
 The ends of the dark slide are furnished with wheels, which, as the 
 slide is moved round, travel in grooves at the bottom of the camera, 
 shown by the dotted lines. These dotted curves are evolutes of the 
 lower circular hoop. The top of the lens tube is continued till it 
 passes over the top of the upper hoop, and the bottom of the lens 
 tube is continued till it passes under the bottom of the lower hoop, 
 the dark slide is then placed between these projecting ends of the 
 lens tube. A piece of wood lined with velvet is screwed to these 
 ends, and, by pressing against the back of the dark slide, keeps it 
 in its place against the hoops. 
 
 It is evident now, that as the lens-tube is turned about its axis, 
 and directed in succession towards the different objects within the 
 field of view, the dark slide moves with it, and is always pressed 
 against the hoops, thus bringing the proper part of the sensitive plate 
 opposite to the lens, — the velvet-lined piece of wood sliding at the 
 same time against the back of the slide, and the wheels travelling 
 along the evolutes. 
 
 The sides of the lens-tube are furnished with folding doors, so as 
 to diminish, at pleasure, the width of the vertical band of picture 
 exposed at any moment ; and its rotatory motion may be regulated 
 by means of a rack and pinion on the top of the camera. The 
 shutter of the dark slide may be withdrawn, and inserted again, 
 either through a slit in the camera at A, or B. The mode of doing 
 this may be left to the ingenuity of the reader. 
 
 The accuracy with which this novel instrument will do its work 
 will depend upon the accuracy of its construction. The optical 
 principles involve no difficulty, and the theory of the instrument is 
 quite correct. All parts of the picture will be equally sharp, and 
 vertical lines will not be bent out of the perpendicular. The per- 
 spective of the picture will, however, be “ panoramic,” and not 
 “ plane,” so that the horizontal lines of objects will vanish in curved 
 and not in straight lines. If this be thought an objection, the 
 picture may either be mounted upon a bent cardboard, or bent 
 round a glass cylinder, and viewed in a diaphanoscope, with the 
 eye in the centre of the cylinder. Waxed paper prints, viewed in 
 this way, would probably be finer than anything that has yet been 
 seen in photography. 
 
 The panoramic camera will no doubt be found a valuable instru- 
 ment for taking skies. 
 
316 
 
 PAP 
 
 Paper-Making. Paper may be made of any fibrous material 
 which can be torn and beaten into pulp. The substances commonly 
 employed are linen and cotton rags, and for coarser kinds of paper, 
 grass and straw. We cannot in this work go at any length into 
 the process of paper-making, but will briefly describe the mode of 
 making hand-made paper, from which the general principles of the 
 manufacture will be understood. 
 
 The rags are mostly imported from Germany and the different 
 ports of the Mediterranean. They are sorted by women ; torn up 
 into shreds, and cut into pieces, then dusted in a machine, and 
 washed, first in water, then in a strong alkaline ley, and afterwards 
 in water again ; they are then ready to be made into pulp. . To 
 effect this, they are placed with water in a machine called the 
 “ engine,” where by the rapid rotation of a complicated piece 
 of apparatus, they are tom into the finest imaginable tatters and 
 shreds. This is called “ stuff.” When the stuff is about half 
 made, it is called “ half stuff,” and is then “ bleached,” generally 
 with chlorine, or chloride of lime, one or two pounds of chloride of 
 lime being sufficient for a cwt. of fine rags. This is added to the 
 “ half stuff” in the engine, and the mixture is beaten together for 
 an hour or so, then the bleaching liquid run off, fresh water con- 
 tinually added, and the half stuff reduced by the continued action 
 of the engine to “ fine stuff.” 
 
 Sometimes the sizing mixture and colouring matter are now 
 added, but in general the sizing is an after operation performed 
 upon the finished sheets of paper in its bibulous state. The stuff 
 is now run off into the “ stuff chest ” or vat. 
 
 Hand-made paper is made as follows : — 
 
 Two men, one called the “vat-man,” the other the “coucher,” 
 stand on opposite sides of the vat, which is covered with a board or 
 “ bridge ” between them. The vat is about five feet square, and four 
 feet deep, with slanting sides, and made of wood or stone. The stuff 
 is then diluted with warm water, and continually stirred. The 
 vat-man takes in his hands a mould, which is the size of the sheet 
 of paper to be made. This mould is a frame of wood, having 
 wooden bars running across, about an inch and a half apart, and 
 across them is laid a wire frame, the wires being from fifteen to twenty 
 per inch ; or sometimes a wove mould is used, covered with fine 
 wire cloth. On the mould a “ deckel,” or moveable edge frame, is 
 used to prevent the edges of the paper from being rough. 
 
 The vat-man puts the deckel upon the mould, and dips it in a 
 vertical position to about half its depth in the stuff, then turns it 
 into a horizontal postion so as to cover it with the stuff, and gives it 
 
PAP 
 
 317 
 
 a peculiar kind of shake, which distributes the stuff evenly upon 
 the mould. The mould must be held perfectly level during this 
 operation, or the sheet will be thicker at one end than the other. 
 The mould is then pushed across the bridge to the “ coucher,” who, 
 after draining off the water, lays the surface of the pulp upon a piece 
 of felt, to which it adheres, and then returns the mould to the 
 vat-man. This piece of felt, with what we may now call the sheet 
 of paper adhering to it, is laid with other similar pieces in a pile, 
 which, therefore, consists of alternate layers of paper and felt. The 
 pile, consisting of six or eight quires, is put, and remains for some 
 time, under a pressure of about 100 tons, to squeeze out all the 
 water, and the sheets of paper are then removed, pressed again 
 without the felts, and hung across a hair line to dry, in the drying- 
 room. 
 
 In this state the paper is bibulous, or blotting-paper, and the 
 next operation is to size it. 
 
 English papers are in general sized with a mixture of gelatine 
 and alum, to which sometimes flour, resin, and yellow soap are 
 added. Erench papers are sized with a less soluble size, consisting 
 for the most part of starch, with a little potass.* 
 
 The sheets of bibulous paper are placed, one at a time, in a 
 vertical position in the tub of size, and pressed into close contact. 
 After a time the papers are taken out, scraped, and pressed to 
 remove the superfluous size, then parted, and pressed again, and 
 afterwards hung up to dry, a process which occupies two or three 
 days, and must not be done too quickly. The paper is then pressed 
 again. 
 
 The paper has now to be glazed or hot-pressed. This is done by 
 
 * With, respect to the sizing of French papers. The following recipe was given 
 some years ago by the Society d’Encouragement of Paris : — 
 
 100 kilogrammes of dry paper stuff, 
 
 12 „ starch, 
 
 1 ,, resin, previously dissolved in 500 kilogrammes of 
 
 carbonate of soda. 
 
 18 pails of water. 
 
 This size evidently renders the paper alkaline. 
 
 The following formula is given by M. Braconnot, in the “ Annales de Chimie,’* 
 Vol. 23 
 
 “ To 100 parts of dry stuff, properly diffused through water, add a boiling uniform 
 solution of 8 parts flour, with as much caustic potass as will render the liquid clear. 
 Add to it 1 part of white soap previously dissolved in hot water. At the same time, 
 heat half a part of resin with the requisite quantity of weak potass lye for dissolving 
 the resin, mix both solutions together, and pour into them 1 part of alum dissolved in 
 a little water. 
 
 This size also renders the paper alkaline. Alum has an acid reaction, and there- 
 fore English paper sized with alum and gelatine is acid and not alkaline. 
 
318 
 
 PAP 
 
 placing a sheet of paper between two glazed pasteboards, alternately 
 in a pile, and between every fifty pasteboards a hot iron plate, then 
 subjecting the pile to the press. Or a pile of sheets of paper placed 
 between pasteboards, may be rolled backwards and forwards upon a 
 plate between cold iron cylinders. This communicates a glaze to 
 the surface of the paper. 
 
 The paper is now finished, and has merely to be trimmed, and 
 the sheets counted and sorted, and tied up in reams each containing 
 480 sheets. 
 
 The above operations of making paper by hand may be success- 
 fully imitated by machinery ; the paper is then said to be “machine- 
 made.” It may be made in sheets of indefinite length. 
 
 Paper frequently contains metallic spots, consisting of particles 
 of iron, brass, or zinc, detached from the machinery, or introduced 
 through carelessness in sorting the rags. The roughness of the 
 felts between which the paper is pressed also occasions inequalities 
 of texture. It is highly desirable to remove these imperfections 
 from paper which is to be used in photography. The practice 
 employed by the Trench paper-makers of colouring their paper with 
 artificial ultramarine, (sulphide of sodium,) is also highly objection- 
 able for photographic purposes, both as regards the appearance of 
 the paper, and from the probability that the introduction of an 
 alkaline sulphide into it might assist the process of fading of positive 
 proofs. 
 
 Should the process of printing positives in carbon ever come 
 into general use, (which it is tolerably certain that it will,) the 
 imperfections now existing in paper for positives will be of less 
 moment. * 
 
 Paper, India or China. India-paper, or Chinese paper, is 
 much used by engravers for printing the first proofs upon, from the 
 plate. It is thin and silky, of a beautiful buff colour, and made of 
 the fibres of the young bamboo, which are triturated, ground, and 
 boiled to a paste. This is set to ferment in a heap covered with 
 mats, and the paper is then made from it. The length of the fibres 
 renders it better adapted for receiving copperplate proofs than the 
 best European paper. This paper is smooth on one side and bears 
 on the other the marks of a brush which is used in the finishing 
 process. It is made in sheets 4 ft. long and 2 ft. broad. When 
 proofs are printed upon India-paper no cement is used to attach the 
 thin sheet of India-paper to the thick plate paper, the mere pressure 
 exerted by the cylinders in the operation of drawing the proof 
 being sufficient to cause perfect adhesion between the two surfaces. 
 
PAR 
 
 PER 
 
 319 
 
 Parchment Paper. See “ Artificial parchment.” 
 
 Paste. Mix well together flour and a little cold water; then 
 add as much more cold water as is necessary to make the paste of 
 the consistence required ; then "boil the mixture, stirring it well all 
 the time; lastly, strain it through muslin. Starch is better than 
 paste for mounting photographs. Sometimes powdered resin is 
 boiled with paste to give it body, and corrosive sublimate added to 
 make it keep. In the latter case it is highly poisonous, and should 
 not be left about carelessly. 
 
 Pendulum. A pendulum suspended in the portrait room may 
 be used with advantage for measuring time by seconds. The pendul- 
 um which beats seconds, that is, which oscillates in a second of 
 time (approximately), whatever be the angle of oscillation, is exactly 
 a metre in length, in these latitudes. A metre is nearly 40 inches. 
 If, then, a 40 inch pendulum, hanging against the wall of the port- 
 rait room be set swinging before removing the cap from the lens, the 
 operator, by counting its vibrations, may regulate the time of ex- 
 posure as accurately as if he consulted a watch. A clock having 
 a pendulum a metre in length would be a useful addition to the 
 portrait room. 
 
 Perspective. There are two kinds of perspective delineation 
 with which the photographer is concerned, viz., “Plane,” and 
 “ Panoramic Perspective.” 
 
 In plane perspective, objects are represented upon a vertical plane 
 placed between them and the spectator. Straight lines, called 
 “ visual rays,” are supposed to be drawn from the various angular 
 points A, B, C, &c., of the objects, to the eye, and where these lines 
 perforate the vertical plane, or “ plane of the picture,” as it is called, 
 are corresponding points a , b, c, &c., through which, if the figure be 
 completed, it is the plane perspective representation of the objects 
 as seen from the point occupied by the eye (not eyes) of the spectator. 
 According to this definition a plane perspective view is nothing 
 more than a plane section of the system of pyramids of which the 
 visual rays are the edges and the eye the common vertex ; the eye 
 being considered a mathematical point. The rules of perspective, 
 therefore, merely relate to the cutting of pyramids by a plane, and 
 are purely geometrical, not referring in any way to the structure of 
 the eye, or the image formed upon the retina, or the rules of optics. 
 Perspective is nothing more than a very simple problem in solid 
 
320 
 
 PEE 
 
 geometry, and it is marvellous to find that so little is accurately 
 known of it by artists, and that so many elaborate and expensive 
 works should have been written about it, when in fact the whole 
 thing lies in a nut-shell, as we shall now show ; not however 
 without calling on the reader for his patient attention, and careful 
 study of our remarks. 
 
 Let us first suppose the object to be represented to be an infinite 
 straight line, making an angle, (j>, with the plane of the picture, and 
 meeting it in the point A. Then, in order to draw the perspective 
 view of this line upon the plane of the picture, it is evident that we 
 should require to join the point A with some other point X. The 
 question becomes how to find this point X. If it were possible to 
 draw a visual ray from the eye to the end. of an infinite straight line, 
 the point where that visual ray would cut the plane of the picture 
 would be the point X required. But is it possible to draw such a 
 line? It is. We have simply to draw through the eye a line 
 parallel to the given infinite straight line, and the point X where 
 this line cuts the plane of the picture is found at once ; for although 
 parallel straight lines do not meet at any finite distance, they may 
 be considered as meeting at an infinite distance, infinite being only 
 another term for “ not finite,” and the second form of the expression 
 being identical with the first. But this is becoming metaphysical. 
 Practically, the problem is solved. The finite line AX is the per- 
 spective view of the infinite line proceeding from A, and making an 
 angle 0 with the plane of the picture. 
 
 Next, suppose any number of other infinite straight lines to make 
 the same angle <p with the plane of the picture, and to meet it in 
 points B, C, D, &c. It is evident that the perspective views of all 
 these straight lines would be terminated in a common point X, and 
 would consist of lines AX, BX, CX, &c. radiating from X ; this point 
 is therefore called the “ vanishing point” of that particular system of 
 parallel straight lines. 
 
 Hence we arrive at the following general rule : — 
 
 The vanishing point of any system of parallel straight lines is the 
 point where a line drawn through the eye parallel to that system cuts 
 the plane of the picture. 
 
 If a horizontal plane be drawn through the eye the line in which 
 it intersects the plane of the picture is called the “ horizontal line 
 and if a line be drawn from the eye perpendicular to the horizontal 
 line, the point in which it cuts it is called the “ point of sight.” 
 
 Hence it follows that 
 
 1st. The vanishing point of a system of parallel horizontal lines 
 is upon the “ horizontal line ” of the picture ; the point being 
 
PEE 
 
 321 
 
 found by drawing through the eye a line parallel to any one of the 
 system of lines. 
 
 2nd. The vanishing point of a system of parallel horizontal lines 
 at right angles to the plane of the picture is the “point of sight.” 
 
 Now we come to the case of the vanishing point of a system of 
 parallel lines which are parallel to the plane of the picture ; that is, 
 to the case in which the angle 0 vanishes. These lines have no 
 vanishing point, because the line drawn through the eye parallel to 
 them never meets the picture. They are consequently represented 
 by parallel lines in the picture. 
 
 Observe the practical conclusions : — 
 
 1st. All vertical straight lines in nature are represented by vertical 
 straight lines in the picture. They do not vanish towards a point 
 in the zenith, as is generally erroneously supposed. In fact, in 
 strict accuracy, vertical lines would vanish downwards towards the 
 centre of gravity of the earth. 
 
 2nd. The horizontal lines of a building which are parallel to 
 the plane of the picture are horizontal lines in the picture. 
 
 Should the reasoning by which these conclusions are established 
 be thought somewhat metaphysical, then we may return to the case 
 of the section of a pyramid. Place a square board vertically behind 
 the plane of the picture, and parallel to it. Then, since the section 
 of a pyramid by a plane parallel to its base is a figure similar to the 
 base, the perspective view of the square is also a square, that is, 
 neither the vertical nor the horizontal lines have any vanishing point. 
 
 We have now discussed the whole theory and mystery of plane 
 perspective. If the reader has carefully followed our reasoning he 
 will not require to spend his money in treatises on perspective, 
 (which are generally full of gross blunders,) but may trust to his 
 own good sense to apply the rules which we have established. 
 
 The following remarks should be borne in mind : — 
 
 In views of marine scenery, the horizontal line is always higher 
 than the sea line, because of the dip of the visible horizon ; and the 
 sea-line is a curve convex to the horizontal line, and most nearly 
 touching it in the point of sight. 
 
 The perspective view of a sphere is an ellipse in every case, except 
 that in which the line 'joining the eye and the centre of the sphere is 
 perpendicular to the plane of the picture, so that the centre of the 
 sphere is on the point of sight. Eor let a visual ray travel round a 
 sphere, it sweeps out a cone with a circular base, and the oblique 
 section of such a cone is an ellipse. 
 
 If the plane of the picture be inclined to the vertical, vertical lines 
 have a vanishing point either above or below the horizontal line. 
 
 Y 
 
322 
 
 PHO 
 
 The reflections of vertical objects in water are vertical, and have 
 no vanishing point, because the image of a vertical line is in the 
 vertical produced beneath the surface of the water, and not in a hori- 
 zontal line lying upon the surface of the water, as it appears to be. 
 When the reflection of a vertical line is not represented as a continua- 
 tion of that line, but as making an angle with it, the perspective is 
 incorrect, no matter in what part of the picture the vertical line may 
 be, or how situated with respect to the point of sight. The reflec- 
 tion of an object which is out of the perpendicular is not necessarily 
 in the same straight line with it, but in general makes an angle with 
 with it. 
 
 The reflection of the sun or moon is always vertically under it, no 
 matter where the point of sight may be. Also, the bar of light pro- 
 duced by the reflection of the sun or moon in rippling water is always 
 vertical, and does not appear to approach the spectator, as it is in- 
 correctly represented to do in many pictures. It is always a good 
 plan, therefore, to take the point of sight immediately beneath the 
 sun or moon, when they occur in a picture. 
 
 Some of the above remarks may be received with surprise and in- 
 credulity by some readers, but a little consideration will shew that 
 they are strictly correct. 
 
 Panoramic Perspective is when the picture is represented upon a 
 vertical cylinder, of which the eye is in the centre. In this kind of 
 perspective the rules are somewhat more complicated, and need not 
 be stated in this work ; it will be sufficient to observe that, in a 
 panoramic picture flattened out, straight lines vanish in curves, not 
 in straight lines. 
 
 When the image is formed upon the focussing screen of a camera 
 having a small pin-hole in front instead of a lens, it is in perfectly 
 true perspective ; for if we consider the pin-hole as the vertex of the 
 system of pyramids formed by lines drawn from it to the objects, and 
 that these lines are produced through the hole so as to form another 
 system, equal and similar to the former, but inverted, it is evident 
 that the section of this second system made by the focussing screen 
 is equal and similar to a section made by a screen placed symmetri- 
 cally with it on the opposite side of the pin-hole, and therefore equal 
 and similar to a perspective view obtained in the ordinary way, but 
 inverted. 
 
 Phosgene Gas. Chlorocarbonic acid. This gas is made by 
 exposing a mixture of carbonic oxide (C 0) and chlorine gases, in 
 equal volumes, to the action of sunshine. They combine quietly and 
 form phosgene gas, the volume being reduced to one half. The 
 
PHO 
 
 823 
 
 gas is an acid, pungent, suffocating compound, which is decomposed 
 by water. See “ Chemistry of Photography. 5 ’ 
 
 Phosphate of Soda. There are three phosphates of soda. 
 The common phosphate is obtained by saturating phosphoric acid 
 with carbonate of soda. It is a salt having an alkaline reaction, 
 efflorescent, and soluble in 4 parts of cold water. It has been used 
 in the first preparation of paper in a printing process with phosphate 
 of silver, discovered by Mr. Maxwell Lyte, and which does not re- 
 quire the use of “ hypo 55 for fixing. See the following article. 
 
 Phosphate of Silver. 3 Ag. O, cP0 5 . This salt is obtained 
 by adding a solution of common phosphate of soda to one of nitrate 
 of silver. A yellow precipitate falls, which is phosphate of silver. 
 It is fusible at a red heat, and soluble in nitric, phosphoric, and 
 acetic acid, also in ammonia and carbonate of ammonia. It is 
 discoloured by exposure to light. The principle of the phosphate 
 of silver printing process is founded on the solubility of the un- 
 reduced phosphate of silver in nitric acid. Unfortunately, the 
 prints are red and require toning, besides being liable to contain 
 chloride of silver and become discoloured in the lights. 
 
 Phosphori. Bodies which emit light in the dark, after having 
 been exposed to light, are called “solar phosphori. 55 When we 
 consider that bodies which have been exposed to heat continue to 
 radiate heat for some time after being removed from the source of 
 heat, and that heat and light are most probably undulations in the 
 same ether, differing only in the length of the wave, the existence 
 of solar phosphori cannot be considered a remarkable phenomenon ; 
 on the contrary, the wonder is that so few bodies should exhibit the 
 property possessed by them. 
 
 The first solar phosphorus on record was discovered about the 
 year 1630 by Cascariolo, a shoemaker of Bologna, who found that 
 calcined sulphate of baryta was luminous in the dark after having 
 been exposed to sunshine, and that it continued luminous for some 
 hours. The matter speedily assumed importance, and considerable 
 quantities of calcined Bolognian spar (the native sulphate employed) 
 were sold as an article of curiosity. 
 
 “ Canton’s phosphorus 55 is another example of the property. It 
 is made by first calcining oyster shells in the open fire for half an 
 hour ; then selecting the largest and whitest pieces, mixing them 
 with about one-third their weight of flowers of sulphur, pressing the 
 whole into a crucible with a closely luted cover, and keeping it at a 
 
 Y 2 
 
324 
 
 PHO 
 
 red heat for an hour. The contents when cold may be turned out 
 and the best pieces selected. They will be found to shine in the 
 dark after having been exposed to sunshine. 
 
 Nitrate of lime fused at a dull red heat is also a solar phosphorus. 
 
 The blue and violet rays appear to be most effective in producing 
 the phosphorescence. It is not found that solar phospfyori generally 
 emit light of the same colour as that to which they have been ex- 
 posed. There are phosphori from heat as well as from light. The 
 native phosphate of lime found near Estremadura, in Spain, and 
 also some varieties of fluor spar, particularly one called “ chloro- 
 phane” are the most remarkable instances. These become luminous 
 when slightly heated, or by friction. 
 
 Some animal substances are spontaneously phosphorescent. The 
 flesh of the tench, carp, herring, and sole is luminous before putre- 
 faction commences. The property is rarely possessed by the flesh 
 of quadrupeds, and has never been observed in that of birds. The 
 phosphorescence of the sea is a beautiful phenomenon frequently 
 observed, but one which has not yet been accounted for. It is 
 probably due to the presence of phosphorescent animal matter. 
 
 The glow-worm and fire-fly are familiar instances of phosphores- 
 cence possessed by living animals. 
 
 Decayed wood and certain mosses have been known to exhibit 
 phosphorescence, but the property is rare in the vegetable kingdom. 
 
 Some salts, (sulphate of potass for instance,) emit light during 
 crystallization. 
 
 Phosphorus. P=32. This remarkable elementary body is 
 contained in the bones and fluids of animals, and also in the vege- 
 table and mineral kingdoms. In bones it exists as phosphate of 
 lime, and is obtained from them by acting on calcined bones with 
 sulphuric acid, and distilling the superphosphate of lime thus pro- 
 duced with the addition of charcoal. The phosphorus, which is 
 volatile, passes over, and its vapour is condensed and drops into 
 water. It is at first a soft translucent yellowish white substance, 
 but becomes red by exposure to light, which is supposed to afford 
 an instance of allotropy. It is insoluble in water, but soluble in 
 oils and in ether. 
 
 Phosphorus is highly combustible and burns slowly and spon- 
 taneously in the air, but magnificently in oxygen. It should be 
 kept, and cut, under water. Its chief use is for making lucifer 
 matches. 
 
 Photo-Galvanography. This is a process for producing 
 
PHO 
 
 325 
 
 copper plates ready for the printer by the joint action of light and 
 electricity. A company was formed at Holloway, in 1856, for carry- 
 ing out the process, under a patent obtained by Herr Pretsch, who 
 superintended operations. 
 
 We paid a visit to the establishment in November of that year, 
 and Herr Pretsch was kind enough to explain the various stages of 
 the process. We subsequently wrote the following article in “ Pho- 
 tographic Notes.” No. 15, describing what we had seen and heard. 
 
 “ A positive photographic print is first taken, — generally on paper. 
 In order to produce from this a copperplate the following operations 
 are employed : — 
 
 “ First ; — A sheet of glass is coated with gelatine containing 
 bichromate of potass, and other chemicals. When dry, the positive 
 is laid upon it, face upwards, and it is exposed to light in a pressure 
 frame for a few hours. The time of exposure of course depends on 
 the intensity of the light. Sunshine is preferred, but is not necessary. 
 The picture upon the gelatine is developed in raised and sunk parts 
 by immersion in a fluid, the principal constituent of which is water. 
 Where the light has not acted, the gelatine swells and forms a ridge, 
 or a series of minute granulations. Where the light has acted, the 
 gelatine is hardened and does not swell. The picture upon the gelatine 
 is very curious, and resembles a positive by reflected light, the sha- 
 dows and dark parts being rough and the lights smooth and polished. 
 
 “ Second — A mould of the picture upon the gelatine is taken in 
 gutta percha. This mould is an intaglio picture, precisely resem- 
 bling the finished copper plate. The gutta percha mould is about 
 half an inch thick. 
 
 “ Third ; — A copper plate is made from the gutta percha mould, by 
 means of the electrotype process. This part of the process is very 
 slow, occupying perhaps a week or two. The copper plate thus ob- 
 tained is called the matrix. It precisely resembles the original 
 gelatine picture. 
 
 “ Fourth ; — The copper plate from which the proof is to be printed 
 is now obtained by the electrotype process from the matrix. This 
 is a slower process than the last, because the copper is much thicker. 
 It occupies about three or four weeks. 
 
 “ The entire process therefore occupies about six weeks. From 
 the final plate four or five hundred good impressions may be struck 
 in the ordinary way ; — a considerable number of plates may be ob- 
 tained from the matrix ; — a considerable number of matrices may be 
 taken from the gutta percha mould ; — • and a considerable number of 
 gutta percha moulds from the gelatine picture. Here then are the 
 means of almost indefinite multiplication. Some idea of the number of 
 
326 
 
 PHO 
 
 proofs which might be obtained from the original gelatine picture 
 would perhaps be got by multiplying 500 by itself four times. This 
 gives more than sixty thousand million impressions; that is, sufficient 
 proofs for six times the present population of the earth. The most 
 elaborate subjects may be engraved by this process in as short a time 
 as the simplest, the amount of detail in a photograph or photo-galvano- 
 graph making no difference, for light, chemistry, and electricity do the 
 work. The time at present required for any subject is a few weeks ; 
 the time frequently spent on engravings is two or three years.” 
 
 The company ceased operations a few months ago, probably from 
 the process not being in a sufficiently perfect state to render them 
 independent of the help of the engraver for retouching their plates. 
 This is much to be regretted, because the process is perfectly suitable 
 for a class of bold artistic subjects which, taken from nature by photo- 
 graphy and multiplied in printers’ ink, would be of great service as 
 studies for artists, or copies for drawing masters to lay before their 
 pupils. 
 
 Photo-Lithography^ This process, like the last, is in a some- 
 what imperfect state, but it will no doubt receive further development 
 and become eventually of much practical utility. It consists in pro- 
 ducing from a negative a positive upon stone which will take printers’ 
 ink, and may be printed from in the same way as an ordinary litho- 
 graph. Two methods have been employed which are totally different 
 in principle, since one depends upon the oxidizing, the other the de- 
 oxidizing power of light. The first method was described by Mr. 
 Macpherson at a meeting of the Photographic Society of Scotland 
 on Dec. 9th 1856, and was reported in “ Photographic Notes,” No. 
 18, as follows : — 
 
 “Take a piece of bitumen of Judea, about 1 J inch square ; pound 
 it in a mortar till it is reduced to a very fine powder ; then put it 
 into a stopped bottle. Pour upon this, 6 ounces of pure sulphuric 
 ether. Shake it for ten minutes, and then let it stand for five min- 
 utes till the powdered bitumen has fallen to the bottom, leaving the 
 ether perfectly transparent, but of a rich brown colour. This liquid 
 is useless, and must be poured off into a bottle, which it is desirable 
 should be kept for such liquids as may afterwards be redistilled, and 
 once more become pure and serviceable ether. To return to the 
 original bottle, containing the sediment of bitumen : — 6 ounces of 
 pure ether must be again poured upon this bitumen. Shake the 
 bottle well for a quarter of an hour, and then pour the contents of 
 it through a paper filter into a clean stopped bottle. This liquid is 
 now in a state for use, and the sediment remaining may be 
 
PHO 
 
 327 
 
 thrown away, as it can never again be used. I may mention that 
 the liquid should be of a beautiful brown transparent colour. Take 
 a lithographic stone, (the blue ones are better than the yellow — the 
 difference between the blue and yellow tints being about the same 
 as in blue and yellow post paper) ; — see that the printer has prepared 
 it with a very fine grain, as if for a chalk drawing. Place it upon a 
 table, in a room with ordinary day -light, — but avoiding the sun’s 
 direct rays. Let it be levelled with a spirit-level, till it is perfectly 
 horizontal. There should be no current of air in the room, and the 
 operator should move as little as possible, either before or after the 
 operation, that any movement of his body may not cause a current. 
 His breath should also be held for a few seconds. He is then ready 
 to begin, and for that purpose should pour out as much of the liquid 
 into a wine glass, as will be required to cover the surface of the stone 
 — a quantity which experience alone can teach him. He then pours 
 it gently and slowly, but regularly, on to the centre of the stone. 
 It immediately runs all over, and spreads itself; and he must be 
 again cautioned against withdrawing his arm quickly so as to produce 
 a current of air. In less than a minute the whole of the ether will 
 have evaporated, leaving a dry looking light brown surface, which, 
 on examination by a powerful lens, ought to appear granular, in 
 other words ought to be composed of myriads of fine dots or stip- 
 ples. 
 
 “ The negative, if of glass, must then be laid down upon the 
 stone ; and great attention is required in laying it down, so that it 
 may be at once put upon that part of the stone which is desired, for 
 it must not afterwards be shifted, or the surface will be destroyed. 
 It is then desirable to surround the negative with slips of pasted 
 paper, so as to paste, and so attach, the negative closely to the stone, 
 and in that way prevent air or dust getting between the stone and 
 negative. It may then be exposed to the light. The exposure will 
 be regulated by the experience of the operator. In my own ex- 
 perience it varies from half an hour to three days ; but I think the 
 three days’ exposure, without the sun, more likely to be successful 
 than the half hour’s exposure with the sun. The stone must then be 
 removed into the operating room, the pasted slips of paper run round 
 with a penknife, and the negative at once lifted cleanly off the stone. 
 The operator can then more leisurely remove the slips of paper with 
 a knife or other sharp instrument. A zinc bath is then placed 
 horizontally on the table ; into the bath is poured as much ether as 
 will give a depth of ^ of an inch. The stone must then be let into 
 the bath, face downwards, exactly as one silvers an albumenized 
 plate — that is gently, but without hesitation or jerk. After it has 
 
328 
 
 PHO 
 
 been there for 8 or 10 seconds, it ought to be raised to an angle of 
 45° two or three times, up and down. Great dexterity is now 
 required. The stone must be at one movement turned over face 
 upwards, and the upper end of it made to rest upon the side of the 
 bath, which ought not to be more than 3 inches high. At this part 
 of the operation an assistant is required, who should be ready with 
 a few ounces of pure ether, which he at once pours over the stone, 
 with a continual movement back and forward, along the upper end 
 of the stone. This ether runs rapidly down, washing away any 
 loose particles of bitumen, which had not been perfectly removed by 
 the washing in the bath. You then place the stone upon a table 
 leaning against a wall, and you see before you the image upon the 
 stone. Should the shadows instead of being dark appear light, and 
 the lights appear dark, the operator will know that it is good for 
 nothing, and that over-exposure was the cause of failure. 
 
 “ The stone should now be placed in the hands of the printer, 
 who will etch it, as he would a chalk drawing, using perhaps a little 
 less acid ; he will then cover it with gum in the usual way, and the 
 next day it will be ready for printing. A great deal depends, as in 
 lithography generally, on its being put into the hands of an ex- 
 perienced picture lithographer. The stone admits of no re-touching, 
 but letters can be added with the use of lithographic ink, as required. 
 I may add that the impressions always look best on India paper, on 
 account of the tint. 
 
 “ I have never worn out a stone. Of each of the two specimens 
 exhibited 500 copies have been thrown off and no difference could 
 be seen between the first and the last.” 
 
 The second process was patented in England by M. Poitevin, of 
 Paris, on April 15th, 1856. The specification of the patent gives 
 the following particulars : — 
 
 “ I print photographically, with ink of a greasy nature, upon 
 paper, lithographic stone, mcjtal, glass, wood, or other suitable ma- 
 terial, in the following manner : — 
 
 “ I apply upon the surface which is to receive the design one or 
 more layers or films of a mixture of equal parts of a concentrated 
 solution of albumen, fibrine, gum arabic, gelatine, or similar organic 
 substance, and a concentrated solution of a chromate or bichromate 
 of potass, or of any base which does not precipitate the organic 
 matter of the first solution. This single or compound layer or film 
 is then dried, if the photographic impression is to be produced by 
 
contact ; or it may be used in a moist state when the photographic 
 impression is to be produced in the camera obscura. In producing 
 the impression by contact, the surface is covered with a photo- 
 graphic negative picture, or an engraving, or other transparent or 
 partially transparent object or screen, and then exposed to light as 
 in the ordinary photographic process. After a sufficient exposure, 
 if the surface has become dry, or has been used in a dry state, it is 
 moistened with water by means of a sponge, and while moist the 
 greasy ink or matter is applied to the surface by a ball or dabber, or 
 by a roller or press, or otherwise, and it will be found to adhere to 
 those parts only which have been affected by the light. Thus, if 
 the screen employed be a negative, having the lights and shades 
 reversed, the print will be a positive, with the lights and darks cor- 
 rect ; and if the screen be a positive, the print will be a negative. 
 The print may be retained on the surface upon which it is first pro- 
 duced, or it may be transferred or printed upon paper, or other 
 suitable material, and the operation repeated. I thus obtain a de- 
 sign upon lithographic stone, or other suitable material, from which 
 I am enabled to multiply impressions by the method of lithographic 
 printing by inking the moistened surface with a greasy ink . 55 
 
 Mr. Macpherson is not the inventor of the bitumen process ; 
 M. Nicephore Niepce first used bitumen in photography, and M. 
 Negre, of Paris, first applied it to photo-lithography. 
 
 Pigments. Positive prints may now be obtained in various pig- 
 ments, by mixing them with an organic substance, and bichromate of 
 potass, — applying the mixture evenly to the entire surface of a sheet 
 of paper, drying it, and exposing it under a negative, — then washing it 
 in water or a suitable solvent, which removes the pigment from those 
 parts of the paper which have not been acted on by light, and leaves it 
 firmly cemented to the paper in the parts which have been so acted on. 
 The process of printing in pigments has not yet received much atten- 
 tion and the results are at present more or less imperfect as compared 
 with those by the old processes ; but since prints by the methods 
 in common use are extremely liable, if not certain, to fade, it is of 
 the utmost importance that the methods of printing in carbon and 
 permanent pigments should be so far improved as to yield results 
 artistically equal to the others. 
 
 The following is a brief account of some of the common pig- 
 ments : — 
 
330 
 
 PIG 
 
 Black . Ivory black is made by calcining ivory dust in a close 
 crucible. Lamp black is the soot produced by the combustion of 
 oils, resins, and other vegetable substances. 
 
 Umber . A brown mineral found in the island of Cyprus ; it is 
 composed of silica, alumina, and oxide of iron and manganese. 
 When calcined for half an hour at a red heat the pigment called 
 burnt umber is produced. 
 
 Asphaltum. A fine rich brown pigment. See “ Asphaltum.” 
 
 Sienna. An argillaceous mineral found in Italy, and also near 
 Wycomb. By calcination it becomes burnt sienna. 
 
 Smalt blue. A glass coloured with oxide of cobalt, and pul- 
 verized. 
 
 Cobalt. Hydrate of alumina mixed with hydrated oxide of cobalt, 
 dried and calcined. 
 
 Sulphate of Indigo. Chemic blue , Saxony blue. Indigo dissolved 
 in about six times its weight of sulphuric acid, then diluted with 
 water, and neutralized with potass. 
 
 Prussian blue. A compound of cyanogen and iron. It is not 
 considered a permanent pigment. 
 
 Stone blue. Finely powdered indigo mixed with starch paste, and 
 made into lumps. 
 
 Copper blue. A mixture of carbonate of copper and chalk, ex- 
 posed to the air until it assumes the proper colour. 
 
 Ultramarine. A pigment composed chiefly of a costly mineral 
 called Lupis lazuli , brought from China and Persia. 
 
 Artificial ultramarine. A pigment containing sulphide of sodium, 
 obtained by fusing together in a crucible, porcelain clay, sulphur, 
 and carbonate of soda. French photographic papers are tinted with 
 this villanous alkaline sulphide, which is enough of itself to cause 
 the fading of any photograph. 
 
 Blue verditer. Nitrate of copper mixed with chalk. 
 
 Copper green. Native sub-carbonate of copper. 
 
 Brunswick green. Carbonate of copper mixed with calcareous 
 matters. 
 
 Vienna green. A mixture of arsenious acid and verdigris.. 
 
 Green verditer. An accidental variety of blue verditer. 
 
 Sap green. The juice of the berries of buckthorn, black alder, or 
 ever-green privet, mixed with lime water and gum arabic, and eva- 
 porated until quite thick. 
 
 Iris green. The juice of the petals of the iris added to quick 
 lime. 
 
 Carmine . An extract from the cochineal insect. 
 
 Lake . The colouring matter of raw shellac. 
 
PIN 
 
 PIP 
 
 331 
 
 Brazil-wood lake. A mixture of a decoction of logwood, alum, 
 and chloride of tin, to which carbonate of soda is added to form a 
 precipitate. 
 
 Madder. A colouring matter obtained from the root of the Rubia 
 tinctorum , which grows in the South of Europe. 
 
 Brown 'pink. To a decoction of French berries and fustic, boiled 
 with potass in a tinned vessel, alum is added. The precipitate is 
 “ brown pink.” 
 
 Dutch pink. Turmeric is substituted for fustic, and whiting for 
 alum, in the preceding formula. 
 
 Orange red. Sandix. White lead calcined. 
 
 Red lead. Minium. Litharge (oxide of lead), roasted in a re- 
 verberatory furnace. 
 
 Indian red. Peroxide of iron. 
 
 Red chalk. Clay iron-ore. 
 
 Venetian red. Oxide of iron. 
 
 Alum white. A calcined mixture of honey and alum. 
 
 White lead. Basic carbonate of lead. 
 
 Permanent white. Carbonate of baryta. 
 
 Zinc white. Oxide of zinc. 
 
 Chrome yellow. 
 Indian yellow. 
 King's yellow. 
 Naples yellow. 
 salt. 
 
 Patent yellow. 
 Queen s yellow. 
 
 Chromate of lead. 
 
 A concretion formed in the intestines of the camel. 
 Sulphide of arsenic. 
 
 A calcined mixture of lead, antimony, alum, and 
 
 Chloride of lead. 
 
 Turpith mineral, or sub-sulphate of mercury. 
 Yellow lake. French berries boiled with potass, and precipitated 
 with alum. 
 
 Ochres. Native oxides of iron mixed with argillaceous and cal- 
 careous earths. 
 
 Verdigris. Acetate of copper. 
 
 Indigo. A product obtained from the indigo plant. 
 
 Sepia. The black liquid contained in the cuttle fish. It consists 
 of carbon, along with albumen, gelatine, and phosphate of lime. 
 
 Vermilion. Cinnabar. Protosulphide of mercury. 
 
 Terra verte . Silicate and phosphate of protoxide of iron. 
 
 Pins. Pins are used in photography for hanging up papers by 
 the corners to dry, or for pinning the corners to a board. Black 
 pins coated with enamel are the best. 
 
 Pipe Clay. A clay analogous to kaolin and found in the Isle of 
 
332 
 
 PLA 
 
 Purbec, and Dorsetshire. It contains a large proportion of Alumina, 
 and is sometimes used for decolorizing old nitrate baths. 
 
 Piaster, of Paris, Gypsum. This useful substance is made 
 by roasting sulphate of lime at a temperature of about 500°, by 
 which the water of crystallization is expelled. When plaster of paris 
 is made into a paste with water, it soon solidifies, and this property 
 constitutes its value for taking casts or moulds. Stucco and sca- 
 gliola are made by mixing plaster of paris, coloured in various ways, 
 with size and water, and polishing the surface. Gypsum, or native 
 sulphate of lime, is frequently used as a manure, particularly for 
 clover crops. Sulphate of lime is soluble in 500 parts of water 
 and it is this salt which principally renders water “ hard.” 
 
 Plate. In optics, a transparent medium bounded by parallel 
 plane surfaces is called a “ Plate.” When a ray of light is refracted 
 through a plate, its direction at emergence is parallel to that at inci- 
 dence, and it does not suffer deviation but only displacement ; the 
 amount of displacement depending on the thickness of the plate. 
 The same thing happens when a ray is refracted through any number 
 of plates of different materials in contact ; — it merely suffers displace- 
 ment and not deviation ; — the medium external to the plates being 
 supposed to be the same, 
 
 Plate Glass. This is made of the same materials as crown- 
 glass, and does not contain lead. Vast quantities of it are now used 
 in photography. Plate glass is made by pouring a quantity of the 
 fused “ metal ” upon a table or cuvette of cast iron, and then pass- 
 ing a roller over the surface. The plate is then annealed or al- 
 lowed to cool slowly in an oven, or carquaise, along with others. 
 When cold the plate is removed, and carried in an upright position 
 to a part of the manufactory where it is to be roughened down and 
 polished. This is accomplished by fixing one side of the plate with 
 plaster of paris to a horizontal stone table, and another plate to a 
 piece of apparatus above it. The apparatus is then put in motion, 
 and the surface of the upper plate rubbed upon that of the under 
 one, with wet sand between them, by a circum-rotatory motion, at 
 the same time that a peculiar lateral motion is given to the table 
 which supports the low r er plate. When the plates are in this way 
 sufficiently worked on one face, the process is repeated on the other. 
 The plates are next smoothed in the same way by substituting moist 
 emery for moist sand, and the polishing is effected by colcothar 
 (oxide of iron) applied by rubbers of felt. The final polishing is 
 
PLA 
 
 POR 
 
 333 
 
 given by women, who mb two plates together with a little moistened 
 putty of tin between them. 
 
 Manufacturers of plate glass should be particular not to pack it 
 up, when intended for photography, with printed papers between 
 the sheets, for it has been found that permanent impressions are thus 
 communicated to the glass, which are reproduced upon the photo- 
 graph. It has been affirmed that these impressions cannot be re- 
 moved by the strongest nitric acid. 
 
 Plate Paper. The thick bibulous paper upon which engravings 
 are printed. 
 
 Plumbago, Blacklead, Graphite. This substance is com- 
 posed of carbon and iron, and contains about 8 per cent, of iron. 
 The finer kind is used for blacklead pencils, and the coarser kinds 
 for polishing grates, diminishing friction in machinery, &c. It is 
 almost exclusively obtained in a pure form from the mine of Borrow- 
 dale, in Cumberland ; it is infusible, very difficult of combustion, 
 and sometimes occurs crystallized in hexangular plates. In an 
 impure form it is not an uncommon mineral, and is found in detached 
 masses, generally among primitive rocks. 
 
 Plumbers’ Solder. Equal parts of lead and tin. 
 
 Polarized Light. See “ Light.” 
 
 Portrait Room. The construction of the room in which 
 portraits are taken is a matter of the first importance. The best 
 arrangement for producing a pleasing expression in the countenance 
 of the sitter, an artistic effect of light and shade, and a portrait 
 free from distortion and disagreeable exaggeration of near objects, 
 is as follows : — 
 
 The room should be at least 30 feet long, 10 feet wide, and 8 feet 
 high at the sides. The roof should have a ridge from end to end. 
 Both ends of the room should be dark, and nearly in the middle 
 there should be a window on each side coming down to within 3 
 feet of the floor and going up to the roof. In the roof there should 
 be two skylights exactly corresponding in breadth with the windows, 
 x and going up to the ridge. The width of the windows and skylights 
 need not exceed 5 feet. They should be provided both with black 
 and white blinds, which can be drawn at pleasure, and also with the 
 means of opening and shutting them in calm or hot weather. The 
 background is placed at that end of the room which is nearest to the 
 
334 
 
 POS 
 
 POT 
 
 windows and the camera at the other. White screens, moveable on 
 rollers, should also be at hand when required to throw a reflected 
 light upon any part. The room should be entirely papered with 
 a very dark blue paper ; and the eyes of the sitter should never be 
 directed towards the light, but into darkness. 
 
 The portrait room should, if possible, be built of wood, and made 
 to turn about an axis in the centre, with two wheels at each end, 
 so that it may be placed in any direction with respect to the light. 
 When this is not practicable it should be placed north and south, 
 and the sittey should face the north. A lens of long focus and large 
 aperture should be used in preference to one of short focus and small 
 aperture ; not because a large aperture is better than a small one, but 
 because it is at present a matter of unavoidable necessity. A lens 
 of too short focus for the size of the picture produces horrible 
 distortion in the image. Photographers generally are too fond of 
 straining their lenses and working with oblique pencils. The only 
 objection which can properly be raised against a lens of long focus 
 and suitable aperture is that which arises from the room being filled 
 with the smoky atmosphere of a town, which renders the picture 
 indistinct and produces fog when the camera is too far from 
 the sitter. To avoid this evil, portraits should always be taken in 
 the suburbs and not in the centre of a large town. 
 
 The floor of the portrait room should be covered with light holland 
 in the neighbourhood of the sitter, and with dark drugget at the end 
 opposite to him. Chairs, sofas, tables, vases, &c., should not be 
 polished, because the high lights look like spots of snow in the 
 picture. Nothing plain, or poverty-stricken, or of ugly design should 
 appear in the portrait room ; on the contrary it should be furnished 
 with articles of taste and luxury, but not with anything gaudy or 
 distracting to the eye. The camera stand should be placed upon a 
 platform mounted on wheels. 
 
 In taking a portrait, one of the side windows should always have 
 either the white or black blind drawn, and the same may in general 
 be said of one of the skylights. The object of this is to throw more 
 light upon one side of the face than the other. 
 
 Positive Collodion Process. See page 120. 
 
 Potass. KO=47. Hydrate of Potass, KO, HO =5 6. 
 
 “ Caustic Potass,” or hydrate of potass, is obtained by boiling 
 together in an iron vessel slaked lime and a solution of carbonate of 
 potass. The carbonic acid leaves the potass and goes to the lime, 
 forming an insoluble precipitate of carbonate of lime, and the potass 
 
POT 
 
 PEE 
 
 335 
 
 remains in solution. When a little of the liquid taken out ceases to 
 effervesce on the addition of an acid, the decomposition is complete. 
 The clear liquid is then drawn off into an iron or silver vessel, eva- 
 porated to dryness, fused in its own basic water, and run into moulds. 
 The sticks thus formed still contain a little carbonate ; this is re- 
 moved by dissolving them in absolute alcohol, when the carbonate of 
 potass is precipitated as insoluble, but the plan is open to objection 
 since the alcohol is liable to be decomposed. 
 
 Caustic potass is soluble in half its weight of cold water. It is 
 highly alkaline and caustic, acting energetically upon most organic 
 substances, and dissolving sulphur, alumina, silica, and several 
 sulphides ; its aqueous solution, also, dissolves the oxides of some 
 of the metals, ns manganese, zinc, lead, tin, antimony, cobalt, nickel, 
 &c., and acts upon glass, particularly when at a boiling heat. It is 
 freely soluble in alcohol and fuses at a red heat. 
 
 Potassium. K=40. This singular metal is obtained by ex- 
 pelling the oxygen from potass either by exposing it to intense heat 
 in contact with charcoal, or by Yoltaic Electricity. It is a bluish 
 white metal of great lustre, which fuses at 150°, boils at a red heat, 
 floats upon water, and takes fire by coming into contact with water 
 in consequence of its great affinity for oxygen. It must be preserved 
 under naphtha. 
 
 ' Preservative Processes. The object of these processes is to 
 preserve the sensitiveness of an excited collodion plate for a longer or 
 shorter time, as may be required, and thereby to do away with the 
 inconvenience of preparing the plate and developing the picture on the 
 spot whence the view is taken. 
 
 The best dry preservative processes at present known are undoubt- 
 edly those of Dr. Hill Norris, and Mr. Eothergill, described under 
 the head of “ Dry Collodion Processes; 5 ’ q. v. With respect to the 
 moist preservative processes, various plans have been suggested, 
 (some right and others manifestly wrong in principle,) which consist 
 in removing more or less of the free nitrate of silver from the sensitive 
 plate by washing it and then pouring over it such a substance as 
 glycerine, golden treacle, oxymel, or honey, which retains its mois- 
 ture for a great length of time. After the exposure, and immediately 
 before the development, the plate is well washed in order to remove 
 the preservative substance, and the picture is then developed, either 
 with gallo-nitrate, or pyrogallo-nitrate of silver, and fixed in the 
 ordinary way. 
 
 Of the substances named as preservatives, golden syrup, or glycer- 
 
 
386 
 
 PEE 
 
 ine, or a mixture of golden syrup, and metagelatine, are probably the 
 best , and oxymel or honey the worst. The objection to honey consists 
 in its containing a large quantitiy of grape sugar, which is a powerful 
 reducing agent and tends to fog the plate without increasing its sensi- 
 tiveness ; while golden syrup is entirely ^crystallizable and a very 
 feeble reducing agent, therefore a much more suitable substance than 
 honey to employ. The reducing action of the grape sugar contained 
 in honey is kept at bay somewhat by adding vinegar to it, and forming 
 oxymel ; but when an acid is present in the preservative, not only is 
 the time of exposure greatly increased, but the latent image is grad- 
 ually destroyed by it between the time of exposing the plate and de- 
 veloping the picture. Some time ago an amusing spectacle was 
 exhibited by two photographers, each contending for the honour of 
 the discovery of the honey process. We believe more honour would 
 have been gained at the time by any one who had then pointed out the 
 ^advantages which attend, and the error of principle which is 
 involved in the use of it. Glycerine appears to be ineit and well 
 adapted for a preservative agent, particularly when the nitrate of 
 silver is not thoroughly washed off. 
 
 In any preservative process it is always well to add a chloride, 
 say chloride of magnesium, to the iodizing solution, and the collo- 
 dion should not be too hard and contractile. 
 
 The deliquescent salts, nitrate of magnesia, and nitrate of zinc, 
 have been tried as preservatives, but they do not appear to answer. 
 
 The theory of preservative processes appears to be this : — The 
 sensitiveness of the excited collodion plate is impaired, but not alto- 
 gether destroyed, by washing off the free nitrate of silver. An 
 image perfect in all its details may be impressed upon a washed col- 
 lodion plate, by sufficiently increasing the time of exposure ; but in 
 the development nitrate of silver must be added to replace that which 
 has been removed. The impressed image cannot be removed by 
 simple washing in water, although it may be dissolved out by im- 
 mersion in an acid nitrate bath, or by acid oxymel, or acid gelatine 
 or any acid developer, or other chemical agents. The effect of a 
 moist preservative solution poured over the plate is therefore (sup- 
 posing it to be quite inert), merely to retain the moisture of the film 
 during the time that it is in contact with it. 
 
 A good moist preservative process is chiefly valuable to the pro- 
 fessional photographer (who must of necessity work collodion in a 
 van or tent near' the spot where the picture is taken), for taking in- 
 teriors, or subjects which require a very long exposure. To the 
 amateur it is useful for enabling him to excite and develop the plate 
 at the inn where he is staying. If an excited plate be required to be 
 
PEI 
 
 337 
 
 “kept” more tlian a few hours, Dr. Norris’s dry process is certainly 
 far better than any moist preservative process. See “ Collodion.” 
 
 Printing Ink. This may be applied as a backing to collodion 
 positives, and it does not crack. To apply it, first rub the back of 
 the plate over with it, then smear a piece of paper with it, press the 
 two blackened surfaces together, and put the plate at once into the 
 passe-partout. It may also be used for blackening the skies of collo- 
 dion negatives. 
 
 Printers’ ink is made by grinding lamp-black in printers’ varnish, 
 which is a mixture of linseed oil, resin, and yellow soap. By adding- 
 more varnish it may be thinned to any extent. 
 
 Prints, Colouring. Before colouring an engraving or photo- 
 graph upon bibulous paper, it must be sized by applying to it the 
 following mixture : — 
 
 Dissolve 4 ounces of glue and 4 ounces of white soap in 3 pints 
 of hot water ; add 2 ounces of powdered alum ; stir well together, 
 and it is ready for use. It is to be applied cold, either with a sponge 
 or flat camel-hair brush. 
 
 Printing Process. By “printing” is meant the reproducing 
 a positive, in which the lights and shades are true to nature, from a 
 negative in which they are reversed. The operation not being at- 
 tended with the destruction of or injury to the negative may be 
 repeated indefinitely, and therefore any number of prints may be 
 taken from the same negative. 
 
 There are two methods of printing ; one consists in copying the 
 negative by means of a lens, the other by pressing it upon a sen- 
 sitive tablet in a pressure frame, and exposing it to direct light. In 
 both cases the light which produces the print is transmitted through 
 the transparent parts of the negative, and stopped by its opaque 
 parts. The particulars of the former method are described in the 
 article on “ Copying,” (q.v.) It only remains therefore to describe 
 the latter mode of proceeding, viz., printing by superposition. 
 
 There are two methods of printing by superposition of the negative 
 upon a sheet of sensitive paper. One is called Sun-printing, the other 
 Development-printing. 
 
 In sun-printing the paper is said to be either “ plain” or “ albu- 
 menized.” The latter process is minutely described at page 13. 
 
 The process of Sun-printing upon Plain paper is as follows : — 
 
 Use the best Papier Saxe, or the papers of Marion, or Canson ; all 
 
 z 
 
338 
 
 PEI 
 
 of which are manufactured on the continent, and have a finer surface 
 than English papers. 
 
 Float the face of the paper for a minute upon a bath composed of — 
 
 Filtered rain water 1 oz 
 
 Gelatine 3 grains 
 
 Chloride of sodium .... 6 grains. 
 
 The ingredients are to be boiled together, strained, and used when 
 cold. Hang the papers up to dry by a pin at one corner. 
 
 Excite the paper by brushing over it, with a Buckle’s brush, the 
 following solution of ammonio -nitrate of silver. 
 
 Distilled water 1 oz 
 
 Nitrate of silver ... 50 grains 
 
 When dissolved add ammonia, a drop or two at a time, until the 
 brown turbidity at first formed is exactly redissolved, and the solution 
 becomes again clear. 
 
 Brush the papers over twice with the above, and hang up to drain. 
 
 When as many have been excited as are required for immediate 
 use, dry them before the fire at a moderate distance, and use them 
 at once, for they turn brown by keeping. 
 
 Expose in the pressure frame until the picture is somewhat over- 
 printed. 
 
 On removing it from the pressure frame, wash it (in the dark 
 room) in several changes of rain water, then with water to which a 
 few drops of ammonia have been added, lastly with water again. It 
 is now of a reddish purple tint. 
 
 Tone it by immersion in a very small quantity of a bath contain- 
 ing 
 
 Distilled water ... 6 ounces 
 
 Sel d’or 1 grain 
 
 Hydrochloric acid . . a few drops. 
 
 From one to five minutes immersion in this bath is sufficient. 
 The tint changes from a reddish purple to an indigo colour, while the 
 whites are but slightly changed. The bath must not be used again. 
 
 Wash the print thoroughly in water changed a dozen times at 
 least, then fix it in a bath containing 1 part of hyposulphite of soda 
 to 20 parts of water. Let it remain 20 minutes in this bath, then 
 throw the hypo away, and wash the print in water, and let it soak 
 in water frequently changed and agitated, for 24 hours. 
 
 Press between cloths, hang up to dry, and the print is finished. 
 
 The simplest Development-printing process is as follows : — 
 
PEI 
 
 339 
 
 Use H oiling worth’s thin photographic paper ; (the thick sort is 
 useless.) Immerse it in the following bath : — 
 
 Filtered rain water ... 1 ounce 
 
 Salt 6 grains 
 
 Lemon juice 1 drop. 
 
 The time of immersion may lie between one minute and 24 hours 
 without producing any marked difference in the result. 
 
 Excite the paper by floating it upon a nitrate bath made thus : — 
 Distilled water .... 1 ounce 
 
 Nitrate of silver . . . .30 grains 
 
 Lemon juice 6 or 8 drops. 
 
 Hang it up to dry, and use it as soon as possible. 
 
 Expose it in the pressure frame until a faint trace of the picture 
 is visible. 
 
 Develop it thus : — 
 
 Turn up the edges of the paper all round so as to make it into a 
 tray. Lay it, with a sheet of blotting paper underneath, upon a hori- 
 zontal sheet of glass, and pour upon the darkest part of the picture 
 a little saturated solution of gallic acid, which spread with a bent 
 glass rod. The development immediately commences and is com- 
 pleted in a few minutes. Do not stop it at too early a stage, before 
 the blacks have acquired the proper intensity. 
 
 Wash the picture once or twice in rain water, and fix it exactly in 
 the manner described in the preceding process. 
 
 When finished it should precisely resemble an engraving in ap- 
 pearance. 
 
 By adding a little iodide of potassium to the salt, the paper is 
 rendered much more sensitive and the print more permanent, its 
 colour is also improved by an admixture of grey or blue ; but the 
 process is less manageable and certain. 
 
 These processes have been somewhat briefly described because the 
 Author has learnt, within a few days, the particulars of a method of 
 printing in carbon, which he feels sure must speedily supersede all 
 the processes with the silver salts, from its economy and certainty, 
 as well the undoubted permanence of the prints. This process, 
 however, (the discovery of Mr. John Pouncy, of Dorchester,) he is 
 not at liberty at present to make public. 
 
 Prism. In solid geometry a prism is a solid described by the 
 motion of a straight line which in passing round the boundary of a 
 plane rectilineal figure always preserves its parallelism, the solid being 
 terminated at the other extremity by a plain figure parallel to the 
 first. In optics, however, the term prism is confined to the case of 
 
 z 2 
 
340 
 
 PEI 
 
 a prism with a triangular base, and its sides rectangles perpendicular 
 to the base. 
 
 In optical experiments with the prism the edge of the prism is in 
 general very sharp, the two adjacent planes which form it being in- 
 clined at a very small angle, called the “ refracting angle of the prism.” 
 When the prism is so placed with respect to a ray of light refracted 
 through it as that the emergent and incident rays make equal angles 
 with the sides of the prism, the deviation of the refracted ray is a 
 minimum. 
 
 Suppose a prism to be placed in its position of minimum devia- 
 tion with respect to a ray refracted through it near its edge, and let 
 D be the deviation of the refracted ray, fj. the index of refraction of 
 the material of which the prism is made, and a the refracting angle 
 of the prism ; then, if the angle made by the incident ray be small, 
 
 D = 0a-l)a. 
 
 A ray of light refracted through a prism is decomposed into rays 
 of different refrangibility and colour, because the deviation of a ray 
 depends upon the refractive index of the prism for that ray, and since 
 white light is not homogeneous but composed of light of different 
 degrees of refrangibility, the refractive index will vary with the dif- 
 ferent rays of which white light is composed, being greatest for the 
 violet and least for the red rays, therefore the deviation will be 
 different for rays of different colours. 
 
 If a second prism, precisely similar to the first, be placed against 
 it in such a way as that its edge is next to the base of the first prism, 
 the two prisms will form a plate, and a ray refracted through them 
 will neither suffer deviation nor decomposition ; that is to say the 
 effects produced upon it by refraction through the first prism will be 
 exactly counteracted by refraction through the second, so that the 
 second prism will re-compose into white light the rays which were 
 dispersed by the first. The second prism therefore achromatizes 
 the first; but the refracted ray does not suffer deviation , there 
 is therefore no optical utility in such an arrangement. But if the 
 second prism be made of a different material from the first, having 
 different refractive and dispersive powers, and a suitable refracting 
 angle be given to it, the first prism will be achromatized by the second, 
 and the ray will suffer deviation . This important result depends on 
 the fact that the dispersive power of a medium is not proportional to 
 the deviation produced by it. 
 
 This being the case, opticians are fortunately able to achromatize 
 lenses and prisms. To be very exact however, it must be remem- 
 bered that in consequence of the irrationality of dispersion 2 prisms in 
 contact can only unite 2 of the coloured rays or lines of the spectrum. 
 
PRO 
 
 PYR 
 
 341 
 
 Proof-Spirit. Alcohol, S. G. *92 at 62° Fahrenheit. /^“Al- 
 cohol.” 
 
 Prussian Blue. A peculiar compound of cyanogen and iron, 
 the exact formula of which has not been determined, but may be 
 represented approximately by 6 K 0 + 4 Pe. 4 Cfy. 3 . 
 
 This substance is much used both as a dye, and pigment. It is 
 made by precipitating solutions of peroxide of iron with ferro- cyanide 
 of potassium, (yellow prussiate of potass.) It is insipid, inodorous, 
 insoluble both in water and alcohol, and not poisonous. The alka- 
 lies decompose it, and it does not therefore, as a dye, resist the action 
 of soap. According to Chevreul, it becomes w T hite by exposure to 
 sunshine, but recovers its colour in the dark. It has a strong attrac- 
 tion for water. 
 
 Prussiate of Potass, Red and Yellow. See “ Perro-, and 
 Perrid-Cyanide of Potassium.” 
 
 Pumice Stone. A grey porous stone found in the neighbour- 
 hood of active and extinct volcanoes, and supposed to have been 
 thrown up bv them. It is used by painters for smoothing surfaces 
 intended to be painted ; and, when pounded, by other artificers for 
 polishing glass, metals, &c. 
 
 Purple of Cassius. This is a fine purple pigment used in 
 enamel painting, and staining glass of a red colour. It is composed 
 of the mixed oxides of gold and tin, and is precipitated by immersing 
 a piece of tinfoil in a solution of chloride of gold. Its composition 
 is Au.O-|- 3Sn.0 2 -f 4 HO. 
 
 Putty. See “ Glazier’s putty.” 
 
 Putty Powder. Polisher’s Putty. Peroxide of tin. 
 
 Pyro-Acetic Ether ; Pyro-Acetic Spirit. C 3 H 3 O. This 
 is a colourless, limpid, inflammable liquid, sometimes called “ Ace- 
 tone.” It is permanent in the air, and mixes in all proportions with 
 water, alcohol, ether, and turpentine. It is obtained by the destruc- 
 tive distillation of acetate of copper, or other acetates. It appears 
 to be of no present use in photography, although like many other sub- 
 stances it has been tried by experimentalists, as an addition to col- 
 lodion. 
 
342 
 
 PYE 
 
 Pyro-Gtallic Acid. C 6 H 4 0 4 . This substance is extensively 
 used by photographers as a developer in the negative collodion 
 process. 
 
 It may be made by exposing gallic acid to a temperature of about 
 420°, when it sublimes and may be collected in the form of white 
 shining scales ; but is apt to be contaminated with empyreumatic oil. 
 
 A better plan is to treat finely powdered galls with successive 
 portions of cold water until exhausted, then to collect all the 
 infusions and evaporate them to dryness. The spongy deliquescent 
 mass thus produced must then be pounded and spread upon the 
 bottom of an iron vessel 3 or 4 inches deep and 1 foot in diameter, 
 the top of it being covered with a piece of blotting paper pierced 
 with pin holes, and surmounted by a paper cap 12 or 18 inches 
 high. The pan is then cautiously and uniformly heated for some 
 hours at a temperature of about 400°. The crystals of pyro- 
 gallic acid collect in the cap, and the other products are absorbed 
 by the blotting paper. 
 
 Pyrogallic acid is not an acid, and does not redden litmus paper. 
 It is white, crystalline, inodorous, and bitter; and very soluble in 
 water, alcohol, and ether. The aqueous solution blackens by long 
 exposure to air, and deposits a brown powder. It gives a deep 
 indigo colour to a solution of profosulphate of iron, if pure and free 
 from persulphate, to which it gives an orange colour. 
 
 Pyrogallic acid is blackened by chlorine, but iodine has no effect 
 upon it. 
 
 It is a powerful deoxidizer, and reduces the oxides of the noble 
 metals ; hence its use as a developer in photography. It combines 
 with oxide of lead, and forms a white powder. 
 
 Pyro-Ligneous Acid. A crude vinegar obtained by the 
 destructive distillation of wood. When purified it is used as a 
 substitute for vinegar in many processes of the arts, and also in 
 making pickles, sauces, &c. 
 
 Pyro-Ligneous Spirit. Pyroxylic Spirit. See “Wood- 
 Alcohol.” 
 
 Pyroxyline. C 24 H 15 N 5 O 40 . This substance, which when 
 dissolved in ether forms collodion, is made by acting on lignin with 
 a diluted mixture of sulphuric and nitric acids represented by the 
 formula HO, N0 5 -f 2 HO, S0 3 -f-3|H0, the temperature being 
 uniformly maintained at from 135° to 150°, according to the quality 
 of the collodion required. 
 
PYR 
 
 343 
 
 It may either be made from cotton wool, or linen rags. The rags 
 must first be boiled in a strong solution of soda, and then thoroughly 
 washed. The mode of proceeding will be understood from the 
 following description, extracted from a treatise by the author on the 
 “ Positive Collodion Process.” 
 
 ‘‘Procure some dry cotton wool chemically clean, some pure 
 sulphuric acid, S. G. 1*84, some pure nitric acid, S. G. 1*5, and 
 some rectified sulphuric ether, S. G. *750. 
 
 “Use the fire-place of an outbuilding for the experiments. Put 
 an old frying pan filled with sand upon the fire, and in this sand- 
 bath place a pie-dish containing water heated to about 170° Paht. 
 Then procure a breakfast cup and a couple of long thick glass rods. 
 
 “1st Experiment. — Put into the cup 
 
 5 drachms Nitric Acid, by measure. 
 
 5 drachms Sulphuric Acid „ 
 
 25 grains Cotton Wool. 
 
 “ Dense suffocating fumes rise from the mixture ; these should 
 escape up the chimney. Keep working the cotton wool about with 
 the glass rods for 5 nfinutes, during which time the temperature of 
 the mixture should be 150°. The temperature of the water in which 
 the cup stands being about 170°, that of the mixture in the cup 
 will be as nearly as possible 150°; but you must test it with a 
 thermometer, the ball of which can be inserted in the mixture ; 
 for the preservation of an even temperature is of the utmost 
 importance. 
 
 “At the end of five minutes, remove the cup, throw away the 
 mixed acids, and put the cotton into a pail of water. Wash it 
 quickly, opening it well, and rousing it about in the water. Then 
 continue the washing in a basin, changing the water several times, 
 and squeezing the cotton after each washing between your hands. 
 
 “ When you have thoroughly washed and squeezed out all traces of 
 the acids in this way, pull the cotton out into a large loose ball, 
 and hang it up to dry gradually in a clean piece of netting. The 
 cotton when dry looks pretty much as it did at first, but you feel a 
 peculiar harshness about it. 
 
 “ The first experiment yields pyroxyline of the most explosive kind. 
 Be careful therefore of accidents. 
 
 “Repeat the experiment ten or twelve times, adding in the 
 second experiment 30 minims of water to the acids, and increasing 
 the quantity of water added by 30 minims in each fresh experiment. 
 The twelfth experiment will therefore contain, in addition to the 
 acids, 330 minims, i. e. 5 \ drachms of water. 
 
 “We will now suppose the various samples of gun-cotton to be dry, 
 and ready for an investigation of their properties. 
 
344 
 
 PYB 
 
 : Pirst- 
 
 -weigh them. 
 Sample 
 
 1 will weigh 43 grains 
 .. 2 „ 43 „ 
 
 jj 3 ,, 43 ,, 
 
 „ 4 „ 42 „ 
 
 » 5 „ 37 „ 
 
 Samples 6, 7, 8 37 „ 
 
 “ The cotton having increased in weight from 75 to 50 per cent., 
 according to the quantity of water added. 
 
 “We now proceed to test the solubility of these twelve samples of 
 gun-cotton in ether, S. Gr. *750, and also to ascertain the various pro- 
 perties of the film produced when the solution is poured upon a glass 
 plate. Observe that ether, at *750, contains a proportion of alcohol 
 and water ; the S. G. of absolute ether, being only *720. 
 
 “Weigh two grains of each sample of gun-cotton, and test their 
 respective solubility in half an ounce of ether. 
 
 “ Samples 1,2, and 3, will be found to be insoluble. Sample 4 
 looks more gelatinous, and seems inclined to dissolve. Sample 5 
 dissolves completely on shaking the bottle. Samples 6, 7, 8, 9, 10, 
 are soluble ; 11 partly so ; 12 not at all. 
 
 “ Now compare the different samples of cotton. The first three or 
 four are long and fibrous, the next three or four are somewhat 
 shorter, the last three or four become very short, and break up into 
 little short shreds, many of which are lost in the washing. 
 
 “ The first three or four samples are called “ Pyroxyline,” and 
 the last three or four “ Xyloidine.” But it will be seen that this 
 nomenclature is imperfect, for it does not include the middle 
 varieties, which are those with which we are concerned in Photo- 
 graphy, viz : Nos. 5, 6, 7 and 8. We will call them photographic 
 gun-cotton. 
 
 “ The first samples of pyroxyline are highly explosive. Place a 
 small tuft upon the hearth, and apply to it the end of a red-hot 
 piece of iron wire. It instantly goes off ‘ puff/ without smoke, 
 and leaving no ash. The last varieties of xyloidine are merely 
 combustible, and not explosive. 
 
 “ Let us next examine the nature of the solutions made with 
 samples between Nos. 5 and 10. 
 
 “ Pour a few drops of No. 5 upon the finger, so that it may run 
 round both ways. It dries quickly (producing a sensation of cold), 
 and, when dry, contracts strongly, looking like a piece of goldbeater’s 
 skin, stuck tightly round the finger. This is the hard contractile 
 collodion. Its use should be avoided in photography. 
 
 “ Now pour a few drops of No. 10 upon the finger. This also dries 
 quickly, but when dry does not contract like the former, and instead 
 
PYB, 
 
 345 
 
 of being transparent, is semi-opaque, or “ opalescent,” or <c papy- 
 raceous,” looking like a piece of tissue paper stuck round the finger. 
 This is also a kind of collodion to be avoided in photography. 
 
 “ The proper variety of gun-cotton for photographic purposes lies 
 between these extremes. 
 
 “ In order to try which is the best, pour a little of each solution 
 upon a clean glass plate. But in order to make the experiment fairly, 
 wait a day or two, until the floating particles in the collodion have 
 settled to the bottom of the bottle ; for you cannot properly filter 
 collodion without a special apparatus. 
 
 “ Examine the films before a strong light, with the help of a 
 magnifier. 
 
 “ Eilm No. 5 is not only hard and contractile, but shows struc- 
 ture, being covered with wavy marks, or lines. 
 
 “ Eilms Nos. 6 and 7 are much better and are nearly structureless ; 
 No. 6 is the best. 
 
 “ Eilm No. 8 begins to get slightly opaque. 
 
 “ In films Nos. 9 and 10 the opacity increases. 
 
 “ No. 6 is therefore the best collodion, and on adding a little 
 alcohol to it, the appearance of structure in the film altogether 
 vanishes. It adheres tightly to the glass, without contracting, and 
 cannot easily be washed off.” 
 
 The proper strength of the nitrosulphuric acid, determined from 
 the above experiments, does not agree exactly with the formula 
 given in symbols in a former paragraph, and which is that of Mr. 
 Hadow ; but some latitude is allowable in that formula, and the best 
 proportions are purely a matter of experiment. 
 
 Before putting the cotton into the acids it should be pulled out 
 into very thin flat pieces. 
 
 Instead of using mixed acids, a mixture of sulphuric acid and 
 nitre may be employed. The ingredients must be pure, and the 
 nitre finely pulverized and dried. 
 
 Mix together — 
 
 Water . . . 1J fluid drachms. 
 
 Sulphuric acid . . 12 „ „ 
 
 And pour it upon 
 
 Pure nitre . . .600 grains. 
 
 Stir it well until it ceases to effervesce, and forms an even pasty 
 mixture free from lumps. This mixture must be kept at a tempera- 
 ture not lower than 135°, and used at once, as it solidifies on cool- 
 ing. The cotton must be kneeded in the mixture for about 10 
 minutes. This plan of making pyroxyline is not nearly so good as 
 with the mixed acids. 
 
 In using commercial acids of unknown strength, the exact 
 
346 
 
 QUI 
 
 quantity of water to be added is a matter easily determined by two 
 or three experiments. 
 
 The theory of pyroxyline is as follows : — Lignin is composed of 
 carbon, oxygen, and hydrogen. When acted on by nitrosulphuric 
 acid, from 3 to 5 equivalents of hydrogen are removed, and their 
 place supplied by peroxide of nitrogen, by a process called in 
 chemistry “substitution.” The only effect of the sulphuric acid 
 appears to be, to prevent the nitric acid from dissolving the pyroxy- 
 line; for sulphuric acid has a strong attraction for water, and 
 pyroxyline is soluble in dilute though not in strong nitric acid. The 
 theory expressed in symbols is CH n O + NO g = C (H n _ 1 , N0 4 ) 
 O + HO. Lignin + nitric acid = pyroxyline + water, in which 
 expression the number of atoms involved in the change is, for the 
 sake of simplicity, omitted. 
 
 Pyroxyline being a substitution compound is very unstable. Bottles 
 containing it are frequently filled with red fumes, and by its decompo- 
 sition in the form of a collodion film, oxides of nitrogen are given off 
 which destroy the picture. It should, therefore, be dissolved in ether 
 as soon as made, and collodion pictures should always be varnished. 
 
 Pyroxyline is sometimes made from Swedish filtering paper ; but 
 the plan is liable to the objection that paper may contain mixed 
 fibres of different kinds, which would be unequally acted on by the 
 acids, at the same time that the fibres in the interior of the paper 
 are not so readily acted on as those at the surface. These constitute 
 weighty objections to the use of filtering paper. 
 
 For further information on this subject see “ Collodion.” 
 
 Quick Lime. Oxide of calcium, free from carbonic acid and 
 water. It has a powerful affinity for water, and when “ slaked” by 
 the addition of water is converted into “ hydrate of lime.” The 
 heat evolved is so great as to reach 500°, and when the operation is 
 conducted in the dark light is also evolved. One part of lime is 
 soluble in 150 parts of water, and the solution is called lime-water. 
 Quick lime is used in several metallurgic processes as a cheap and 
 powerful flux ; and in many chemical operations as a means of 
 depriving other substances of their water. It is highly alkaline and 
 caustic. 
 
 Quickness of Lenses. The comparative “quickness of lenses,” 
 (as it is termed,) depends 'partly on the colour of the glass, the 
 number of glasses in the combination, the number of reflecting sur- 
 faces, &c. ; but mainly on the aperture of the lens, and its focal length. 
 
 It is evident that, other things being equal, the intensity of light in 
 the image depends first on the quantity of light admitted, and secondly 
 
EAM 
 
 EEC 
 
 347 
 
 on the area over which it is distributed. It varies, therefore, directly 
 as the aperture, and inversely as the size of the picture. 
 
 But the size of the picture given by a lens varies directly as the 
 square of its equivalent focal length ; and the area of the aperture, or 
 diaphragm, varies as the square of its diameter. Therefore the time of 
 exposure varies directly as the square of the equivalent focal length, 
 and inversely as the square of the diameter of the aperture or stop. 
 
 In the same lens the time of exposure varies inversely as the 
 square of the diaphragm used. Eor instance, with a diaphragm of 
 half an inch diameter the time of exposure must be four times as 
 great as with a diaphragm of one inch. 
 
 The equivalent focal lengths of two different lenses may be com- 
 pared by directing them towards the same objects from the same spot, 
 and comparing the distance between the same objects in each picture. 
 In this method, however, allowance is not made for distortion. 
 
 Eamseen’s Eye-Piece. This telescopic eye-piece is used as a 
 focussing magnifier to magnify the image formed on the focus sing- 
 screen of the camera. It is composed of two plano-convex lenses, 
 equal in all respects, and mounted in a tube with their plain sides 
 outwards, at a distance apart equal to two-thirds of the focal length 
 of either. When using this magnifier, the image on the ground- 
 glass should be nearly in its principal focus. It is used in telescopes 
 when spider-lines are placed in the focus of the object glass. It is 
 sometimes called the Positive Eye-piece, and is not achromatic. 
 
 Ee algae. As. S 2 . Eed sulphide of arsenic. This substance is 
 used in making “ White Indian Eire,” (y. v.) It is easily fused, and 
 sublimed. 
 
 Eeaumur’s Thermometer. In this scale 0° is taken as the freez- 
 ing point, and 80° as the boiling point of water. See “ Thermometer.” 
 
 Eecovery of Waste Silver. A great deal of silver is wasted 
 by photographers. Most of it might be saved by a little care and 
 management. Washings containing nitrate of silver should be col- 
 lected in a pan and common salt added, which throws down insoluble 
 white chloride of silver. When this has settled to the bottom, the 
 clear liquid should be drawn off by a syphon, and the chloride col- 
 lected and put into a bottle. Old nitrate baths may be treated in 
 the same way. When a tolerable quantity of chloride has been col- 
 lected in this way it may be sent to the refiner, who will give nitrate 
 of silver in exchange for it. Messrs. Johnson, of Hatton Garden, 
 are purchasers of chloride of silver. Old hypo-baths frequently 
 
348 
 
 BED 
 
 BEE 
 
 contain a large quantity of silver, both in solution as a double hypo- 
 sulphite of silver and soda, and as a black precipitate of sulphide of 
 silver. In order to obtain the silver, sulphuric acid should be added 
 to the bath ; this throws it down as a brown or black sulphide of 
 silver, which may be sent to the refiner, who will give nitrate of 
 silver in exchange for it. 
 
 The best mode of reducing either the chloride or sulphide of 
 silver, is to dry it, and mix it in a crucible with about twice its 
 weight of carbonate of soda, (common washing soda,) then place 
 it in the hottest part of the kitchen fire, and urge the fire with the 
 bellows until the silver melts and forms a button of pure silver at 
 the bottom of the crucible. No other mode of reduction is so simple 
 and trustworthy. ' But before having recourse to this process, the 
 chloride or sulphide should be well washed in many waters. 
 
 Bed Heat. 980° Fahrenheit, according to Daniell. The heat 
 of a common fire is about 1150°. 
 
 Bed Lead. Pb. 3 0 4 . This fine pigment is made by exposing 
 litharge to the action of air at a temperature of about 560°, by which 
 it absorbs oxygen and becomes converted into red lead. The 
 brilliancy of the colour is reduced by exposure to light. 
 
 Beflecting Prism. Suppose EGiH to be a glass prism having 
 equal sides EG, GIL, and the angle at G a right angle; and let 
 AB be a ray of light incident at B 
 perpendicularly to the side GH. 
 
 This ray will not suffer deviation 
 on entering the glass, but will 
 proceed in the same straight line 
 till it comes to C. What will then 
 happen to it P 
 
 Draw Cn at right angles to EH, 
 and make the angle nCo equal to 
 the “ critical angle ” of the glass. 
 
 (See “ Befraction.”) This angle 
 will lie between 39°, and 42°, ac- 
 cording to the refractive index of 
 the glass, being the least for flint 
 and the greatest for crown glass. Therefore all rays within the 
 glass incident at C and not lying within the angle nCo will suffer 
 total internal reflexion. Now the angle ^CB = 45°, and is there- 
 fore greater than nCo, consequently the ray BC is internally reflected 
 at C, and follows the rectilinear course CDE, CE being at right 
 angles to CA. 
 
REE 
 
 349 
 
 It appears therefore that the back of a glass prism may be used as 
 a reflector. 
 
 With respect to oblique rays incident at B. It is evident that all 
 rays incident at B and lying within the angle ABH will suffer inter- 
 nal reflexion, but between A and G there will be a limit, for a ray 
 PB whose direction BQ within the glass makes an angle BQH 
 greater than oCH, will not suffer total internal reflexion, but will 
 pass through the prism in the direction QR. In order for this to 
 happen, the angle PBA must be greater than about 9° when the 
 prism is made of dense flint glass. All rays therefore lying within a 
 space PH, nearly equal to 100°, suffer internal reflexion ; and 
 those lying within PG pass through the prism. 
 
 Reflexion. When a ray of light is incident on a polished surface 
 of any kind, it is turned out of its course, and suffers “reflexion.” 
 
 The law is, that the reflected ray lies in the same plane with the 
 incident ray and the normal to the reflecting surface, at the point of 
 incidence ; and that it makes with the normal an angle equal to that 
 made by the incident ray, on the opposite side of it, but on the same 
 side of the reflecting surface. 
 
 In all cases of reflexion a part only of the light is reflected, the 
 remainder being scattered or absorbed. 
 
 Reflexion at a Plane Surface. When a virtual image of a 
 luminous object is formed by reflexion at a plane surface, there is no 
 spherical aberration in the pencils, nor any distortion in the image, 
 and the image is situated in exactly the same relative position with 
 respect to the reflector, behind it, as the luminous object m front of it. 
 As these points should be clearly understood we shall demonstrate 
 them with the help of a diagram. 
 
 Let P Q, be a luminous object placed 
 before a plane reflector ED ; and let 
 PC be any one of the rays of the 
 pencil proceeding from P. This ray 
 after reflexion at C will follow a course 
 C K such that CK and PC make equal 
 angles with CD. Draw the line PA 
 perpendicular to the plane of ihe re- 
 flector, and produce it to^?, making Ap 
 =AP. Join jt?C. Then in the tri- 
 angles PAC, jpAC, which lie in the 
 same plane, joA=P A, C A is common 
 to both, and the included angles at A 
 are right angles, therefore the angle 
 p C A = P C A. But the angle E C R = 
 
350 
 
 REE 
 
 PC A, and is in the same plane with it; therefore p C A=ECR ; 
 CR is consequently in the same straight line with C p. Hence it 
 follows that the reflected ray CR, if produced backwards, passes 
 through p. But the position of the point p does not depend upon 
 the distance AC, or the angle PC A; it is therefore the same for 
 every reflected ray of the pencil from P. Therefore p is the virtual 
 image of P, and the reflected pencil is entirely free from aberration. 
 
 In the same way it may be shewn that if Q, be any other point of 
 the object, and Qq be drawn perpendicular to the reflector, BQ, being 
 equal to B q, q is the virtual image of Q,. 
 
 Hence it follows that pq the virtual, image, and PQ, the object, 
 are symmetrically situated with respect to the plane of the reflector 
 DE, on opposite sides of it. 
 
 Refraction. When a ray of light passes out of one transparent 
 medium into another of different density it is bent out of its course, 
 and suffers deviation. 
 
 The law is, that the refracted ray lies in the same plane with the 
 incident ray and the normal to the surface at the point of incidence ; 
 and that it lies on the opposite side of the normal, and makes with 
 the normal an angle of refraction the sine of which bears to the sine of 
 the angle of incidence a constant ratio, depending on the nature of 
 the two media. 
 
 When refraction takes place from vacuum into a medium, this 
 constant ratio is called the “ refractive index” of the medium, and is 
 generally denoted by the Grreek letter p . It is always greater than 
 unity. 
 
 If then, q> be the angle of incidence 
 <p ' “ refraction 
 
 the law of refraction is expressed by the equation 
 sine <p=p sine q> 
 
 which is called the “ Law of Sines.” 
 
 The sine of an angle is a decimal fraction less than unity, and may 
 be found by consulting a table of natural sines. The sine of 0° = . 
 0 ; of 90° = 1 ; of 30°= ‘5 ; and so on. See “ Sine.” 
 
 Suppose then the refractive index of a piece of glass to be 1*54, 
 and the angle of incidence of a ray upon its surface to be 37° 18' ; 
 required to find the angle of refraction. 
 
 By consulting the table we find that the sine of 37 Q 18' = *60599: — 
 Therefore *60599 = 1*54 x sine 
 which gives sine = * 3 9 3 5 
 
 Consulting the table again, we find that *39341 is the natural sine 
 of 23° 10', and *39367 the natural sine of 23^11'; therefore the 
 angle of refraction, is equal to 2 3° 10' 20". 
 
KEF 
 
 351 
 
 The following table gives the value of fi for a few different sub- 
 stances. 
 
 Chromate of lead 
 
 . 
 
 2.974 
 
 Diamond. . 
 
 
 2439 
 
 Nitrate of silver . 
 
 . 
 
 1-788- 
 
 Flint glass . 
 
 .from 1-625 to 1*58 
 
 Crown glass 
 
 . from 1‘542 to 1 - 514 
 
 Canada balsam 
 
 m 
 
 .1-55 
 
 Castor oil . 
 
 , . 
 
 1-49 
 
 Turpentine . 
 
 
 1-475 
 
 Nitric acid 
 
 
 1*41 
 
 Alcohol .... 
 
 . 
 
 1-372 
 
 Acetic acid . . . 
 
 
 1-36 
 
 Ether ..... 
 
 
 1-358 
 
 Water .... 
 
 
 1-335 
 
 Air 
 
 • 
 
 1-000276 
 
 We have now to consider the case of a 
 
 ray about to pass from a 
 
 dense medium into vacuum. 
 
 A ray of light on having its direc- 
 tion reversed returns by the same path 
 as that by which it came ; so that if 
 PNB be a vacuum, P^B a dense me- 
 dium, and pAq the bent course of a ^ 
 
 N ; 
 
 j f p 
 
 / 
 
 1/ 
 
 i «==£• D 
 
 ray proceeding in the direction of the 
 
 
 i--. - ■■■■ - 1 r 
 
 i 
 
 arrow, if this ray be reversed and bs ^ 
 
 / 
 
 
 turned back it will follow the course 
 
 / 
 
 qAp. If then the angles A N=<p, and 
 qAn^ty', the equation sine $ = ^usine <j>' 
 
 Q 
 
 n# 
 
 1 
 
 
 
 becomes sine ty 1 = — sine (j > . 
 
 If <t> r be such that (p= 90°, sine (j> r will be equal to — ; and 
 
 P 
 
 the angle whose sine is — is called the “ Critical Angle.” 
 
 P 
 
 Suppose, now, PA to be a ray whose angle of incidence differs 
 from 90° by a quantity less than any assignable quantity ; it 
 will then after refraction follow the course AQ, and QA^ will be the 
 “ critical angle.” This angle for plate glass is about 4 2°, and for 
 flint glass about 39°, therefore less than 45° in both cases. 
 
352 
 
 REF 
 
 RES 
 
 Now observe what follows : — 
 
 Any ray within the dense medium, proceeding towards A and lying 
 within the angle n Q, will emerge and take a direction somewhere 
 within the angle PN ; but no ray incident at A whose direction lies 
 within the angle QB will be able to get out of the glass ) but will 
 suffer total reflexion, as shown in the figure by the ray L AM ; the 
 angle LA^ being equal to the angle MA». 
 
 The meaning and importance of the critical angle will now be per- 
 ceived ; for the inner surface of a transparent medium may become 
 a totally reflecting surface ; and a block of transparent glass may be 
 as perfectly opaque to light as a sheet of iron. It is on this princi- 
 ple that the reflecting prism is constructed; ( q . v.). 
 
 Refractive Index. See the preceding Article. 
 
 Practical opticians generally determine the mean refractive index 
 of their glass by making it into a lens of known surfaces, and find- 
 ing by trial the focal length of the lens. 
 
 A more scientific way is to make it into a prism of small angle, 
 decompose a ray of white light through it, close to the edge, and at 
 the angle of minimum deviation ; receive the spectrum upon a 
 telescope, and measure the deviation of a ray corresponding with 
 a certain dark line of the spectrum, by means of a suitable instru- 
 ment furnished with verniers. 
 
 The different coloured rays of the spectrum have different re- 
 fractive indices, that for violet being the greatest, and for red the 
 least. 
 
 Rennet. The stomach of the calf, salted and dried. It is used 
 for curdling milk. See “ Whey,” and “ Casein.” 
 
 » 
 
 Resins. Resinous substances are an abundant vegetable pro- 
 duct, and are frequently obtained by making incisions in the bark 
 of trees. In some cases the resin, which is then called a “ balsam,” 
 occurs mixed with volatile oil, which either evaporates or becomes 
 oxidized and resinified by the action of air and light. In other 
 cases it occurs mixed with gum, and is then called a gum-resin. 
 Bitumens and fossil resins are supposed to be products of ante- 
 diluvian vegetables. 
 
 Most of the resins are soluble in anhydrous alcohol, and many of 
 them in ether, sulphide of carbon, and the oils. 
 
 The alcoholic solutions of some of the resins are acid, of others 
 neutral, and of others alkaline. Ammonia does not precipitate the 
 acid solutions, but forms with them an ammonio-alcoholic solution, 
 
EEV 
 
 EOC 
 
 353 
 
 which in general precipitates a white powder on the addition of 
 nitrate of silver. 
 
 Acetic acid, and also nitric acid, dissolve many of the resins. 
 
 Common resin, or Colophony, is the residue of the distillation of 
 turpentine. It contains water at first, and is then yellow. When 
 deprived of this by fusion it becomes black. 
 
 Some of the resins are of great use in making varnishes. See 
 “ Varnish.” 
 
 Eeverse Action of Light. Sometimes those parts of a 
 ‘negative which should be the most opaque come out perfectly 
 transparent through over-exposure. This is called the “reverse 
 action of light.” It happens most frequently when the bath is acid 
 with free nitric acid. Solarization, in all the processes, may be 
 considered as the first step towards this extreme action of light 
 upon iodide of silver, and if the exposure were sufficiently continued, 
 the complete effect of the reversion of the lights and shades would 
 probably occur in all cases. This reversion may be produced by 
 exposing a plate to diffused light for a second or two after pouring 
 on the developer. 
 
 If the latent, or invisible image upon iodide of silver, be produced 
 by actual reduction by light, (which is highly probable,) then it is 
 not difficult to understand how over-exposure may carry the reduc- 
 tion to such a stage as that the reduced material may cease to be 
 a substance for which the decomposing pyrogallo -nitrate has an 
 affinity. If, by the excessive action of light, the developable material 
 in the sensitive film be reduced nearly to the metallic state, then it 
 is easy to conceive that it could not be intensified, (for we know how 
 impossible it is to intensify the metallic precipitate of a glass posi- 
 tive) ; while, from its existing in extremely minute quantity, it might 
 be removed along with the iodide of silver in the fixing process. At 
 any rate this appears to be at present the most plausible way of ac- 
 counting for the “reverse action of light,” as it is improperly called. 
 
 Eice Water. This is sometimes added to the iodizing solution 
 in the waxed-paper negative process. It is made by boiling whole 
 rice in water for a few minutes and then straining the liquid. The 
 proportions are quite empirical, and the operation of very questionable 
 utility. Serum of milk is a much better organic substance to add 
 to the iodizing solution when it is required to obtain the effects due to 
 organic matter. 
 
 Eock Crystal; Quartz. This may be considered as pure 
 
 A A 
 
35*4 
 
 ROT 
 
 SEA 
 
 silica.' It occurs in fine large six-sided prismatic crystals, which 
 are extremely hard. The finest specimens . come from Madagascar, 
 and the Alps ; the Bristol and Cornish diamonds are also good 
 specimens. Rock crystal is used for spectacles, and sometimes for 
 lenses. It has about the same refractive index as glass, and is 
 beautifully transparent, and very cold to the tongue. 
 
 Rotten Stone. A mineral found in Derbyshire. It is reduced 
 to fine powder, and used for polishing metals, daguerreotype plates, 
 &c. When used for the latter purpose, it should be sifted upon- 
 the plate through a fine muslin strainer. 
 
 Rouge ; Colcothar ; Crocus. Red oxide of iron. Used for 
 polishing glass, metals, &c. 
 
 It is prepared thus : — 
 
 Make a boiling solution of sulphate of iron, filter it, and add to 
 it a concentrated solution of oxalic acid ; this throws down yellow a 
 oxalate of iron. Wash the precipitate, and heat it, while still moist, . 
 upon an iron plate, over a charcoal fire. At a temperature of about 
 400° the salt is decomposed, and brown-red peroxide of iron, or 
 “ rouge” formed in a very finely divided state. 
 
 Daguerreotype plates should not be polished with rouge, because 
 the iron clings to the silver and injures the tone of the picture. • 
 
 Sal-Ammoniac. Chloride of ammonium; q. v. 
 
 Saltpetre. Nitrate of potass; q. v. 
 
 Sal-Volatile. Carbonate of Ammonia, in powder, mixed with 
 ethereal animal oil : 4 parts of the former to 1 part of the latter. 
 
 Sandarach. Juniper resin. A resin much used in varnishes, 
 and the produce of the Tlmia articulata which grows in Barbary. 
 Sandarach usually occurs in small yellow drops, easily fusible, and 
 soluble in alcohol. 
 
 Saponification. Soap is a combination of a fatty acid con- 
 tained in oil with a strong alkali. The principal acids contained in 
 oils are, the stearic, margaric, and oleic ; and when existing in oils 
 they are combined with a peculiar base called “ Glycerine,” ( q . v.) 
 or oxide of glyceryle ; so that when an oil or fatty substance is 
 boiled with a strong alkali, as soda or potass, the feeble base glyce- 
 rine is displaced by the more powerful one, and the results are soap 
 and glycerine. This is the theory of saponification. 
 
 Sealing Wax. The best sealing-wax is made with shellac, or 
 dammar, the inferior sorts with common resin. 
 
SEE 
 
 355 
 
 SEA 
 
 The following are the compositions of the best kinds of coloured 
 sealing wax : — 
 
 Red Sealing-wax — - 
 
 Shellac . . . 2lbs. 
 
 Venice turpentine . . .lib. 
 
 Vermilion, or sub-chromate of lead . -|lb. 
 
 Black Sealing-wax — • 
 
 Shellac . ... 21bs. 
 
 Venice turpentine . . . lib. 
 
 Lamp black . . . |lb. 
 
 Melt the shellac ajid turpentine together with heat, and add the 
 pigment as the mixture cools. 
 
 Common black bottle wax is made thus : — 
 
 Blagk resin . . . 61b s. 
 
 Bees’ wax . . . . ^lb. 
 
 Lamp-black . . i l|lb. 
 
 Neither Venetian red nor red-lead should be substituted for lamp- 
 black, because the latter is inert, and not acted on by chemicals. 
 
 Sea-Watee,. Sea-water has been sometimes used in photography 
 instead of a solution of common salt. The composition of the water of 
 the English Channel, according to the analysis of Schweitzer, is as fol- 
 lows : — Water .... 
 
 Chloride of so r dium 
 
 potassium 
 
 magnesium 
 
 Bromide of magnesium 
 Sulphate of magnesia 
 
 • lime 
 
 Carbonate of lime 
 magnesia 
 
 964*74372 
 
 27-05948 
 
 •76552 
 
 3-66658 
 
 •02929 
 
 2*29578 
 
 1-40682 
 
 •03301 
 
 1000-00000 
 
 Sea-water contains, therefore, about 16 grains of chlorides' t(T the 
 ounce of water. When used for positive-printing it would therefore, 
 in general, require to be diluted with an equal bulk of fresh-water. 
 
 The water of the Mediterranean contains a trifle more saline 
 matter than that of the English Channel. 
 
 The clamminess and stickiness of sea-water is due to the presence 
 of the magnesian salts. Its average specific gravity is 1*027 ; and 
 the average of its saline contents 3J per cent. 
 
 Seed Lac. See “ Lac.” Seed-lac is said to be more soluble in 
 alcohol than shellac, and therefore to make clearer varnish. 
 
 a a 2 
 
356 
 
 SEL 
 
 SEP 
 
 Sel d’Or. An. O, S 2 0 2 +3 [Na. O, S 2 0 2 ]-f4 HO. 
 
 This salt is a double hyposulphite of gold and soda, containing 
 4 atoms of water of crystallization. It is made by adding 1 part of 
 perchloride of gold to 3 parts of hyposulphite of soda, each dissolved 
 in 500 parts of water. The liquid is colourless, and contains several 
 salts besides sel d’or. The solution of gold must be added to that 
 of soda, and not the solution of soda to that of gold. In the reaction 
 which takes place, 8 atoms of hyposulphite of soda, 1 atom of chloride 
 of gold, and 4 atoms of water, form 2 atoms of a sulphur salt of 
 soda, 1 atom of sel d’or, and 3 of chloride of sodium. Thus : — 
 
 8 [Na. O, S 2 0 2 ] 4- Au. C1. 8 + 4HO= 
 
 2 [Na. O, S 4 0 6 ] + 2 Sel d’or + 3Na. Cl. 
 
 The sel d’or is precipitated from the solution made as described 
 above by the addition of alcohol. It crystallizes in fine needles, 
 which are very soluble in water. 
 
 Sel d’or sometimes contains a considerable proportion of common 
 salt and hyposulphite of soda. 
 
 Neither hyposulphite of gold, nor hyposulphite of silver can be 
 isolated. They only exist in double salts. 
 
 Sel d’or is used for toning positive prints. It acts very energe- 
 tically if used before the print has been fixed in hypo. The rationale 
 of its action appears to be, that the gold of the sel d’or is substituted 
 for the silver of the print, so that oxide of silver, plus sel d’or, 
 becomes oxide of gold, plus the soluble double hyposulphite of soda 
 and silver. In this method of toning no sulphide of either metal is 
 formed, and it cannot be considered in the light of a sulphur- 
 toning process. When chloride of gold is added to hyposulphite of 
 soda in excess, the sulphur salt Na. O, S 4 0 5 , which is produced 
 would, if not removed, constitute a sulphur -toning bath. Hence 
 the advantage of using sel-d’or instead of the toning and fixing 
 bath commonly employed, and described in the article “ Albumen- 
 ized-paper Printing Process.” 
 
 Sepia. (Gr. o-r? 7ria, a cuttle-fish.) Sepia is a fine, rich, brown 
 pigment, obtained from the black liquid which is ejected by the 
 cuttle-fish, in order to darken the water when pursued. The sac 
 which contains the colour is extracted from the fish, and the juice 
 dried as quickly as possible. 
 
 The colouring matter consists of carbon in an extremely divided 
 state, along with albumen, gelatine, and phosphate of lime. The 
 brown colour is obtained by acting on it with a caustic alkali. 
 
 Sepia may probably be found a valuable pigment for the 
 printing processes in carbon and pigments, in which bichromate cf 
 potass is used as a mordant when reduced by light. 
 
SEP 
 
 SIL 
 
 357 
 
 Sepometer. (Grr err] irio to putrefy.) An instrument invented 
 by Mr. Angus Smith for determining, by means of the decoloration 
 and decomposition produced in permanganate of soda, the amount 
 of organic impurity existing in the atmosphere of towns. These 
 organic impurities in the atmosphere no doubt considerably affect 
 photographic operations, and the photographer should not be without 
 the means of testing their presence. The aqueous solution of per- 
 manganate of soda is of a purple colour, and is decolorized by agit- 
 ating it in contact with air containing ammonia, sulphuretted hydro- 
 gen, and sulphurous and phosphorous acids. It is also decomposed 
 by organic matter, being a powerful oxidizer. 
 
 Serum op Milic. Serum, or whey, is the watery liquid which 
 remains after the cream and cheese have been removed from milk. 
 It contains a sugar called sugar of milk and some soluble salts, also 
 a small quantity of uncoagulated casein. It is of great use in photo- 
 graphic printing upon plain paper for giving surface vigour to the 
 proofs without any disagreeable glazed appearance ; — and in the paper 
 negative process it seems to act better than any other substance in 
 giving density to the blacks, probably from the presence of the salts 
 of lactic acid which it contains. 
 
 The simplest and best mode of making whey for the paper processes 
 is to add lemon juice to skimmed milk in the proportion of about 2 
 spoonsful of lemon-juice to a quart of whey. Boil them together 
 and strain the liquid through a cloth, which separates it from the 
 curd. It should be of a greenish colour and slightly opalescent. 
 About 6 grains of salt may then be added for positive printing paper, 
 and about 5 grains of salt and 10 of iodide of potassium for negative 
 paper. The acidity of the whey thus prepared preserves the whites 
 of the paper beautifully. 
 
 Whey may be made by adding a piece of rennet about 3 inches 
 square to a quart of skimmed milk, and putting it in a bason on a 
 hob so as to raise the temperature to about 120°. In half an hour 
 • or so the curd is formed. This is called sweet whey, from its not 
 being so acid as the other. It contains rather more casein. 
 
 Shellac. See “ Lac.” 
 
 Silica. Si. 0 2 . Oxide of Silicum. Silicic acid. This is a 
 substance existing abundantly in nature, and occurring in nearly a 
 pure form as flint, and rock crystal. When perfectly pure, and free 
 from water, silica exists as a white insipid powder, insoluble in wat el- 
 and most solvents, and nearly infusible ; but when existing in the 
 form of a hydrate its properties are very different ; it is then soluble 
 to some extent in water and in acids, and at a high temperature is 
 
358 
 
 SIL 
 
 itself capable of acting as a powerful acid which decomposes even 
 some of the sulphates. By evaporating an aqueous solution of silica 
 gelatinous silica is precipitated ; this on being dried becomes again 
 quite insoluble. 
 
 Silica, and the silicates, enter largely into the composition of glass. 
 
 Silver. Ag. = 108. The Luna, or Diana, of the alchemists. 
 
 This valuable metal occurs native and also in a variety of combi- 
 nations, but principally as sulphide. It' is chiefly found among 
 primitive rocks. The richest mines are in Peru and Mexico, and the 
 richest in Europe are those of Saxony, Bohemia, Swabia, and that 
 of Konigsberg in Norway. 
 
 Silver at high temperatures resists the action of air for a long time, 
 and does not oxidize ; but it is easily tarnished by sulphuretted hydro- 
 gen, particularly when it is alloyed with copper. It is sometimes 
 found crystallized in cubes and octohedra, and is the whitest of all 
 known metals. It fuses at a bright red heat. 
 
 Silver, Oxide of. There are three oxides of silver, viz. 
 
 Suboxide 
 
 Protoxide 
 
 Binoxide 
 
 Of these the protoxide only forms salts. 
 
 Suboxide of silver is produced as a grey film when ammonio- 
 nitrate of silver is exposed to air. It is also produced when citrate 
 of silver, (or any analogous organic salt of silver,) is exposed to the 
 action of hydrogen at a temperature of 212°. The protoxide of 
 silver contained in the citrate then loses one half of its oxygen, and 
 the suboxide remains combined with one half of the acid. At least 
 this is Wohler’s theory ; but the probability is that the acid is also 
 decomposed, and the compound not exactly a subcitrate of silver. 
 
 Dr. Graham says, “ The solution in water of the suboxide salt is 
 dark brown, and the suboxide is precipitated black from it by potass ; 
 when the solution of the subsalt is heated it becomes colourless, and 
 metallic silver appears in it. The salt dissolves of a brown colour in 
 ammonia. Several other salts of silver, containing organic acids, 
 comport themselves in the same way as the citrate when heated in 
 hydrogen.” 
 
 Protoxide of silver may be obtained by adding a dilute solution of 
 caustic potass to a solution of nitrate of silver. It is thrown down 
 as a dark olive coloured precipitate. 
 
 Protoxide of silver is, like oxide of lead, soluble to a slight extent 
 in pure water, to which it communicates an alkaline reaction. The 
 solution is reddened by exposure to light. This oxide is reduced to 
 pure metal at a dull red heat, and to black suboxide by the action 
 
SIL 
 
 SIN 
 
 359 
 
 of light. It stains glass of a yellow colour, and is used in painting on 
 glass and enamel. Ammonia dissolves it completely, and forms a 
 colourless solution called ammoniacal oxide of silver. 
 
 The peroxide of silver is a curiosity of no value in photography. 
 
 Silver-Meter. This is an instrument for testing accurately the 
 quantity of nitrate of silver in an old nitrate bath containing impur- 
 ities which affect the S. G. of the liquid, and therefore render the 
 hydrometer useless as a means of testing its strength in silver. 
 
 The principle on which this meter acts consists in ascertaining how 
 much of a solution of pure chloride of sodium of definite strength is 
 necessary to precipitate the whole of the silver contained in a definite 
 quantity of the nitrate bath. The following is the mode of pro- 
 ceeding : — 
 
 Make a solution of pure chloride of sodium by adding pure hydro- 
 chloric acid to a solution of carbonate of soda until effervescence ceases, 
 and the acid is a little in excess. Evaporate the solution to dryness, 
 and fuse the pure chloride of sodium in a porcelain capsule over a 
 spirit lamp. Then make a solution of it in distilled water of the 
 strength of 17 grains of the salt to 12 fluid ounces of water. This 
 is the standard test solution ; which must be made very accurately. A 
 fluid drachm of it will exactly precipitate half a grain of nitrate of silver. 
 
 In order to test the strength of the bath, put a small measured 
 quantity, say a drachm, into a clean stoppered bottle, and add to it 
 a little distilled water. Then add the test solution from a graduated 
 measure, a little at a time, shaking after each addition and allowing 
 the white chloride of silver to settle each time, until the salt ceases 
 to produce any more cloudiness in the clear liquid. The quantity 
 of the test solution employed, estimated at the rate of one drachm 
 to half a grain of nitrate of silver, will then determine the exact quan- 
 tity of nitrate of silver contained in a fluid drachm of the nitrate bath. 
 
 Mr. G. Wood, of 117, Cheapside, manufactures a neat form of 
 apparatus on the above principle, by which the operator is saved all 
 the trouble of calculation. We have one of these instruments, and 
 find it of great use, and very accurate. 
 
 Sine of an Angle. The size of an angle does not depend 
 upon the length of the lines which contain it, but upon their inclina- 
 tion to one another. This may be measured in a variety of ways. 
 One mode is to find what is called the Sine of the Angle, which is 
 done as follows : — 
 
 In either of the straight lines which contain the angle take any 
 point P, and from P draw a perpendicular PN upon the opposite 
 side. Then, if we call the angular point A, the ratio of the perpen- 
 
360 
 
 SIN 
 
 SOL 
 
 dicular PN to the hypothenuse AP, of the right angled triangle 
 APN, is the sine of the angle at A. 
 
 It matters not where P is taken upon the line AP, for take any 
 other point P' and draw a perpendicular P'N' upon the opposite 
 side; then the triangles APN, A P'N' are similar, and the ratio 
 PN : AP is equal to the ratio P'N' : AP'. 
 
 Since the perpendicular is less than the hypothenuse (except A be 
 a right angle, in which case they are equal), the sine of an angle is 
 a decimal fraction less than unity. It may be found by consulting 
 a table of natural sines. 
 
 In old treatises on Trigonometry the sine is generally defined by 
 reference to a circle of which the radius is unity, but that plan is now 
 exploded. In the modern system of mathematics the trigonome- 
 trical ratios have nothing to do with a circle. The sine of an angle 
 is not a line, but a ratio, or number , and has no linear dimension. 
 If, for instance, a man wants half an orange he may say “ Give me 
 sine 30° of an orange,” the sine of 30° being one half, or *5. 
 
 Sines, Law of. The “law of sines” in Optics connects the 
 angle of refraction with that of incidence, in the case of a ray of 
 light which suffers refraction. See “ Refraction.” 
 
 Size. See “ Gelatine.” 
 
 Sizing of Paper. See “ Paper Making.” 
 
 Soap. See “ Saponification.” 
 
 Soda. Na.O. Oxide of Sodium. 
 
 Sodium, the metal of which soda is the protoxide, has a rather 
 less affinity for oxygen than Potassium, but if laid upon a piece of 
 ice it immediately takes fire and burns brilliantly. 
 
 Soda, which is the oxide of the above singular metal, may exist 
 either in a pure form, or as a hydrate. 
 
 Caustic soda is the hydrate of soda, and its formula is Na. O, 
 HO. It resembles in most of its properties caustic potass ; and is 
 white, opaque, brittle, and deliquescent ; also very soluble in water 
 and alcohol. Its affinity for acids is rather less than that of potass. 
 
 Caustic soda is made from the common carbonate of soda used in 
 washing, by driving off the carbonic acid by heat in contact with 
 quick lime. The common carbonate of soda was formerly obtained 
 from kelp, but is now manufactured almost exclusively from common 
 salt mixed with sulphuric acid. 
 
 Solarization. See “ Exposure.” 
 
SPE 
 
 361 
 
 Specific Gravity. The specific gravity of any substance is 
 the weight of a unit of volume of that substance, at a temperature 
 of 60° Eaht. 
 
 The unit of volume, in the common table of specific gravities, is 
 the volume of that quantity of pure distilled water which at 60° 
 Eaht. weighs 1000 grains ; and in the same table the unit of specific 
 gravity is 1000 grains. 
 
 The specific gravity of water is therefore 1. 
 
 If a vessel containing the unit of volume be filled with absolute 
 alcohol, its increase in weight will be 794 grains, therefore the S. G. 
 of absolute alcohol is *794. 
 
 If the same vessel be filled with pure concentrated sulphuric acid, 
 its increase in weight will be 1842 grains, therefore the S. G. of 
 sulphuric acid is 1*842 ; and so on. 
 
 Specific Gravity Bottle. This is a glass bottle shaped like 
 a decanter, and furnished with a stopper which is drilled with a 
 hole, also with a counterpoise, or brass box filled with shot, which 
 exactly balances it when empty. The specific gravity bottle holds 
 exactly 1000 grains of pure distilled water at 60° Eaht. Its capa- 
 city is therefore the unit of volume of the specific gravity table. 
 
 To use it, fill it quite full with the fluid to be tested, and put in 
 the stopper. The superfluous fluid overflows through the hole in 
 the stopper. Wipe the bottle quite dry, and weigh it, together 
 with its contents, in a delicate balance, the counterpoise of the 
 empty bottle being placed in the opposite scale. The number of 
 grains required to be added to the scale which contains the counter- 
 poise, and which consequently represent the weight of the fluid, 
 divided by 1000, is its specific gravity. 
 
 Specific Heat. By the specific heat of a body is meant the 
 time it takes to cool from a certain given temperature to another 
 given temperature, when placed in vacuo in a polished silver vessel. 
 By some writers the specific heat of a body is supposed to be its 
 “ capacity for heat,” as if heat, which is the undulation of an ethereal 
 medium, could be stowed away among the particles of a body. 
 Surely the notion of “ capacity for heat” is absurd. That different 
 bodies should require different times to pass from one temperature 
 to another involves no difficulty of comprehension, and specific heat 
 is simply the measure of the time required. 
 
 Dr. Graham says, “ Of all liquid or solid bodies water has much 
 the greatest capacity for heat ; hence the sea, which covers so large 
 a proportion of the globe, is a great magazine of heat , and has a 
 beneficial influence in equalizing atmospheric temperature. Mercury 
 
362 
 
 SPE 
 
 lias a small specific heat , so that it is quickly heated or cooled ; 
 another property which recommends it as a liquid for the ther- 
 mometer, imparting as it does great sensibility to the instrument.” 
 
 The reader may amuse himself by trying to state in a different 
 form, and on the assumption of the undulatory theory of heat, the 
 facts implied in the above sentence: 
 
 The time which a body takes either to become colder or hotter 
 depends in great measure on the condition of its surface. If the 
 surface be smooth, polished, and white, the time is increased; if 
 rough and black, the time is diminished. When an ethereal undula- 
 tion strikes the surface of a body, it depends upon the condition of 
 the surface whether the undulation be continued among the particles 
 of ether within the body, or whether it be reflected among those 
 which are external to the body ; and conversely, when a body is 
 contracting, and therefore radiating heat, it depends upon the con- 
 dition of its surface, whether the undulations of the ether within 
 the body be communicated to the ether without it, or returned by 
 internal reflexion among the ether within it. A smooth polished 
 surface is favourable to the reflexion of undulations, and unfavourable 
 to their direct propagation. Colour is no doubt owing to a pecu- 
 liarity of surface. See “ Latent Heat.” 
 
 Spectrum. Suppose a small hole of any shape made in the 
 window shutter of a darkened room, and that sunshine be ad- 
 mitted through it, and the light received upon a white screen, 
 placed perpendicularly to the line joining it and the hole ; then, a 
 round image of the sun will be formed upon the screen, the diameter 
 of the image depending upon the distance of the screen from the 
 hole. Observe that whatever may be the shape of the hole, the 
 image of the sun will be round. A triangular hole would not give 
 a triangular image, nor a square hole a square image ; the sun being 
 round, its image is round ; for the image is not produced by a ray 
 of sunshine which enters through the hole and falls upon the screen, 
 producing a spot of light the same shape and size as the hole, but 
 by pencils of light which diverge from every part of the sun, and 
 after crossing each other in passing through the hole, proceed till 
 they reach the screen, where they form a round image of the sun, 
 the size of which increases as the distance of the screen from the 
 hole increases. Thus, the sun being about half a degree in angular 
 diameter, if the screen be placed 10 feet from the hole, the sun’s 
 image will be about 1 inch in diameter; if 20 feet from the hole, 
 2 inches in diameter, and so on. 
 
 This being understood, let a glass prism be placed with its edge 
 immediately behind the hole. Then, since white light is not homo- 
 
SPE 
 
 363 
 
 geneous, the pencils will all be decomposed by refraction through 
 the prism into pencils of the different coloured lights of which 
 white light is composed ; so that the screen, instead of receiving a 
 single round image of the sun in white light, will receive upon a 
 different part of it as many different coloured round images of the 
 sun as there are different kinds of light in white light separable by 
 refraction. These images will partly overlap one upon the other, 
 and produce a long image of the sun, having belts across it of differ- 
 ent colours, arranged in the following order, (if the refracting angle 
 of the prism be suitably taken,) viz., red, orange, yellow, green, 
 blue, indigo, violet; which colours are called the “prismatic 
 colours,” and the entire coloured image the “ prismatic spectrum.” 
 Should, however, the refracting angle of the prism be too small, 
 there will be a space of white light in the centre of the spectrum, 
 produced by the coincidence of a portion of each of the coloured 
 images. 
 
 The reader will perhaps find this account of the way in which 
 the prismatic spectrum is generally produced somewhat different 
 from the accounts given of it in popular treatises on Optics ; these 
 popular explanations generally proceeding on the assumption that 
 the light which is admitted through the hole is a single beam of 
 light, — which is manifestly incorrect. Every photographer knows 
 that an image is formed of external objects by light admitted 
 through a small hole in the front of a dark box, and received upon 
 a focussing screen ; and that this is true, however small the hole 
 may be and whatever its shape. We have, therefore, to deal with an 
 image of the sun, and not with a single ray of white light. 
 
 It appears, then, that when the spectrum is formed by admitting 
 the light through a hole, however small, the bands of different 
 colours contain an admixture of lights of different refrangibilities. 
 In order to obviate this evil, M. Eraunhofer admitted the light 
 through a long and extremely fine slit, instead of a hole, and placed 
 the prism with its edge parallel to the slit, and at a considerable 
 distance from it. But even this arrangement is not sufficiently 
 exact, for the additional precaution must be taken of covering the 
 prism with an opaque diaphragm, having an extremely fine slit 
 parallel to the edge of the prism and therefore parallel to the other 
 slit, so that the light from the first slit may also pass through the 
 second slit. In this way the spectrum is rendered nearly pure, and 
 the different parts of it free from the admixture of other colours. 
 And here it is important to observe that in the experiments of Sir 
 David Brewster, in which he detected white light in every part of 
 the prism in a state of admixture with his supposed simple colours, 
 red, blue, and yellow, allowance had not been made by him for the 
 
364 
 
 SPE 
 
 impurity of his spectrum, so that his conclusions that Newton’s 
 theory was wrong and that the seven colours of the spectrum may 
 be reduced to three was founded on experiments conducted in 
 ignorance apparently of an elementary principle in geometrical 
 optics. 
 
 When a pure spectrum is examined by a telescope, it is discovered 
 to be intersected by a great number of dark lines, as shown in the 
 following figure, and which are called 
 “ Fraunhofer’s Lines,’’ he having first dis- 
 covered them. There are nearly 400 of 
 these lines. 
 
 These lines are produced by the inter- 
 ference of undulations of light. In the 
 spectra produced by light from different 
 sources the lines do not occur in the same 
 order ; neither do they occur at the same 
 relative distances when light from the 
 same source is refracted through prisms 
 made of different materials. In the light 
 from the sun and planets the spectral lines 
 occur in the same order ; but in the spectra 
 formed by light from the fixed stars, or by 
 the electric light, or by light produced by 
 the combustion of different substances, the 
 spectral lines do not occur in the same 
 order, nor are they equally numerous. 
 
 With respect to the calorific, luminous, 
 and actinic properties of different parts of 
 the solar spectrum ; — It has been shown 
 in the article on light, that light, heat, and 
 actinism are most probably undulations in 
 the same ethereal medium, differing only 
 in the length of the wave ; and it is found 
 by experiment, that both heat, light, and 
 actinism, are capable of producing chemical 
 changes in bodies ; so that the term “ ac- 
 tinism” merely means that certain sub- 
 stances are chemically affected by certain 
 rays (called the actinic,) residing at a cer- 
 tain part of the spectrum. 
 
 It is therefore somewhat unscientific to 
 call certain rays “ actinic” because they 
 produce chemical changes in certain bodies, 
 and then to say generally that the chemical rays chiefly lie at the violet 
 
 Bed. 
 
 Orange. 
 
 Yellow. 
 
 Green. 
 
 Blue. 
 
 Indigo. 
 
 Violet. 
 
SPE 
 
 365 
 
 part of the spectrum, when we know that every part of the spectrum is 
 capable of producing important chemical changes in some substance 
 or other. To say that the heat rays reside mostly at the red end of 
 the spectrum, luminous rays mostly in the yellow part, and chemical 
 rays mostly in the violet part’, is so far unscientific that it is not the 
 statement of a universal law ; for the effects exhibited by the 
 different parts of the spectrum upon a substance placed in it, depend 
 upon the nature of that substance, and are different with different 
 substances. This being the case, we have not included in the fore- 
 going figure of the spectrum the three wave lines of light, heat, and 
 actinism, with which it is generally adorned in popular works on 
 photography. 
 
 In the following table the lengths of the waves corresponding to 
 the principal lines of the solar spectrum are expressed in millimetres. 
 
 Extreme red 
 
 
 . * 0 0 0 7 5 millimetres . 
 
 A . 
 
 , 4 
 
 . *00074 
 
 B 
 
 
 . *0006879 
 
 C 
 
 
 . *0006559 
 
 D 
 
 , 
 
 . *0005888 
 
 E . 
 
 . , 
 
 , . *0005265 
 
 E 
 
 , 
 
 . *0004856 
 
 Gr . 
 
 . , 
 
 . *0004296 
 
 H 
 
 
 . *0003968 
 
 I 
 
 . , 
 
 . . *00037 
 
 Extreme violet . 
 
 * 
 
 . *00036 
 
 Speculum Metal. This is generally a compound of about 
 6 parts copper, 2 parts tin, 1 part arsenic. It is used for metal 
 reflectors. The great reflector of Lord Rosse’s telescope is made of 
 126*4 parts copper and 58*9 parts tin, without the addition of any 
 arsenic. 
 
 The word “ Brass” was omitted in the letter B. We may 
 observe in this place, that brass is a compound of copper and zinc, 
 with the addition sometimes of a little lead, tin, and iron. The 
 latter metal should not be introduced in the brass used for philo- 
 sophical apparatus. The proportions of the metals in brass for turn- 
 ing are as follow : — 
 
 Copper 
 
 . 61*6 
 
 Zinc 
 
 . 35*3 
 
 Lead . 
 
 . 2*9 
 
 Tin 
 
 . 0*2 
 
 Spermaceti. This substance is sometimes used instead of wax 
 
366 
 
 STA 
 
 in photography. It is a soft white crystalline substance, precipitated 
 from the oil of the spermaceti whale on cooling after the death of 
 the animal. It is purified by pressure and boiling in a weak solu- 
 tion of caustic alkali, after which it is washed, melted in boiling 
 water, and cast into blocks or cakes. It is soluble in about 50 
 parts of alcohol S.G. *820. Pure spermaceti is certainly better than 
 impure wax in photography ; and, from its being softer and more 
 easily melted, it penetrates the pores of paper more readily. 
 
 Standard Gold. A sovereign weighs 5 dwts. 3*27 grains, and 
 is composed of 11 parts gold to 1 part copper. The standard gold 
 of Prance contains 9 parts gold to 1 part copper. 
 
 Standard Silver. Standard silver consists of IT 10 parts of 
 silver and *90 parts of copper. A shilling weighs 3 dwts. 15*27 
 grains. 
 
 Standard Spirit. A mixture of alcohol and water, having the 
 S.G. ‘92 at 62° Pahrenheit. It contains very nearly equal parts of 
 absolute alcohol and water. 
 
 Starch. C 12 H 10 O 10 =12 atoms of carbon and 10 of water. 
 This substance occurs abundantly in vegetables, and is generally 
 obtained by steeping the powdered grain or seed, or the raspings of 
 the root, bulb, or stem in cold water, which becomes white and 
 turbid, and, after being strained, deposits the starch in the form of 
 a white granular substance which is then dried at a gentle heat. 
 
 Common starch is manufactured from flour. Arrowroot, tapioca, 
 and sago are different forms of starch. Starch is frequently made 
 from potatoes. 
 
 When starch is heated to a certain temperature, it becomes con- 
 verted into a gum called “ Dextrine ” (q. v.). It forms a blue com- 
 pound with iodine, called iodide of starch, an aqueous solution of 
 which is bleached by light : it combines also with sulphuric acid, 
 forming sulphate of starch, and with lime and baryta; also with 
 tannin. Starch becomes converted into sugar by the action of an 
 azotized principle, called “Diastase,” and also by the action of 
 dilute acids. 
 
 Starch is insoluble in cold water, alcohol, and ether. When 
 boiling water is poured upon it, clots are formed which cannot 
 afterwards be diffused through water. Solution of starch is best 
 made by pounding the starch, and mixing it thoroughly with cold 
 water ; then adding hot water, or boiling it, stirring it uniformly 
 
STE 
 
 367 
 
 until a gelatinous mixture is obtained. A solution of starch is 
 supposed to consist of the granules considerably distended and dif- 
 fused through the water. When, however, the indurated envelope 
 of the starch granule bursts, the contents are distributed through 
 the water, and form a transparent gelatinous liquid which, on cool- 
 ing, throws down an opalescent deposit. The substance held in 
 solution in the clear liquid has been called “ Amidine.” 
 
 Stearine. (Grr. ortap suet.) The principal constituent of solid 
 fats. It is composed of stearic acid in combination with glycerine. 
 
 Stereoscope. (Gt. (yrepeog solid, (jkotew I see.) This is an 
 instrument for exhibiting two plane perspective views of an object, 
 taken from different stations, as one having the appearance of 
 solidity. It is the invention of Professor Wheatstone, and was first 
 made public and the theory of it explained by him in the year 1839. 
 At that time the pictures were exhibited by means of reflectors, but 
 the inventor suggested that lenses might be used instead. Some years 
 after this, an instrument was brought out by Sir David Brewster, 
 in which small photographic pictures are placed in a dark box, and 
 viewed through half-lenses mounted in tubes ; and, soon after the 
 introduction of that very imperfect instrument, Messrs. Knight, of 
 Poster Lane, patented an improved form of stereoscope, in which 
 large semi-lenses are used, mounted in the front of the instrument, 
 without tubes ; and this, in the course of time, became so generally 
 preferred to the other, as to supersede it to a great extent. But it 
 was soon found that the semi- lenses, whether large or small, pro- 
 duced an unbearable amount of distortion in the solid picture, 
 making straight lines look as if they were concave to the spectator ; 
 so, in order to obviate this evil, whole lenses were tried, and these 
 were found to answer in certain cases. But as the theory of the 
 lenticular stereoscope is even now but little understood by opti- 
 cians or the public at large, it was not perceived that, in order to 
 render that form of instrument perfect, it was necessary not only to 
 use whole lenses to get rid of the distortion, but to take the 
 pictures in a camera suitably constructed, and mount them pro- 
 perly. The popular form of the stereoscope as manufactured by 
 Mr. Knight, and called by him the Cosmorama Stereoscope, is 
 however, notwithstanding its defects of principle, a very amusing 
 instrument, and it is not till the spectator has become familiar with 
 the true principles of stereoscopic representation that the defects are 
 perceived. We shall endeavour, then, in the present article to 
 explain the theory of the stereoscope in a clear and intelligible 
 
368 
 
 STE 
 
 manner. But the reader must first consult and study the article on 
 “Binocular Vision,” for it is here assumed that the principles 
 of binocular vision are clearly understood. 
 
 Theory of the Stereoscope. 
 
 The theory of the stereoscope consists in showing — first, how 
 the instrument may be employed to represent things truthfully as 
 we see them in nature ; secondly, how it may be employed to repre- 
 sent them falsely as we should see them if our eyes were wider 
 apart ; and, thirdly, to explain how it happens that distortion is 
 introduced by using an instrument of improper construction — 
 straight lines being represented by curves, and so on. 
 
 Let L, B be the eyes of a spectator; A, C, B lamp posts of 
 different heights having lights, or luminous points, A, B, C, at the 
 top. Draw the visual rays L A, L C, L B, B A, B C, B B. Cut 
 them by a vertical plane P Q, parallel to the line which joins L and 
 B. Then the points a,c,b , a,c,b, where the visual rays pass through 
 this plane will be the images of A, B, C, as seen from the stations 
 L and B, P Q being supposed to be the plane of a perspective 
 picture. (See “ Perspective.”) If the plane P Q be placed as 
 shown in the figure, so that the perspective view as seen from L may 
 be completely exterior to that seen from B, the pictures will be as 
 represented on a card beneath L and B. (See the Lower Part of 
 the Figure.) 
 
 Now if the perpendicular distance between L and the plane P Q, 
 be such that the points a , b, c , can be distinctly seen by an eye at L, 
 — that is to say if this perpendicular distance be not too short for 
 distinct vision, and if we place the card P Q before the eyes, as in the 
 figure, the right picture being cut off from the left eye and the left 
 picture from the right eye by a partition, as shewn in the figure, then 
 instead of seeing two pictures abc , abc , only one image will be seen, 
 and that one image will appear to be the lights A,B,C in their na- 
 tural position, and at their true distance. For when the left eye is 
 directed to a along the line L a, the right eye is directed to a along 
 the line B a, and the optic axes L a 3 B a, being produced meet at A, 
 so that the spectator instead of seeing two images a 3 a } upon a plane 
 P Q, sees one image A at the true distance and in the true position 
 of the light A. Similarly with respect to the other images b ) c i b ) c i 
 which combine and produce single images at B and C. 
 
 It is evident that the perpendicular distance of P Q, from L or B, 
 (which we will call F,) will depend upon the size of the angle ALB, 
 or ABB ; for the smaller that angle is the further the plane PQ may 
 be placed from L and B, and therefore the more easy it will be- 
 
STE 
 
 369 
 
 B B 
 
370 
 
 STE 
 
 come for the images a,b,c, to be seen distinctly by persons of ordinary 
 vision. Most persons can see a thing distinctly at a distance of 8 
 inches. If then E = 8, the angle ALB may be about 16 Q . 
 
 Now we come to the principles of the stereoscope. 
 
 Suppose we place at each of the stations L,R, a photographic 
 camera, the lens of which is 8 inches focus, and adjusting these ca- 
 meras with their axes strictly parallel, take the pictures of A,B,C, 
 (including an angle of 16°). Then when these pictures are properly 
 mounted they will be identical with the perspective views of A,B, 
 C, upon the plane P Q, ; and if placed in a box having a partition 
 in the middle and holes to look through at P and Q, at a dis- 
 tance of 8 inches from the pictures , the spectator on looking through 
 the holes will perceive a single image of the points A,B,C, at their 
 true distance and in their true relative positions. 
 
 A box so constructed may be called a “ Simple Stereoscope,” 
 because it does not involve the use either of lenses or reflectors, and 
 the pictures properly taken and viewed in it have the inexpressible 
 charm of truthfulness. 
 
 Of what use then, it may be asked, are lenses and reflectors ? To 
 this question we will now endeavour to reply. 
 
 In the first place the simple stereoscope, when adapted to persons 
 of ordinary sight, does not include an angular field of more than 16° ; 
 which is in general too small. A photographic picture should include 
 at least an angular field of from 25° to 30°. This can only be effec- 
 ted by bringing the pictures as near to the eyes as 5 inches, at which 
 distance most persons find a difficulty in seeing them distinctly ; but 
 this difficulty is overcome by placing a whole lens of 5 inches focus 
 in each of the holes L,B, and viewing the pictures, (which must be 
 taken with a lens of 5 inches focus,) through them. This instrument is 
 called the “ Lenticular Stereoscope.” In the following description 
 of it we shall recapitulate what has been said, and discuss the matter 
 more fully. 
 
 The Lenticular Stereoscope. 
 
 This is an instrument for exhibiting a pair of stereoscopic pictures, 
 which include an angular field of about 26°, and have been taken in 
 a stereoscopic camera provided with lenses of 5 inches equivalent focal 
 length, having their axes parallel, and mounted at a distance L ft 
 (=2J inches) from centre to centre. [The reader will observe that 
 the stereoscopic camera described at page 67 of this work is calcu- 
 lated to exaggerate the relief of objects, because the stations are 
 taken at 5 inches apart ; in all other respects however the principle 
 of it is correct.] 
 
STE 
 
 371 
 
 The best construction of the Lenticular Stereoscope for exhibiting 
 paper pictures, (kata-positives,) is shewn in the following figure, 
 which scarcely needs explanation. 
 
 <^1 
 
 
 The whole lenses fixed in the front of the box are placed 2^ inches 
 from centre to centre, and should not be less than 1 inch in diameter. 
 They should be of 5 inches focus, and achromatized meniscus lenses 
 with the hollow side outwards, in fact the same lenses as those used 
 for taking the pictures, which may be unscrewed from the camera, 
 and screwed to the stereoscope. The back of the solid box should 
 have holes in it as represented by the dotted lines, and also a parti- 
 tion in the middle. The holes should be 2\ inches from centre to 
 centre, and their diameter determined by trial. If the front box be 
 2\ inches deep, the back diaphragms should be a trifle more than 1 
 inch in diameter. A set of diaphragms of different sizes to fit in the 
 back of the front box would be a useful addition to the instrument. 
 The inside of the front box should be properly blackened. The 
 pictures may be circular, mounted 2\ inches from centre to centre 
 and about 2\ inches in diameter. By the centre of each picture is 
 meant the point where the axis of the lens cuts it. In mounting 
 the pictures this is the point to be considered, and no reference what- 
 ever is to be made to any of the objects in the pictures. These are 
 always nearer together than 2\ inches, when mounted. The moun- 
 ted pictures are fastened by springs to a slider which moves in a slit 
 in the bottom of the stereoscope, so that the distance of the pictures 
 from the lenses may be varied to suit different sights, or for trying 
 
 B B 2 
 
372 
 
 STE 
 
 experiments ; the proper distance, however, being 5 inches, which is 
 the focal length of the lenses. The pictures should be mounted upon 
 black cardboard. In order to make the objects appear more distant 
 than the cardboard, that is, in order to make them appear as if viewed 
 through a circular hole nearer to the spectator, the following rule in 
 mounting them should be attended to. Just before trimming the 
 edges, mount them temporarily 2 \ inches from centre to centre, upon 
 a piece of cardboard, with pins at the corners ; then measure the 
 distance between the nearest object in each picture, (this will be less 
 than 2-§- inches, probably not much more than 2-g- inches,) and take 
 a dimension a little less than this for the diameter of the pictures. 
 With half this diameter as radius, and the centre of the picture as 
 centre of the circle, strike a circle on each picture, which will be the 
 margin required. 
 
 In conclusion, we would observe that the object of taking the 
 lenses as large as an inch in diameter is not that the margins of the 
 lenses may be looked through, but simply because when they are too 
 small their circumferences are seen by the eyes and form two inter- 
 secting circles upon the solid picture. Were it not for this circum- 
 stance the lenses need not exceed half an inch in diameter. 
 
 The stereoscope for exhibiting transparent pictures, (dia-positives,) 
 is of the same general form as Knight’s Cosmorama stereoscope, the 
 the other points being the same as that of the Stereoscope described 
 above. 
 
 It now only remains to explain the exact effect of the lenses 
 upon the pictures. Turning to the figure at the commencement 
 of this article ; if a lens of focus E were placed at L, and the eye 
 pressed close to it, a pencil diverging from b would after refraction 
 through the centre of the lens be converted into a pencil of parallel 
 rays, without suffering deviation, so that the eye would see the point 
 b in the same direction as if no lens were interposed, that is along 
 the line L5B, but would have less difficulty in bringing to a 
 focus upon the retina a parallel pencil than one diverging from a 
 point so near as b. When therefore the eye is placed so close to the 
 lens as to see things exactly through its centre there is no magnifi- 
 cation. If however the eye be placed at a little distance from the 
 lens it does not look at the side objects exactly through the centre, 
 and therefore the lateral pencils suffer a slight deviation, which in- 
 creases the apparent angle ALB, and produces magnification, which 
 may however be counteracted by putting the pictures a little further 
 from the lens than its focal length. 
 
 If then we leave out of consideration the unavoidable defects of 
 all lenticular optical instruments, it appears that the form of stereo- 
 
STE 
 
 373 
 
 scope that has been described exhibits pictures, when properly taken , 
 in such a way as to represent objects precisely as they would appear 
 to unassisted vision, so that natural truth is perfectly realized. Arid 
 the reader will particularly observe that in this form of stereoscope 
 the images are not displaced or caused to overlap by any optical 
 contrivance. How then, it may be asked, is the effect produced ? 
 
 Let us examine carefully the pictures upon the card P Q. 
 
 In the first place the points a, a, — bjo—c.c, are upon the same 
 horizontal lines, which is simply because the cameras had their axes 
 parallel, and not converging to a point ; {See remarks on page 65). 
 
 In the next place, if we measure the distances a a, bb , cc, we find 
 a a the greatest because A is the most distant object ; bb the next 
 because B is the next object in point of distance ; and cc the least 
 because C is the nearest object. But all these distances are less than 
 LB. If, however, in addition to the three lamps, a fixed star D were 
 introduced in any part of the picture, the visual rays LD, ED would 
 be parallel, and the points d , d , where they cut the pictures would be 
 at the same distance apart as L and B. If then we join L,B, with 
 points c,c, nearer together than L and B, (and in the same plane 
 with them) the lines Lc Be meet at a finite distance C ; but if we 
 join L,B with points d,d, at the same distance apart as L and B the 
 lines Ld, B d are parallel, or only meet at an infinite distance. Hence 
 it follows that in mounted stereoscopic pictures the furthest objects 
 are the widest apart, and the nearest objects the nearest together ; 
 objects at an infinite distance only being as far apart as the distance 
 between the eyes. These considerations will explain at once why it 
 is that there is no necessity for displacing the images of the pictures 
 by half lenses ; the objects being sufficiently displaced by the pers- 
 pective in the pictures themselves, and any further displacement being 
 wrong in principle. The Brewsterian stereoscope has therefore 
 been a step in the wrong direction, and the error has to be pointed 
 out to the public, and the work begun over again. 
 
 We have now done with the Lenticular stereoscope. Its defects 
 are those which are inseparable from all optical instruments in which 
 lenses are used, and the pictures are so small that it is impossible to 
 include in them the same amount of detail as in larger pictures. But 
 at the same time for many purposes the smallness of the pictures, 
 involving but a small expenditure of material and permitting the 
 use of a light and portable form of apparatus in which both pictures 
 may be taken simultaneously, is a real advantage. 
 
 We now proceed to an account of the reflecting stereoscope. 
 
374 
 
 STE 
 
 The Reflecting Stereoscope. 
 
 Beferring to the figure at the commencement of this article. If the 
 visual rays are cut by a plane pq instead of PQ, the pictures will be 
 larger than before, and instead of being entirely exterior one to the 
 other, will overlap, and be mixed together so to speak. But if we 
 take the pictures in cameras placed at L and B, with lenses of focal 
 length L m or B^, having their axes parallel, and then by means of 
 reflectors throw the virtual images of the pictures so taken into their 
 proper positions on the plane pq , and view these images by eyes at 
 L and B, a truthful solid image will be produced, as in the former 
 case ; because the left eye will not then see the picture from the 
 right station, nor the right eye that from the left station. 
 
 The arrangement is exhibited in the following figure : — 
 
 A 
 
STE 
 
 375 
 
 After what has been said on the subject of the lenticular stereo- 
 scope the foregoing figure will only require a few words of expla- 
 nation. 
 
 The reflectors are placed at right angles to each other, and the 
 pictures at right angles to the dotted line, or base, passing through 
 the angle formed by the reflectors, the distance from that angular 
 point being equal to Lm or R^, and therefore to the focal length of 
 the lens with which the pictures are taken. The distance of the 
 middle points of the pictures, (that is the point where the axis of 
 the lens cuts them,) viz. m, n, from the dotted base is half the dis- 
 tance LR. The pictures are taken in a non-reversing slide, so that 
 their images on the plane pq as seen in the reflectors, are not re- 
 versed ; this is an important point to attend to. 
 
 The image of the left hand picture is pmb ; that of the right hand pic- 
 ture qua, the lines p b, qa , being separated for the sake of distinctness, 
 but in point of fact the images lie on the same plane. The left eye 
 cannot of course see the image of the right picture, and vice versa , 
 so that the images overlapping produce no confusion. The image 
 of each picture and the picture itself are symmetrically situated with 
 respect to the reflector by which it is viewed. 
 
 All this being understood we come to the mode in which the 
 _ instrument acts. 
 
 A pencil from b in the left picture, after reflexion at the left mirror 
 enters the left eye as if it had come from the point b on the line pb ; 
 the point b is therefore seen by the left eye in t^e direction Lb. 
 Similarly, a pencil from b in the right picture, after reflexion at the 
 right mirror enters the right eye as if it had come from the point b 
 on the line qb ; the point b is therefore seen by the right eye in the 
 direction RA These two lines R b, Lb, are the instantaneous direc- 
 tions of the optic axes, and being produced they meet at B, which is 
 the true position of the object B. Similarly with respect to the other 
 objects A, C. Therefore by means of the reflecting stereoscope a 
 true representation is afforded in natural relief, and actual distance, 
 of the objects in the picture. 
 
 The reflecting stereoscope is not open to any single practical or 
 theoretical objection. As an optical instrument it is absolutely 
 perfect, being subject to no defects of distortion or aberration. Eor 
 any scientific purpose, therefore, the reflecting stereoscope should 
 always be preferred to the other. The reflectors may be made of 
 polished speculum metal if objection be raised to glass mirrors, and 
 the pictures may be taken simultaneously in a camera with double 
 lenses 2i inches from centre to centre, constructed as shown in the 
 following figure, which explains itself. 
 
376 
 
 STE 
 
 In tliis double-reflecting camera the non-reversing slide need not 
 be used. 
 
 It now only remains to add a few remarks on taking stereoscopic 
 pictures. 
 
 In the first place, the effect of taking the stations wider apart 
 than 2 \ inches (the parallelism of the cameras being still preserved) 
 is to bring the $iear objects in the solid image nearer to the spec- 
 tator than they were before, and nearer than they are in nature, 
 while the most distant objects remain in their true position. This 
 may be in some cases allowable, because the stereoscope is intended 
 to serve certain educational purposes ; and it may happen sometimes 
 that, by giving bolder relief to objects than they really have, the 
 thing to be explained may be rendered more intelligible. 
 
 In the next place, the effect of directing the axes of the lenses to 
 the same point at a finite distance introduces distortion in the 
 solid image, unless the pictures are placed in the stereoscope at the 
 same angle of inclination to one another as the focussing screens of 
 the cameras. When, however, the stations are taken very wide 
 apart (several feet, for instance), if the parallelism of the cameras 
 be preserved, the pictures are partly thrown outside the focussing 
 screen, besides being taken with very oblique pencils; so that, 
 when correct principles of operation are once departed from, for any 
 purpose, other changes in the arrangement of things become neces- 
 sary. We cannot, however, within the compass of the present work, 
 do more than explain completely the theory of the correct form of 
 
STE 
 
 377 
 
 the instrument ; it would require a separate treatise to follow up 
 the subject through all the modifications which it may assume. 
 Enough has been said to enable any intelligent reader to think out 
 the remainder for himself. 
 
 In printing stereoscopic pictures from a negative, taken in a 
 double-lens camera, by contact in the pressure frame, it must be 
 borne in mind that the print requires to be cut in half and the pic- 
 tures transposed, in order to bring the picture that was taken from 
 the left station before the left eye in the stereoscope, and vice versa . 
 If this be not attended to, a pseudoscopic effect is produced. 
 
 In printing stereoscopic transparencies by means of a lens, the 
 following plan may be adopted : — 
 
 A copying camera, rather more than double the length of the 
 stereoscopic camera, is provided, and the lenses of the stereoscope 
 are fixed in the middle of it ; there must also be a partition divid- 
 ing the camera in half lengthways. In this way the left lens copies 
 the left picture at the same time that the right lens copies the right 
 picture. The negative must be placed with its back next to the 
 lenses at one end of the box, and the sensitive positive plate in a 
 common slide at the other end. The camera is then directed 
 towards the sky, and the wet collodion process employed. An 
 exposure of a few seconds is sufficient. The positive need not be 
 divided and the pictures transposed, for, when placed in the stereo- 
 scope with its plane side next to the lenses, and a ground glass laid 
 against the film, the pictures are in their right position to be viewed. 
 By putting the lenses midway between the negative and positive, 
 the positive becomes of the same size as the negative. The best 
 lenses to employ are portrait lenses, with a small stop between the 
 back and front lenses in each. 
 
 In the stereoscopes and stereoscopic pictures commonly sold 
 there are the following serious defects : — 
 
 1st. The pictures are frequently taken in converging cameras, 
 and then mounted upon the same flat surface. 
 
 2nd. The pictures are generally mounted so wide apart that the 
 most distant objects in each are wider apart than the distance between 
 the centres of the eyes. 
 
 3rd. An attempt is made to obviate the evil produced by the 
 above practice, by using semi lenses in the stereoscope which dis- 
 place the images. This of necessity produces distortion, because 
 straight lines are always represented by curves when the outside 
 part of a lens is used to view objects through, instead of the centre. 
 
 4th. The displacement of the images is in general so great as to 
 cause the optic axes to converge to points situated within two or 
 
378 
 
 STE 
 
 sue 
 
 three feet from the nose, instead of the true distance of the objects. 
 The effect of this is to make the solid picture look like a small 
 model of the object, which the spectator could, if he chose, lay his 
 hand upon, or touch with a yard measure. 
 
 Lastly. The focal length of the lenses of the stereoscope is in 
 general 6 inches, while that of the lenses in the camera is only 4 
 inches. This makes objects appear much smaller than they do in 
 nature. 
 
 To sum up. The common stereoscope and pictures make objects 
 look very near, very small, and distorted. The stereoscope described 
 and recommended in the present article makes them look of their 
 true size, at their true distance, and without perceptible distortion . 
 
 In this article the term “ solid image” has been several times 
 used. The employment of this term may perhaps be thought ob- 
 jectionable ; we do not, however, know of a better, and if the thing 
 meant has been clearly understood, the end has been answered. 
 
 Stereomonoscope. This is an instrument invented by M . Clau- 
 det for exhibiting upon a screen of ground glass a single picture 
 having the true effect of solidity. This result, which may at first 
 sight appear paradoxical, is accomplished thus : — 
 
 A pair of stereoscopic pictures is first taken in the usual way. 
 Magnified images of them are then thrown, by means of a pair of 
 lenses (one for each picture) upon the same part of a large upright 
 screen made of coarsely ground glass, the axes of the lenses con- 
 verging at a suitable angle. The spectator then stands at a 
 distance of a few feet on the opposite side of the screen, and looks 
 with both eyes at the image formed upon it. The image formed by 
 the right hand lens is seen by the left eye, and that formed by the left 
 hand lens by the right eye, in consequence of the roughened state 
 of the glass, which is filled with minute transparent spots. A true 
 stereoscopic effect is said to be produced in this way by the com- 
 bination of the images. 
 
 Stone Blue. A mixture of indigo and starch, moistened with 
 water, made into cakes, and dried. Sometimes Prussian blue is 
 used instead of indigo. 
 
 Succinic Acid. (Latin, Succinum amber.) An acid obtained 
 rom amber by distillation with sulphuric acid. It is one of the 
 most powerful organic acids, and forms salts with the alkalis and 
 metallic oxides called succinates. Succinate of silver is darkened by 
 light, and has been used in photographic experiments. 
 
SUG 
 
 SUL 
 
 379 
 
 Sugar. This is a substance extracted from the sweet juices of 
 a great number of plants. There are two principal varieties, viz., 
 cane sugar and grape sugar. The latter variety is described in a 
 separate article, q. v. 
 
 Cane sugar is for the most part obtained from the sugar cane, 
 but it exists also in beet-root, in the sap of certain species of maple, 
 and in the stem of indian corn. It is cry stalliz able, and the crystals 
 usually occur as six-sided prisms with flat and irregular ends. It is 
 soluble in about one-third of its weight of water, but more sparingly 
 in alcohol, boiling absolute alcohol only taking up about the one- 
 eightieth part of its weight of cane sugar. It is dimorphous, exist- 
 ing in the crystalline form as sugar candy, or white sugar, and in an 
 amorphous concrete state as barley sugar, which is produced by 
 rapidly boiling down the syrup till it solidifies in a transparent mass 
 on cooling. 
 
 Cane sugar is powerfully acted on by sulphuric acid, which takes 
 a portion of the elements of water from it, and produces a black 
 magma consisting chiefly of carbon. It is converted into grape sugar 
 by the action of dilute acids, and also of certain azotized principles 
 called “ ferments.” 
 
 Cane sugar combines with some of the alkalies and metallic ox- 
 ides. 
 
 Sugar of Lead. Acetate of Lead ; q. 
 
 Sugar of Milk ; Lactine. A sugar contained in whey, and 
 obtained by evaporating the whey to a syrup, and allowing it to 
 crystallize. It has a feebly sweet taste, is very hard, and feels gritty 
 between the teeth. It is slow and difficult of solution in cold water. 
 It combines with protoxide of lead. 
 
 Sulphate of Iron. There are two sulphates of iron, the proto- 
 sulphate, or sulphate of the protoxide, and the persulphate, or sul- 
 phate of the sesquioxide. 
 
 Profo-suLPHATE of Iron. Ue. O, S 0 3 +7 H 0 = 139. This 
 salt, sometimes called “ Green vitriol,” is obtained by acting on iron 
 filings with dilute sulphuric acid, evaporating, and crystallizing. 
 When pure, and free from persulphate, the crystals are of a bluish 
 green colour, free from red stains, and in the form of oblique rhombic 
 prisms. They are insoluble in alcohol, which deprives them of water 
 and precipitates the salt as a white powder ; — and soluble in rather 
 more than an equal weight of water. In dry air they effloresce 
 
380 
 
 SUL 
 
 
 but in moist air become reddened and oxidized into a persulphate, 
 They fuse in their own water of crystallization. 
 
 Native green vitriol is frequently found in coal mines associated 
 with iron pyrites. 
 
 Protosulphate of iron forms double salts with the sulphates of 
 ammonia and potass. Its aqueous solution absorbs binoxide of 
 nitrogen, and becomes of a deep olive colour. 
 
 Protosulphate of iron is a powerful deoxidizer, and is used exten- 
 sively in Photography as a developer. When oxidized it becomes 
 converted into persulphate. The proto- and persulphates are capable 
 of crystallizing together, and forming a double salt of a grass-green 
 colour, while the pure protosulphate is of a bright bluish green. 
 
 Per- sulphate or Iron. There are several persulphates of 
 iron, some of which are soluble in water, others not, yielding either a 
 brown mass, or dingy grey salt. They are compounds of sulphuric 
 acid with Sesquioxide of iron, Pe. 2 0 3 . 
 
 Sulphate of Silver. A g. 0, S0 3 . Silver may be dissolved 
 in its own weight of boiling sulphuric acid. A white salt is formed, 
 which is sulphate of silver. It is also produced by adding sulphate of 
 soda to a solution of nitrate of silver. It is soluble in 90 parts of 
 cold water ; and also in ammonia, which forms a double salt “ am- 
 monio-sulphate of silver.” 
 
 Nitrosulphuric acid formed by dissolving 1 part of nitre in 10 
 of sulphuric acid dissolves silver at a temperature below 200°, and 
 the solution may be moderately diluted before sulphate of silver 
 separates from it. 
 
 Sulphides. Sulphur combines in various proportions with most 
 of the metallic and non-metallic elements, and forms compounds 
 called “ sulphides. 55 
 
 Sulphide of Ammonium. See “ Hydrosulphate of Ammonia. 55 
 
 Sulphide of Sodium. See “ Artificial Ultramarine. 55 
 
 Sulphide of Silver. Ag. S. This fine mineral occurs native 
 in various forms, sometimes crystallized in cubes, octohedra, and 
 dodecahedra. When combined with sulphide of antimony it forms 
 the red or ruby silver ore. It may be produced easily as a grey 
 crystallizable compound, by heating together finely divided silver and 
 sulphur, the compound being much more fusible and softer than silver. 
 
SUL 
 
 381 
 
 Sulphuretted hydrogen, and hydrosulphate of ammonia, throw down 
 a yellowish black precipitate from a solution of nitrate of silver, which 
 is believed to be sulphide of silver. 
 
 Sulphide of silver is decomposed by a solution of chloride of copper 
 containing common salt, the products being chloride of silver and 
 subsulphide of copper. No decomposition, however, takes place 
 unless salt be present. 
 
 Sulphur. S= 16 . A yellow, crystallized elementary body, 
 found chiefly in the neighbourhood of volcanoes. About 20,000 tons 
 are consumed annually in England, and are imported chiefly from 
 Sicily. It exists .abundantly in combination with lead, copper, and 
 iron. 
 
 Sulphur is highly combustible, and bums with a blue flame, pro- 
 ducing sulphurous acid. It is insoluble in water, and permanent in 
 the air. It volatilizes at 180°, and fuses at 220°, When heated 
 to 300°, it becomes viscid and of a brown colour ; if it be then poured 
 into warm water it becomes soft like wax, and may be used for taking 
 impressions ; it becomes hard when cold. 
 
 Sulphur is soluble with difficulty in alcohol, but is more soluble 
 in sulphuric ether, and still more so in volatile oils, turpentine, and 
 alkaline solutions. Milk of sulphur is sulphur precipitated from an 
 alkaline solution by an acid. 
 
 Sulphur Toning. WTien an argentine photograph having the 
 reddish tint produced by the combination of suboxide of silver with 
 organic matter is placed in water containing a small quantity of sul- 
 phide of ammonium, the tint gradually changes from red to purple, 
 and thence to green-yellow. This is called sulphur toning, the 
 yellow substance being supposed to be sulphide of silver in an allo- 
 tropic state ; or it may possibly be a bisulphide of silver; or a double 
 sulphide of silver and ammonium. A similar result occurs when the 
 print is placed in a bath of hyposulphite of soda containing unstable 
 sulphur salts, or unstable compounds of sulphur and oxygen, or free 
 sulphur in a nascent state or state of fine division, exhibiting a milky 
 turbidity in the bath produced by the addition of an acid to it. 
 
 The chemistry of the sulphur toning of a red argentine photograph, 
 although probably a very simple matter, has not yet been investigated 
 by any chemist of eminence, and the subject clearly made out, although 
 one of great importance, for thousands of poimds are annually wasted 
 by photographers in the production of perishable prints. 
 
 The ordinary fading of positives appears to be nothing more than 
 the sulphur toning process carried to the yellow stage, in consequence 
 
382 
 
 SUL 
 
 of the presence of a destructive sulphur salt which cannot be removed 
 from the paper. The combination of organic matter with the silver 
 may have something to do with the result, and with the composition 
 of the yellow substance. Photographs of a black tint, produced by 
 development, and containing a much greater quantity of material 
 than sun-prints, and that in a form much more nearly metallic, are 
 found to be vastly more permanent then the latter. 
 
 Sulphuretted Hydrogen. See “Hydrosulphuric Acid.” 
 
 Sulphuric Acid. Oil of Vitriol. S0 3 ; or when hydrated, 
 HO, S0 3 . 
 
 This important article of manufacture, of which nearly a hundred 
 thousand tons are made annually in Great Britain, is in general pro- 
 duced by burning together in a furnace a mixture of sulphur and nitre, 
 the proportions varying from the one-eighth to the one-twentieth part 
 of nitre. 
 
 The fumes are collected in immense leaden chambers, the bottoms 
 of which are covered with water, which absorbs the vapour and 
 becomes impregnated with sulphuric acid. When the water has 
 acquired the S. G. of about 1*25, it is drawn off and evaporated by 
 boiling in shallow leaden vessels till it acquires the S. G. of 1* 7, 
 when it would begin to act upon the lead. The evaporation is then 
 completed in platinum vessels till the acid reaches the S. G. 1*845. 
 The manufacture is then finished and the concentrated acid is trans- 
 ferred to carboys, (large glass bottles covered with wickerwork,) 
 which contain 100 lbs. 
 
 Sulphuric acid of the S. G. 1*845 contains anhydrous sulphuric 
 acid S0 3 plus one atom of water. It is a limpid, inodorous, colourless, 
 and very heavy liquid of an oily consistence. It boils at a heat of 
 620°, and distils over without decomposition. No fumes arise from it. 
 A piece of polished zinc suspended above it in a closed bottle remains 
 untarnished for many months. It has a strong affinity for water, 
 and if 4 parts of it are added to one of water at the freezing point, the 
 temperature of the mixture speedily rises to 212°. 
 
 Commercial oil of vitriol is tolerably pure. The principal impurity 
 is sulphate of lead, which is detected by its producing a white 
 turbidity when the acid is diluted with distilled water. 
 
 Nordhausen oil of vitriol is a dark coloured fuming sulphuric acid, 
 composed of a mixture of anhydrous and hydrated sulphuric acid. 
 It is manufactured in Germany. 
 
 Anhydrous sulphuric acid is a tenacious crystalline mass, which in 
 the absence of all moisture has no acid properties. W~hen water 
 
SUL 
 
 TAN 
 
 383 
 
 is added to it, combination takes place, with heat, light and ex- 
 plosion. 
 
 Sulphuric Ether. See “ Ether. ” 
 
 Sulphurous Acid. S0 2 . This acid is produced by the combus- 
 tion of sulphur in oxygen. At ordinary temperatures it exists as a gas, 
 but at the freezing point becomes liquid. At a lower temperature it 
 may be solidified, and then forms a white mass. Water which has 
 been recently boiled absorbs about 30 volumes of it. The solution 
 possesses bleaching and deoxidizing properties. 
 
 Sumach. A yellow dye, and tanning material, extracted from the 
 dried and bruised branches of certain species of Rhus cultivated in 
 Italy and the South of Erance. 
 
 Tangent. The tangent to a curve at any point P is defined 
 thus : Take any other point Q, and draw a straight line through 
 P, Q. This straight line cuts the. curve in the points P, Q, and is called 
 a “ secant.” Now let the point Q move along the curve towards 
 P. When it has approached to within a distance less than any 
 assignable distance from P, but without actually coinciding with P, 
 the secant P Q becomes a tangent to the curve at P. 
 
 This definition applies equally,' either to the case of a curve of 
 double curvature (like a corkscrew), or to a curve which lies upon a 
 plane. 
 
 The Tangent-Plane to a surface at a point P is found by 
 cutting the surface by any two planes which pass through P, finding 
 the tangent lines through P to each of the plane curves thus pro- 
 duced, and drawing a plane through those two tangent lines. 
 
 If two points, Q, R, be taken on the surface, and a plane PQR 
 drawn through them, and we suppose Q, R, to move towards P, 
 then the plane PQR, when Q and R have approached P to within 
 a distance less than any assignable distance, is not necessarily the 
 tangent plane at the point P, as it is sometimes stated to be in 
 books of geometry. 
 
 Tannin; Tannic Acid. Tn.=C 18 H 5 0 9 +3H0. An astrin- 
 gent principle contained in various vegetable substances, but prin- 
 cipally in infusion of galls. It is obtained in a pure form by treating 
 powdered galls with washed ether, i.e., ether containing 10 per cent, 
 of water ; this is allowed to filter through the galls, and the filtered 
 liquid divides itself into two strata, the upper one being ether, and 
 
384 
 
 TAB 
 
 the lower a concentrated aqueous solution of tannic acid. This is 
 evaporated in vacuo over sulphuric acid, and pure tannin remains 
 as a bulky pale-yellow residue, which is exceedingly soluble in water, 
 but less soluble in absolute ether and alcohol. 
 
 The aqueous solution of tannic acid reddens litmus paper, and 
 exhibits the properties of an acid, displacing carbonic acid from the 
 carbonated alkalis with effervescence, and forming salts called 
 tannates. 
 
 Tannin combines energetically with gelatine, and forms an inso- 
 luble precipitate when added to solutions of isinglass, or glue. When 
 oxidized, tannin becomes converted into gallic and carbonic acids. 
 Its chief use in photography consists in producing a leather varnish 
 upon paper positives ( q.v .). 
 
 A concentrated solution of tannin is precipitated by nitric and 
 hydrochloric acids, but not by oxalic, tartaric, lactic, acetic, or citric 
 acids. Tannin, when added in excess, gives a dark blue or black 
 colour to solutions of the ^?ersalts of iron, but produces no immediate 
 change in solution of the pure ^rofosalts. The black precipitate 
 produced in the former case is common writing ink, and is composed 
 of one atom of peroxide of iron and three atoms of tannic acid. 
 
 Tartar. Bitartrate of Potass. A white salt nearly insol- 
 uble in cold water, and quite insoluble in a cold mixture of alcohol 
 and water. It has an acid reaction. It exists abundantly in the 
 juice of the grape, and is deposited in wine casks in the form of an 
 incrustation called “ argol,” which is purified and whitened by solu- 
 tion in boiling water, and crystallization. 
 
 Neutral tartrate of potass is produced by neutralizing a solution 
 of bitartrate of potass with lime. 
 
 Tartaric Acid. Tar.=C 8 H 4 O 10 +HO. This acid exists 
 free in many acid fruits and plants, and is generally obtained from 
 cream of tartar, in the form of white crystals, which are soluble in 
 about 4 parts of water at 60°, and also in alcohol. It is a very 
 powerful organic acid in photography, and should be used with 
 caution. It is deliquescent in damp air. 
 
 Paper washed with a solution of tartaric acid is said to be slightly 
 sensitive to light. 
 
 # 
 
 Tartrate of Silver. This salt is produced by adding' tartrate 
 of potass to solution of nitrate of silver ; the white precipitate thrown 
 down being tartrate of silver. It is darkened and decomposed both 
 by heat and light. 
 
TAU 
 
 TEN 
 
 385 
 
 Taupenot’s Process. This is a dry preservative process in 
 which a glass plate is first collodionized and excited, then coated 
 with iodized albumen, and excited a second time. Three years ago, 
 before the problem of dry collodion was solved, this process, disco- 
 vered by Dr. Taupenot, Professor of Chemistry at a Erench college, 
 was brought forward and excited so much attention that it has not 
 even yet been completely abandoned by amateur photographers for 
 the better process of Dr. Hill Norris, of Birmingham, since disco- 
 vered and perfected. 
 
 The operations are as follow : — 
 
 A plate is first collodionized and excited in the usual way. Tt is 
 then washed with distilled water for the purpose of removing the 
 free nitrate of silver. A mixture containing about equal parts of 
 albumen and water, and 1J per cent, of iodide of potassium, is then 
 poured over it. This completely destroys the sensitiveness of the 
 plate, which may then be dried in daylight before a fire. It is now 
 ready to be excited a second time. This is effected by immersing 
 it in a bath of aceto-nitrate of silver, containing 50 grains of nitrate 
 of silver and 1 dram of acetic acid. The plate is then washed with 
 distilled water, and either dried by artificial heat, or allowed to dry 
 spontaneously. It is now ready to be exposed in the camera, and 
 may be kept for several days, or perhaps weeks, in a sensitive state. 
 
 The picture is developed by first steeping the plate in a dish of 
 distilled water for a minute or two, then laying it upon a levelling 
 stand, and pouring over it a saturated solution of gallic acid to 
 which a few drops of aceto-nitrate of silver are added. It is fixed 
 in the usual way with hyposulphite of soda, not cyanide of potassium ; 
 for the latter salt should never be used with albumen films, as it 
 acts too energetically upon albumen. 
 
 The pictures obtained by this process have a yellowish or greenish 
 tint, and are not therefore suited for transparencies. 
 
 The collodion should be porous and adhesive, and not hard and 
 contractile, or blisters will be produced when the film is wetted a 
 secdnd time. 
 
 The objections to the process are, — the multiplicity of operations, — 
 the bad colour of the pictures, — and the discoloration of the 
 second nitrate bath. As a set off to these objections it has no 
 advantage whatever over dry collodiqn. 
 
 The plates may be developed more quickly with pyrogallo-nitrate 
 of silver, but the definition is not quite so good, and stains more 
 liable to occur. 
 
 Tent. When views are taken by the wet collodion process, in 
 
 c c 
 
386 
 
 TEN 
 
 which the free nitrate of silver is not removed from the plate by 
 washing, it becomes necessary to operate either in a dark room, or 
 van, or tent, at or near the spot whence the view is taken ; for 
 otherwise the latent image is destroyed by the evaporation of the 
 moisture from the sensitive plate, and the consequent solution of 
 the iodide of silver by the concentrated nitrate in the film. 
 
 The dark tent used by travelling photographers is of various forms 
 and sizes, and more or less portable: For large pictures 12x10, 
 or so, perhaps the best form is that of the ordinary military tent, 
 having a pole at each end and fastened to the ground with ropes 
 and pegs. It should be made with black palico, lined with yellow. 
 
 In one of the gable ends there should be a yellow window, and the 
 entrance should be in the opposite gable. Inside there may be 
 a table and all the necessary conveniences. This kind of tent has 
 of course no pretensions to portability, and a travelling van is pre- 
 ferable, since much time is necessarily occupied in erecting and 
 taking ilown a tent of this form. 
 
 For stereoscopic pictures, or pictures not exceeding 8 X 6, a much 
 simpler and more portable form of tent will answer the purpose. A 
 deal tray about 2'ft. 6in. long, 1ft. 9in. wide, and 4 ins. deep, is 
 screwed upon a short tripod stand. At the corners the four up- « 
 rights of a light iron frame are inserted, which is made thus ; — 
 
 the rods being about the thickness of stair rods. 
 
 A covering made of black calico lined with yellow is thrown over 
 this frame, and h&ngs down to the knees. In this covering at the 
 back is a yellow window nearly the whole height of the iron rods 
 and about 6 inches wide ; the yellow curtain of this window or 
 aperture may be drawn backwards and forwards at pleasure. The 
 tray has a shelf at the back which carries the bottles, etc. 
 
 To use this tent the operator stands beneath the projecting part 
 of the top of it, with the tray in front of him, and draws the lower 
 part of the curtains tightly round him under his elbows, and fastens 
 
TES 
 
 THE 
 
 387 
 
 them by means of hooks to the inside of the front part of the tray 
 so as to exclude day-light. He has then his hands at liberty, and 
 manipulates in the usual way. By putting a tent of this kind, 
 together with the chemicals and apparatus, upon a wheelbarrow, or 
 suitable truck, or basket with wheels which ship and unship, the 
 photographic tourist becomes independent of help from others. 
 
 Test Papers. Make an infusion of commercial litmus, and 
 steep blotting paper in it ; dry it, and cut it into narrow strips. It 
 is of a deep indigo colour, and is reddened by being immersed in 
 any acid solution, or exposed to acid fumes. This blue litmus 
 paper, as it is called, is therefore a test for acidity. It may also be 
 used as a test for alkalies by dipping it into very dilute sulphuric acid, 
 and drying it. In this state it is called reddened litmus paper, and 
 its original blue colour is restored by immersion in any alkaline 
 solution. A volatile acid, such as acetic, should not be used in 
 making reddened litmus paper. Another kind of test-paper for 
 testing alkalinity is made by steeping blotting paper in an infusion 
 of turmeric ; this is of a yellow colour, which is changed to brown 
 by the action of an alkali. It is not considered so good as reddened 
 litmus. But the best kind of test paper for testing alkalinity is 
 made by steeping blotting paper in a strong infusion of the petals 
 of the red rose. The red colour of this kind of paper is changed 
 to green by an alkali. 
 
 When a solution to be tested is very feebly acid or alkaline a few 
 minutes must be allowed before the change of colour in the test- 
 paper is perceived. 
 
 Test-papers, as usually sold by chemists, are made up in little long 
 narrow books. They are indispensable to the photographer for 
 testing the condition of the nitrate bath. 
 
 Thermometer. (Gr. Srep/arj heat, fierpov a measure.) This is 
 an instrument for measuring temperature by recording the expansion 
 produced in a liquid by heat. 
 
 The mercurial thermometer consists of a glass tube of fine and 
 equal bore, having a bulb at one end. This tube is filled with 
 mercury at a high temperature, above boiling point, and its end 
 hermetically sealed. On cooling, the mercury contracts and leaves 
 what is called a Torricellian vacuum above it in the tube, i. e. a space 
 filled with the vapour of mercury. 
 
 To graduate the thermometer, it is first immersed in melting 
 snow, the temperature of which is found to be invariable, and the 
 height of the mercury marked ; and next in water boiling in a thin 
 
 c c 2 
 
388 
 
 TIN 
 
 TEA 
 
 
 polished metallic vessel at a barometric pressure of 30 inches, a 
 temperature which is also found to be invariable, and the height of 
 the mercury marked again. These two graduations are called the 
 freezing and boiling points of water. The space between them is 
 divided differently in different thermometers, as shown in the table 
 at the end. 
 
 The thermometer used for testing the temperature of liquids is 
 furnished with a hinged back, so that the bulb and lower part of 
 the tube only can be inserted in the liquid. 
 
 If equal parts of hot and cold water are mixed together the 
 thermometer indicates accurately the arithmetical mean between the 
 temperatures. 
 
 Tincture of Iodine. This is composed of 48 grains of iodine 
 added to one fluid oz. of alcohol S. Gr. 835. The iodine is dissolved 
 and the tincture poured off into a well stoppered bottle. 
 
 ToninGtBatii. In printing positives by direct light, the purple 
 tint due to the subchloride of silver is removed by the fixing agent, 
 and nothing remains but a red compound of silver and organic 
 matter. In order therefore to render a sun-print presentable as a 
 work of art the thin red tint of the shadows must be blackened or 
 intensified by some means. This is effected by the toning bath, 
 which either darkens the print by sulphurating the silver in the 
 image, or substitutes gold for silver, according to its composition. 
 Developed prints do not necessarily require a toning bath, since the 
 material of the image is sufficiently black and intense without it. 
 The most suitable toning-bath to be employed in the various printing 
 processes is described in the articles which treat of those processes. 
 
 Tournesol. The Trench term for Litmus, q. v. 
 
 Tracing-Paper. There are two kinds of tracing paper, viz., 
 transparent, and black. Transparent tracing-paper is made by 
 smearing the paper with boiled oil, or magilp, or colourless dam- 
 mar resin dissolved in turpentine or benzole ; or better still, with 
 Canada balsam diluted with turpentine. Black tracing-paper is 
 made by saturating a piece of blotting paper with a mixture of 
 lamp-black ground in honey ; or by rubbing a piece of sized paper 
 with black-lead or black chalk. The tracing is first made upon the 
 transparent paper in lead pencil, this is then laid upon the black 
 paper, and that with its blackened side upon the paper which is to 
 receive the final drawing. The lines on the tracing-paper are then 
 gone over with a porcupine’s quill, or other hard point, exerting a 
 gentle pressure. A corresponding outline is thus obtained upon 
 
TEA 
 
 389 
 
 the paper beneath the black one. Sometimes blue paper is used 
 instead of black ; this is made by substituting indigo or prussian 
 blue for lampblack. 
 
 The transparent paper made with Canada balsam takes ink and 
 water colours freely. A tracing-paper is made in France from raw 
 flax, and is called “ Papier Yegetale.” 
 
 Transferring. A collodion picture which is not too porous 
 and powdery may be transferred from the glass plate, when wet after 
 the final washing, in the following manner : — 
 
 Lay a thick and wet piece of blotting paper upon the film in such 
 a way as to cover the plate all but about a quarter of an inch at 
 one end. Turn the narrow edging of film which is outside the 
 blotting-paper over it by means of a penknife, and then, beginning 
 at that end, raise the blotting-paper gently off the plate ; the film 
 will come off with it. It may be permanently fixed to a sheet of 
 dry gelatinized paper, by laying the blotting-paper bearing the film 
 upon the gelatinized paper, pressing the two into close contact, 
 and letting them dry spontaneously, when the blotting-paper will 
 come off, leaving the film attached to the gelatinized paper so 
 strongly as to resist all attempts to remove it again by scratching 
 or rubbing. The paper may then be waxed. 
 
 Collodion positives may be transferred from glass to glazed leather 
 by damping both the film and the leather with alcohol, pressing 
 the two into contact, and in a few minutes peeling off the leather, 
 which brings the film with it, apparently so incorporated with the 
 black glaze as to be incapable of being removed by scratching with 
 the nail, &c. 
 
 Collodion negatives may be transferred to gutta-percha in the 
 following planner described by M. Leon Cassagne at a meeting of 
 the French Photographic Society on June 19, 1857. 
 
 “It is generally known that at the Imperial Printing Office of 
 Vienna, when a good collodion negative has been obtained on glass, 
 it is the custom to transfer it by means of a double film of gelatine, 
 and gutta-percha dissolved in chloroform. The process which I 
 have adopted, and which has never been described in the Bulletin 
 of the Society, consists in first dissolving — 
 
 Pure gutta-percha . . . 1*92 grammes 
 
 Chloroform, or Benzole . .31*09 „ 
 
 or, 
 
 Gutta-percha 
 Chloroform, or Benzole 
 
 . 2*56 grammes 
 
 . 31*10 „ 
 
390 
 
 TEA 
 
 “ You perceive that the quantities are not invariable. There are 
 cases in which it is necessary to vary them. I shall not enter into 
 details ; the operator, in each particular case, will be able to decide 
 for himself. 
 
 “ When the negative on the glass is dry and in good condition, 
 pour on the collodion side a coating of the above solution. Let it 
 run slowly and uniformly, that it may have time to penetrate and 
 unite with the collodion film. As soon as this coating is completely 
 dry, strengthen it with a second, formed of the following sub- 
 stances : — 
 
 Gelatine of commerce, (very white) . 30 grammes. 
 
 Filtered water, as much as the gela- 
 tine can absorb, until it has swelled 
 to the utmost. 
 
 Isinglass . . . . 5 „ 
 
 Alcohol . . . 15 „ 
 
 “ Melt the gelatine in the water which it has absorbed, by plac- 
 ing the vessel containing it in hot water. Melt the isinglass in the 
 same way in the alcohol. Mix by degrees, and with care ; stirring 
 with a wooden spatula this species of varnish. Warm it with pre- 
 caution, that it may not be injured by too much heat. Hold the 
 negative, the coating of gutta-percha upwards, before a clear fire, 
 or over a spirit-lamp, until it is heated to 10 or 20° centigrade; 
 then pour over it immediately (removing it from the flame of the 
 lamp), a coating of gelatine, as thin as its density will allow. It is 
 unnecessary to say that the gelatine must be warm and perfectly 
 liquid at the time. Leave it for an instant to cool and dry, shel- 
 tered from dust, and you will be able to remove easily, by means of 
 the steam from boiling water, the triple film of collodion, gutta- 
 percha, and gelatine. This operation, which is very easy, is per- 
 formed as soon as you see that the film is slightly softened by the 
 steam, and you should then begin to remove it from the glass at the 
 comer from which the excess of collodion was poured off when the 
 plate was collodionized. It often happens that the film disengages 
 itself at this corner of the glass. It is a good plan to facilitate the 
 entire removal of the film with a thin blade of flexible polished horn, 
 on which, with the help of fingers, you support the film, while you * 
 detach it by degrees, either with, or without, the aid of a thin thread 
 of water, running drop by drop from a tap, and which insinuates 
 itself by degrees under the collodion, between it and the glass. As 
 soon as the entire film is raised, flatten it between two pieces of 
 glass, having good surfaces, and sufficiently thick to act by their 
 own weight. The collodion used must have sufficient consistency, 
 not so much however as to leave striae or lines on the plate when dry. 
 
TEA 
 
 391 
 
 “ The chloroform or benzole solution, should be allowed to stand 
 several days before being used, in order that the colouring matter, 
 or any impurities in it, may be deposited. Filter through paper, 
 that the solution may be sufficiently thin, shutting the top of the 
 funnel to prevent too much evaporation, which would have the effect 
 of thickening the solution. Benzole, of specific gravity much less 
 than the chloroform, gives good results, but inferior to those ob- 
 tained by chloroform, which gives a solution almost colourless, and 
 adheres firmly when the evaporation is completed ; which also takes 
 place more rapidly than with the benzole. 
 
 “ The density of the solution of gutta-percha, which is always 
 slightly coloured, retards considerably its complete clarification. It 
 is necessary to avoid all impurities in this solution.” 
 
 The following is a method of transferring dry collodion negatives 
 to paper, described by M. Bayard at a meeting of the French 
 Photographic Society on Feb. 20th, 1857. 
 
 “ Among the specimens which I have the honour to lay before you 
 are some which have been obtained from very old negatives, and 
 even from negatives which have been varnished. They have all 
 been easily transferred. I cannot however promise you that it will 
 always be so. It is probable that certain varnishes, and particularly 
 fatty varnishes, may offer an impediment to the softening and removal 
 of the collodion film. 
 
 “I am afraid also that albumen and gelatine spread upon the 
 negatives may interfere with the success of the operation; and I 
 must warn you that I have not yet succeeded in transferring with 
 certainty negatives produced by Taupenot’s process, either simple or 
 modified. 
 
 “ The following is my process : — 
 
 “ In order to detach the film of collodion from the glass, I use 
 paper coated with gelatine. 
 
 “To prepare it, dissolve in one litre of filtered rain water, 40 
 grammes (about 4 per cent.) of colourless gelatine. When the 
 gelatine is dissolved, pour the solution into a dish which has been 
 previously heated. Float the papers on the bath for one or two 
 minutes, and hang them up by a corner to dry. When dry, keep 
 them in a portfolio until required for use. The kind of paper 
 which appears most suitable for this operation is Canson’s tliin 
 negative paper. 
 
 “If the negative to be transferred has just been taken, and is 
 still wet, place the glass on a horizontal support, collodion side 
 upwards, and cover it equally and evenly with water. Then, take 
 
a sheet of the prepared paper, (which should be of the same size as 
 the glass,) float the gelatinized side for three or fbur minutes on a 
 bath of water, and having carefully removed it, lay it on the water 
 with which the glass has been covered. Then, by inclining the 
 glass, allow the water to drain off and the paper to become attached 
 to the collodion. Place the glass perpendicularly, and allow it to 
 dry spontaneously. 
 
 “ When the negative which you wish to transfer is old, and has 
 not been varnished, immerse it for about a quarter of an hour, film 
 upwards, in a dish full of water. Ten or twelve minutes after 
 putting it into water, lay a sheet of gelatinized paper on the same 
 water for three or four minutes. Then, remove the glass by the 
 corner^ in such a way as to remove with it the paper which floats 
 above, (the edges being of course properly adjusted to those of the 
 glass). By proceeding cautiously the paper will adhere to the 
 collodion. Drain and dry as before. 
 
 “ When the negative has been varnished, proceed as before, with 
 this difference, viz. : — add 3 or 4 per cent, of alcohol to the water, 
 and let the glass remain in it half an hour. , 
 
 “ When the paper which has been glued to the collodion has 
 become perfectly dry, (it should not be dried by the fire,) make an 
 incision with the point of a penknife all round it, pretty close to the 
 edges of the glass, and then immerse the negative in a dish filled 
 to about an inch deep with water. A quarter of an hour after, 
 you may endeavour to raise a corner of the negative film with the 
 point of a knife. Should the film not come off with the paper, 
 leave it immersed a little time longer. As soon as you find that the 
 collodion will leave the glass, raise the paper carefully, without re- 
 moving the glass from the water, which always moistening the 
 collodion renders the operation more easy. When the paper has 
 been removed with the collodion film adhering to it, press it between 
 blotting paper, and dry it. 
 
 “Negatives transferred in this way acquire great vigour for 
 printing, and if the prints from them are found to be too strong in 
 the contrasts, the negative should be waxed on the reverse side of 
 the paper in the ordinary way.” 
 
 After M. Bayard had made the above communication, he, in order 
 to shew with what ease these transfers could be made, placed in 
 water a collodion negative on glass, having a sheet of gelatinized 
 paper adhering to the film. 
 
 M. Le Gray, who had lent this negative, said that he had 
 warned M. Bayard that it was in very bad condition for transferring, 
 as the film wanted consistency; but that if the experiment sue- 
 
TEA 
 
 393 
 
 ceeded it would be the more conclusive. Notwithstanding these 
 unfavourable conditions M. Bayard effected the transfer with 
 complete success. 
 
 Collodion positives may be transferred to paper by the following 
 process, communicated about a year ago to the 44 Photographic 
 Notes” by Mr. Manson, of Edinburgh. 
 
 44 To make the transfer varnish : — take of boRX 1 dram, shellac 
 4 drams ; digest them in about 5 ounces of water, nearly boiling, in 
 a covered vessel, till the whole is dissolved. When cold it is ready 
 for use. 
 
 44 To transfer the film : — Apply a coat of the varnish to the sur- 
 face of the picture with a large and soft camel’s hair brush, and dry 
 it quickly by holding it over a flame, or at a fire. When cold, 
 applying a second coating of varnish, as before. 
 
 44 Then take a piece of black paper a little larger than the glass. 
 Coat it, and also the picture, with varnish, and lay the two wet 
 surfaces together, beginning at one end, and carefully excluding 
 every bubble of air. WheiPnearly dry lift one end, and strip the 
 whole from the glass. It is now ready for mounting.” 
 
 Daguerreotypes may be transferred to paper by the following 
 process described by Mr. Belfield Lefevre, of Exeter, in the Pho- 
 tographic Notes, Vol. 2, page 343. 
 
 44 To obtain a negative by transferring to the surface of some 
 more or less transparent substance the loose particles which form 
 the lights and half-tints of Daguerre’s image, is an idea which must 
 have suggested itself to the minds of many, whilst the means by 
 which this transfer may be effected are as simple as the idea itself 
 is obvious; and yet, although from the day on which M. Arago 
 communicated M. Daguerre’s process to the Academy of Sciences, I 
 have been a votary of the Photographic art, and am not unfamiliar 
 with Photographic literature, I have seen no allusion, however 
 remote, to any such process. Is it that the results so obtained have 
 not been found available for the purposes of photographic printing ? 
 However this may be, as it is not in my power to pursue these 
 researches any further, I submit the process to your judgment in its 
 present imperfect state, and my sincere desire to contribute to the 
 progress of photography must be my excuse. The following then 
 is a short, but I believe sufficient description, of the modus operandi. 
 I purposely omit mentioning those precautions which are familiar to 
 all careful operators. 
 
 44 1st. — Dissolve one part of pure gelatine, and one part of clarified 
 
394 
 
 TEA 
 
 uncrystallizable sugar (golden syrup of the grocers), in ten parts of 
 boiling water, and pour out the hot solution in a shallow pan. 
 
 “ 2nd. — Float for a few minutes on the hot solution a sheet of 
 H oiling worth’s thin negative paper, previously well dried, 
 
 “ 3rd. — Draw off the paper, holding it vertically, at a short dis- 
 tance from the fire, until the superabundant liquid has ceased to 
 drain off, 
 
 “ 4th. — Lay it out horizontally on a cold slab, until the gelatine 
 has firmly set. 
 
 “ 5th. — Meanwhile take the image to be transferred fresh from the 
 mercury box, and having washed it first in the solution of hypo- 
 sulphite, and then in water, put it on end to drain, until the forma- 
 tion of the horizontal water line marks that the liquid on the surface 
 is reduced to a mere film. 
 
 “ 6th. — Lay the gelatine paper on the image, pressing it down 
 firmly and evenly with a soft cloth, until it is brought at every 
 point in perfect contact with the surface of the metal. 
 
 “ 7th. — After a few minutes peel off the paper. Some caution will 
 be required, as it will be found to adMfre rather firmly. 
 
 “ If the proof has been well selected, ana the manipulation success- 
 ful, every particle of reduced silver will be found transferred to the 
 surface of the gelatine, and a faint vestige of the original image will 
 alone be traced on the black and polished surface of the silver. 
 
 Ctf I say, if the proof has been well selected, for this is a point of 
 much importance. Of course the choice would fall on a full-bodied 
 proof, with thick and creamy lights, and rich opaque middle tints, 
 but this alone will not be found a sufficient guide, and it will be 
 advisable to pass a camel’s hair brush gently over some portion of 
 the washed image ; if the passing brush leaves a tract of black and 
 burnished metal behind, the transfer may be attempted, if not the 
 operation will not be successful. 
 
 “ On examining the transferred image by reflected light, it will 
 appear as a faint and somewhat shadowy transcript of the original 
 drawing, in which a careful inspection in a favourable light will 
 detect many details re-produced with great sharpness and delicacy. 
 By transmitted light, however, the semi-transparent nature of metallic 
 films of extreme tenuity will be found painfully evident. It is 
 indeed a faint negative, but it differs from those obtained by ordinary 
 processes in two most important particulars. In the first place its 
 lights are perfectly and absolutely pure, and in the second its half- 
 tints, however faint, are all represented by a metallic equivalent, 
 really and substantially existing on the surface of the gelatine, and 
 which, therefore, may become the basis of a chemical action, although 
 
TEA 
 
 TEE 
 
 395 
 
 too minute to be detected by the most careful inspection. The 
 colour of the metallic film varies greatly, generally approaching to a 
 reddish brown where it is most dense. This clearly points out a 
 fact for which we should hardly have been prepared, viz., that the 
 high lights in Daguerre’s image are in reality formed of two distinct 
 layers, the upper stratum being blanched by the action of the 
 mercury, and probably amalgamated with it, whilst the lower retains 
 the reddish hue which reduced silver sometimes assumes. The rosy 
 tint which is observable in the high lights of the finest proofs, when 
 seen obliquely, is thus explained. Considered as a basis of chemical 
 action, the transferred image is a sheet of gelatine, on which particles 
 of pure metallic silver, or of silver amalgam, are more or less densely 
 strewn. To increase the opacity of these particles, so as to render 
 them less permeable to the rays of transmitted light, is the problem 
 still to be solved, and for the solution of which three methods are 
 open : — First, to transform the metallic particles into some binary 
 compound, such as an oxide, a sulphuret, an iodide, or a chloride ; 
 secondly, to substitute for them thin chemical equivalents of 
 platinum or gold; and thirdly, to render them the centre of a 
 catalytic action, which shall group around them fresh molecules of 
 reduced silver. The very few experiments which I have been able 
 to make in these different directions have impressed me with the 
 belief that no very serious obstacle is to be apprehended. Thus I 
 have found that the action of iodine transforms the metallic film 
 into a saffron-coloured compound which is not altered by exposure 
 to light. Bi-chloride of mercury changes it into a greyish powder, 
 which is again darkened by a weak solution of ammonia, and the 
 tercliloride of gold increases considerably the intensity of the image, 
 but forms unfortunately, with the gelatine, a compound of a truly 
 Tyrean purple tint. 
 
 <c All these, however, are topics on which I need not dwell, as they 
 will naturally suggest themselves to the minds of those who may 
 deem the subject worthy of investigation.” 
 
 Tragacanth. See “ Gum Tragacanth.” 
 
 Transparent Cement. Dissolve 75 parts of caoutchouc in 
 60 parts of chloroform, and add 15 parts of mastic. 
 
 Treacle. The uncrystallizable sugar or syrup obtained from 
 the sugar cane. Golden syrup is a thin light-coloured treacle, better 
 adapted for photographic purposes than common treacle. 
 
396 
 
 TEI 
 
 ULT 
 
 Tripoli. The waxen veins, or Septorice ludi Helmonti , found on 
 the east coast of England, calcined; also the curl-stone of the 
 Staffordshire mines, calcined. It contains 80 per cent, of silica, and 
 is used for cleaning and polishing metals, &c. 
 
 Turmeric. The root of an Indian plant, the Curcuma longa. 
 The powder is orange yellow, and the tincture used*for making test- 
 papers, The colouring matter of turmeric is called “ curcu- 
 
 mine.” 
 
 Turpentine. Crude turpentine is a kind of balsam composed 
 of a resin and a volatile oil, and obtained as an exudation from the 
 wounded bark of various trees, but particularly the fir. It is 
 imported chiefly from America. 
 
 Oil of turpentine is obtained by distilling crude turpentine with 
 water ; the residue left in the still is common resin, and the volatile 
 oil passes over with the steam, with which it mixes. It is a limpid, 
 colourless liquid, S. Gr. *86, and boiling point 314°. It is neutral 
 to test paper, and almost insoluble in water, but is taken up to a 
 greater extent by absolute alcohol and ether. It mixes readily with 
 oils, and is very inflammable, depositing a dense soot, which is lamp 
 black, or carbon in a finely divided state. Its composition is C 10 H g . 
 It is a solvent of the resins, and to some extent of caoutchouc. 
 
 Oil of turpentine is acted on energetically by sulphuric and nitric 
 acids, and chlorine ; with hydrochloric acid it forms a curious com- 
 pound, called artificial camphor. It combines with iodine and 
 bromine. 
 
 Ultramarine. A magnificent blue pigment obtained from a 
 rare mineral, called “ Lapis lazuli.” 
 
 Ultramarine, Artificial. This substance, which is chiefly 
 composed of sulphide of sodium, is extensively used in the arts, 
 particularly by paper makers for giving a blue tint to paper. There 
 is a very large manufactory of it at Dusseldorf, where some hundreds 
 of men and women are employed. The following is the account of 
 the process, as described by Dr. Eedwood. 
 
 “Mix together 1 part of porcelain clay, 1| part of sulphur, 
 1 part of anhydrous carbonate of soda, and keep the mixture at a 
 dull red heat in a covered crucible as long as vapours are given off. 
 On opening the crucible it will be found to contain a spongy mass 
 part of which will be of a dark-blue colour, and this is to be sepa- 
 
UKA 
 
 397 
 
 rated from the other part. The results of this process are not 
 uniform, yet it is considered the best that has yet been published.” 
 
 Since photographic prints are readily destroyed by an alkaline 
 sulphide, it is evident that the above colouring matter should on no 
 account be added to photographic papers, and yet all the foreign 
 papers manufactured for photography are tinted with it. 
 
 Uranium. U=60. This metal is obtained from the mineral 
 termed Pechblende, which is an impure oxide of it, and also from 
 uranitic mica. The process consists in acting on the oxide with 
 potassium. It is obtained as a black powder which has a powerful 
 affinity for oxygen. The protoxide of uranium was for some time 
 mistaken for the metal itself, and is not by any means a costly 
 substance. 
 
 There are five oxides of uranium, viz. : — 
 
 Suboxide . 
 
 • • U 4 0 3 
 
 Protoxide 
 
 . . U 0 . 
 
 Black oxide 
 
 • • 0 5 
 
 Green oxide . 
 
 • • u 3 0 4 
 
 Peroxide . 1 
 
 
 Sesquioxide > . 
 
 , . U 2 0 3 
 
 Uranic acid. ) 
 
 
 The protoxide is a grey or brown powder obtained by passing 
 hydrogen over peroxalate of uranium at a red heat. The salts of 
 the peroxide are reduced to salts of the protoxide by the action of 
 light, as in the case of the iron persalts, so that in this respect 
 uranium and iron are analogous. The hydrated peroxide is a yellow 
 powder. 
 
 Uranium Glass. Glass is frequently coloured yellow by the 
 addition of oxide of uranium. It possesses the property of “ Fluo- 
 rescence,” q. v. 
 
 Uranium-Printing Process. A sheet of paper is first 
 rendered sensitive to light by immersing it in a strong solution of 
 a salt of the peroxide of uranium, (the nitrate is probably the best). 
 It is then dried, and exposed under a negative to direct light for 
 about the same time as an ordinary sun-print upon a chloride of 
 silver paper. A very faint visible image is thus obtained, which is 
 perceived by holding the paper against the light. The print is 
 then placed either in a weak solution of chloride of gold, or in a 
 strong solution of aceto-nitrate of silver. In the former case a pic- 
 ture is obtained of a purple inky tint, and in the latter casfe of a 
 
398 
 
 YAB 
 
 chocolate brown tint. The print is then washed in abundance of 
 water, several times renewed, and the operation is complete. 
 
 The theory of the process appears to be as follows : — 
 
 The uranium persalt is reduced by light to a protosalt, which, 
 when the print is placed in the gold or silver developer, becomes 
 again oxidized, and the gold or silver reduced, either to & purple 
 substance in the case of gold, or a brown substance in the case of 
 silver. The redundant chemicals are then removed by washing, no 
 fixing agent being necessary. 
 
 The principal objection to this process appears to be the difficulty 
 of obtaining good surface vigour, and fine definition ; there is also 
 the fear of the lights becoming discoloured in consequence of the 
 imperfect removal of the chemicals from the paper. Since no 
 hyposulphite of soda is employed in fixing, the prints may be con- 
 sidered much more permanent than ordinary sun-prints that have 
 been fixed and toned by hypo and gold. 
 
 Uranium prints developed with silver may be intensified by im- 
 mersing them in a solution of protosulphate of iron acidified with 
 acetic acid ; but the lights of the picture are very liable to become 
 discoloured if no fixing agent, such as hypo or cyanide, be employed. 
 
 The uranium printing process is- identical in principle with the 
 Chrysotype process of Sir John Herschel, published in 1842 ; q. v. 
 
 Yaknishes. An excellent practical account of the manufacture 
 of varnishes, by Mr. J. W. Neil, will be found in the 49th volume of 
 the Transactions of the Society of Arts. But the photographer is 
 not so much concerned with knowing the particulars of this manu- 
 facture in its various branches, as with knowing the best formulae 
 for making the two or three different kinds of varnish which are 
 used in photography. These are as follow : — 
 
 Benzole Varnish. This may be applied to glass plates without 
 the application of heat, and it dries very quickly, leaving a tolerably 
 hard film, which does not become sticky at ordinary temperatures. 
 It is made by adding finely pulverized gum dammar to pure benzole. 
 The dammar readily dissolves, and the varnish may then be filtered 
 through cotton wool to separate any solid particles there may be in 
 it. The proportions are about one ounce of dammar to a pint of 
 benzole. The varnish is applied to the plate exactly in the same 
 way as collodion. 
 
 Instead of dammar finely powdered amber may be employed, 
 and this varnish will be found better than amber dissolved in 
 chloroform, though probably not so good and more costly than that 
 made with dammar. 
 
VAR 
 
 399 
 
 Spirit Varnish. This is the best varnish for photographs upon 
 glass, but there is some little trouble and risk in applying it. It is 
 made thus : — 
 
 Put into a glass flask 
 
 Alcohol S. G. *825 . . 20 fluid ounces. 
 
 Pulverized white lac . . ounce. 
 
 Sandarach . . . li drachm. 
 
 Put the flask into hot water, having previously wrapped a piece 
 of paper loosely over the mouth of it. The ingredients are soon 
 dissolved, and may then be filtered by passing the varnish through 
 cotton wool. 
 
 Instead of white lac, seed-lac may be employed, but the varnish 
 is then of a darker colour. Methylated spirits may be used as 
 the solvent. Prench polish diluted with an equal part of alcohol 
 makes a good spirit varnish for negatives. 
 
 Before applying the varnish the plate must be dried and heated 
 before the fire to a temperature of about 100°; not more, or there 
 will be a risk of destroying the picture by causing it to run in 
 smears down the glass when the superfluous varnish is poured off 
 the plate into the bottle. On the other hand, if the plate be not 
 heated sufficiently, the varnish will be chilled, and produce the effect 
 of ground glass. The plate must be warmed again while the varnish 
 is drying. When spirit varnish is properly applied it forms an 
 exceedingly hard and tough film, which it is difficult to scratch or 
 injure during the process of printing, and which dries not become 
 sticky by heat. 
 
 Spirit varnish must never be applied a second time to a plate. 
 If this be attempted the picture will most probably be spoiled. 
 
 Black Varnish. This is made by dissolving in one bottle 
 powdered asphaltum in benzole, and in another, india-rubber in 
 benzole ; then adding the latter to the former in such proportion as 
 may seem best, the object of the india-rubber being to prevent the 
 black varnish from cracking. 
 
 The india-rubber should be cut into small pieces, and left two 
 or three days to dissolve in the benzole, which it does without heat. 
 
 Varnishes may be divided into two classes, — viz. fat varnishes, 
 and spirit varnishes ; and the latter class may be subdivided into 
 two, in one of which alcohol is the solvent, and in the other 
 turpentine. 
 
 The principal fat varnish used by painters is copal, and the 
 principal spirit varnish mastic, but neither of these is suitable for 
 photographic purposes. Varnish for maps and drawings may be 
 
400 
 
 YIN 
 
 YIS 
 
 made by adding turpentine to Canada balsam in about equal parts, 
 and gently heating the mixture. The paper should be sized with 
 gelatine before applying the varnish. Another paper varnish may 
 be made by digesting together — amber 300 parts, camphor 1 part, 
 alcohol 1500 parts. White lac dissolved in borax may also be used 
 as a paper varnish. Another kind of paper varnish may be made 
 from Xyloidine ; (q. v.). Paper may be varnished by floating it upon 
 albumen, drying it, and then coagulating the albumen by floating the 
 back of the paper upon boiling water. See also “ Leather Yarnish.” 
 
 Yinegar. See “Acetic Acid.” 
 
 Yision. (Latin, videre to see.) The phenomena of human 
 vision will be described under two heads, viz. monocular vision, and 
 binocular vision. 
 
 Monocular Vision. 
 
 Yision is said to be “ monocular” when only one eye is employed. 
 
 The human eye may be considered as a sort of spherical camera 
 obscura, in which the pupil, (or little black dot in the centre of the 
 eye), is the diaphragm, and the retina the focussing screen; this 
 admirable natural camera being then placed within a socket lined 
 with fat, in which it works by means of voluntary muscles that are 
 attached to it, and by which its axis is directed towards any point 
 with astonishing rapidity and precision. 
 
 The following account of the human eye is so excellent, that we 
 extract it verbatim from a Treatise on Optics published by the 
 Society for the Diffusion of Useful Knowledge. 
 
 “ The human eye, of which a vertical section is given in the follow- 
 ing figure, is nearly of a globular form, with a slight elongation 
 or projection in front. It con- 
 sists of four coats or membranes, 
 viz. the Sclerotic , the Cornea , 
 the Choroid , and the Retina ; 
 of two fluids or humours, the 
 Aqueous and the Vitreous ; and 
 of one lens, called the Crystal- 
 line. The Sclerotic coat, a a a, 
 is the outer and strongest coat, 
 to which the muscles for giving 
 it motion are attached. It con- 
 stitutes the white of the eye. It 
 is joined to the Cornea , b b } or the clear and transparent circular 
 
YIS 
 
 401 
 
 membrane through which we see. The cornea , which is equally 
 thick throughout, is very tough, and consists of several layers or 
 folds to give it strength, so as to defend the delicate parts within 
 from external injury. On the inner surface of the sclerotic coat 
 is a delicate membrane, called the Choroid coat, which is covered 
 with a black pigment. On the inner side of this lies the Retina , rrrr, 
 which is the innermost coat, and is a tender reticular membrane* 
 formed from the expansion of the optic nerve, which enters the eye 
 at O, a little more than one-tenth of an inch from the axis on the 
 side towards the nose. At the end of the axis of the eye, and in 
 the very centre of the retina, there is a small hole, with a yellow 
 margin. It is called the foramen centrale , or central hole, though 
 it is not a hole but merely a transparent spot, free of the soft 
 pulpy matter of which the retina consists. 
 
 “ A flat membrane of a circular form, e f called the iris, and seen 
 through the cornea b b , divides the interior globe of the eye into two 
 very unequal parts. It has a circular opening, in its centre, called 
 the pupil , which expands when the light which enters the eye is 
 diminished , and contracts when the light is increased. The space 
 before the iris, called the anterior chamber of the eye, contains the 
 aqueous humour , from its resemblance to pure water ; and the space 
 behind the iris is called the posterior chamber , and contains the crys- 
 talline lens, c c, and the vitreous humour, which fills all the rest of 
 the eye. The crystalline lens is suspended in a transparent capsule, 
 or bag, by what are called the ciliary processes, g g. This lens is 
 more convex behind than in front, as the figure shows ; and it con- 
 sists of concentric coats composed of fibres. It increases in density 
 from its circumference to its centre, for the purpose of correcting its 
 spherical aberration. The vitreous humour, Y Y, occupying the largest 
 portion of the eye, lies immediately behind the crystalline lens, and 
 fills the whole space between it and the retina, r r r r. 
 
 “ The following are the dimensions of the eye, as given by Dr. 
 Young and M. Petit : — 
 
 English inches. 
 
 Length of the optical axes , . . , . 0 91 
 
 Vertical chord of the cornea . • • . 0'45 
 
 Versed sine of ditto . . . , . . Oil 
 
 Horizontal chord of the cornea . „ 0‘ 47 
 
 Opening of pupil seen through the cornea . 0'27 to O' 13 
 
 Diminished by magnifying power of cornea to . 0'25 to 0'12 
 
 Radius of the anterior surface of the crystalline lens . . 0'30 
 
 Radius of the posterior surface .... 0*22 
 Principal focal distance of the lens . . . . 1’73 
 
 Distance of the centre of the optic nerve from the central hole at 
 
 the end of the axis . . . . .O'll 
 
 Distance of the iris from the cornea . . . . O' 10 
 
 Distance of the iris from the anterior surface of the crystalline . 0'92 
 Range of the eye, or diameter of field of vision . . 110° 
 
 D D 
 
402 
 
 VIS 
 
 “ Dr. Brewster and Dr. Gordon took the following measures of the 
 crystalline and cornea from the eye of a woman above fifty years of 
 age, a few hours after death. 
 
 Diameter of the crystalline .... 0-378 
 
 Diameter of the cornea . * • . 0-400 
 
 Thickness of the crystalline .... 0'172 
 Thickness of the cornea . . . . . 0-042 
 
 “ The following are the refractive powers of the humours of the eye, 
 according to different observers : — 
 
 Aqueous 
 Humour. 
 Hanksbee . 1*33595 
 Jurin .... 1*3333 
 Rochon . . . 1*329 
 Young . . . 1*3333 
 Brewster . . 1'3366 
 
 Crystalline Lens 
 
 Outer Coat Centre Mean 
 
 1-3767 1-3990 1*3839 
 
 Vitreous 
 
 Humour 
 
 1-33595 
 
 1-332 
 
 1-3394 
 
 “ From the last of these measures we may deduce the following in- 
 dices of refraction : — 
 
 Index of Refraction. 
 
 For rays passing from the aqueous humour into the outer coat 
 
 of the crystalline lens .... 1*0466 
 
 For rays passing from the aqueous humour into the crystal- 
 line, taking its mean index, of refraction . . . 1*0353 
 
 For rays passing from the outer coat of the crystalline into 
 
 the vitreous humour . . . . 0*93 
 
 cc From the dimensions of the eye given above, and by means of the 
 preceding indices of refraction, it will be easy to trace, by the 
 method already described, the progress of rays through the humours 
 of the eye, whether they fall upon it in a parallel or a diverging con- 
 dition. 
 
 “ Let MN, for example, be an object at a considerable distance from 
 
 the eye, EDO. Bays of light diverging from the points M, N, will 
 be converged by the refraction of the humours to points m , n, upon 
 the retina, where they will form an inverted image of it, in the 
 same manner as an image is formed in a camera obscura. That 
 such an image is actually formed on the back of the eye may be 
 easily proved by paring away the sclerotic coat of the eye of an ox, 
 with a sharp knife, till it is sufficiently thin to allow the image to 
 be seen through it. 
 
VIS 
 
 403 
 
 “ In what manner the retina, thus impressed with a distinct image 
 of an external object, conveys to the mind, through the medium 
 of the optic nerve, of which it is the expanded termination, a know- 
 ledge of the existence, the position, and the magnitude of that 
 object, is not known, and probably never will be. Certain facts, 
 however, or laws of vision, have been deduced from observation, 
 and merit our attentive consideration. 
 
 “ 1. On the direction of visible objects. — When the mind sees the 
 extremity M of any object M N, by means of rays flowing from M 
 and collected at m> the retina receives these rays at different degrees 
 of obliquity, and yet the point M is seen only in one direction, 
 namely, in the direction of the central ray of the cone whose apex 
 is at m. This however does not arise from the ray being the result- 
 ant, as it were, or the mean of the directions of all the other rays ; 
 for if we close up all the pupil excepting a small opening at its 
 margin, the point M will be represented at m only by the most 
 oblique rays of the conical pencil, and yet it will still be seen in the 
 same direction as before. Hence we conclude, that when a ray of 
 light falls upon any point m of the retina, in any direction, however 
 oblique to its surface, the object will be seen in the direction of a 
 line perpendicular to the retina at the point m. As the surface of 
 the retina is a portion of a sphere, these perpendiculars must all pass 
 through one point, which may be called the centre of visible direc- 
 tion ; because every point of an external object will be seen in the 
 direction of a line joining that centre and the given point. The 
 truth of this we have established by marking the perfect stability of 
 the image of any object, when it is seen by different points of the 
 retina when the eyeball alone is moved. Hence the centre of visible 
 direction is a fixed point in the vitreous humour ; and as it never 
 changes its place during the rotation of the eyeball, it must be 
 coincident with the centre round which that rotation is performed. 
 In consequence of this coincidence, and in virtue of the law of visible 
 direction, an arrangement of consummate skill, the great Author of 
 nature has provided for the perfect stability of every point in the 
 images of external objects. 
 
 “ 2. Cause of erect vision. — As the humours of the eye act 
 exactly like a convex lens of an equivalent focal length, an inverted 
 picture of external objects will, for the reasons already assigned 
 be formed upon the retina. Many philosophers of eminence have 
 perplexed themselves very unnecessarily, in attempting to deduce 
 erect vision from inverted images. The law of visible direction 
 removes at once every difficulty ; N for as the lines of visible direction 
 must necessarily cross each other at the centre of visible direction, 
 
 D D 2 
 
404 
 
 VIS 
 
 those from the lower part of the image must go to the upper part 
 of the object, and those from the upper part of the image go to the 
 lower part of the object, and hence an erect object is the necessary 
 result of an inverted image. 
 
 “ 3. Distinct and indistinct vision in the same object. — When we 
 look intensely at any point of an object in order to examine it with 
 care and attention, we direct to that point the axis of the eye, and 
 consequently, the image of that point falls upon the central hole in the 
 retina. Every other point of the same object is seen indistinctly, 
 and the indistinctness increases with the distance of the point from 
 that which is seen distinctly. The only perfectly distinct point of 
 vision, therefore, is that where there is no retina ; but we are not 
 entitled to ascribe this to the absence of the nervous matter, as the 
 gradual increase of distinctness towards the central hole does not 
 appear to be accompanied with a gradual dimunition in the thickness 
 of the retina. 
 
 “ 4. Indistinctness of vision at the base of the optic nerve. — It was 
 discovered by M. Mariotte, that when the image of any object fell 
 upon the base of the optic nerve, the object disappeared. In order 
 to prove this experimentally, fix on the side of a room, and at the 
 height of the eye, three wafers, two feet distant. Stand opposite 
 to the middle wafer with one eye shut, and, beginning near the 
 w T all, retire gradually from it, (looking always at the outside wafer 
 which is on the same hand as the covered eye,) till the middle wafer 
 disappears. This will be found to take place at about five times the 
 distance at which the wafers are placed, and when it does happen, 
 the other wafers will be plainly seen. If we use candles in place of 
 wafers the middle one will not disappear, but it will become a 
 cloudy mass of light. The base of the optic nerve, therefore, is not 
 insensible to light, it is only unfit for giving distinct vision of those 
 objects whose images fall upon it. M. Le Cat considered the size 
 of this portion of the retina to be about one-third or one-fourth of 
 a line ; but Daniel Bernoulli found it to be about one-seventh part 
 of the diameter of the eye.” 
 
 The foregoing explanation is clear and accurate, with the excep- 
 tion of some remarks on the direction of visible objects, in para- 
 graph 1. These it is important for us to correct. When a pencil 
 of light from a luminous point M enters the eye and is brought to 
 a focus at m upon the retina, the line of visible direction is not the 
 axis of the cone of rays within the vitreous humour of the eye, but 
 a line drawn perpendicular to the retina at the point m, and which 
 passes through the centre of the eyeball, or, which is the same 
 thing, through the centre of visible direction, as correctly stated in 
 
VIS 
 
 405 
 
 the latter part of the paragraph referred to in terms which evidently 
 contradict the former part of the same paragraph. If the reader 
 will draw a figure showing the course of an oblique pencil through 
 the eye, he will perceive that the axis of the final cone of rays within 
 the vitreous humour is a line joining the point m, with a point some- 
 where near the pupil of the eye, while the line of visible direction 
 is a line joining m with a point near the centre of the eyeball. 
 When the retina receives a blow at m, no matter how obliquely, 
 the direction of the supposed force is referred by the mind to a line 
 perpendicular to the retina at m. 
 
 Binocular Vision. 
 
 When both eyes are employed simultaneously vision is said to be 
 “Binocular.” The principles of it are easily explained, and will be 
 understood by means of the following figure, and a few words of 
 explanation. 
 
 Let L, R, be the two eyes employed simultaneously. Then only 
 one point can be seen distinctly at any instant of time, and that is 
 the point to which both the optic axes 
 are directed and at which they meet, as 
 shown by the point A in the figure. If 
 B, C, be other points, then the mind is 
 simply warned of their existence by their 
 images upon the sides of the retinae, and 
 distinct vision of them is not obtained. 
 
 But the eyeballs are capable of being 
 turned in their sockets with extreme ra- 
 pidity and precision, and the optic axes 
 may be united at several points in suc- 
 cession by muscular motions which are 
 almost instantaneous, so that it is by the 
 comparison which the mind is able to 
 form of the efforts made by the voluntary 
 muscles in enlarging or contracting the 
 angles LAR, LBR, LCR, that the most 
 certain estimate of the comparative dis- 
 tance of near objects is obtained. Hence 
 arises the important difference between monocular and binocular 
 vision, for by means of the latter additional certainty is given to 
 our appreciation of distance, and hence arise all the remarkable 
 phenomena of stereoscopic effect. 
 
 As a proof that only one object is seen distinctly at a time, it is 
 
 
406 
 
 VIS 
 
 WAS 
 
 only necessary to hold up the forefinger at a distance of a foot from 
 the nose, and look intently at some object several yards beyond it ; 
 the finger will then appear double and transparent, distant objects 
 being seen through it indistinctly. 
 
 No idea of the distance of an object can be obtained from the 
 changes which take place in the focussing arrangements of a single 
 eye to suit the different distances of objects between the limits of a 
 few inches and infinity. 
 
 Visual E ays. See “ Perspective.” 
 
 Vitriol. “ Blue vitriol ” is sulphate of copper ; “ green vitriol ” 
 proto-sulphate of iron ; “ oil of vitriol ” sulphuric acid ; and “ white 
 vitriol ” sulphate of zinc. 
 
 Volume. The space occupied by any solid body is called its 
 “ volume.” 
 
 Volume has three dimensions, viz., length, breadth, and thick- 
 ness; Area two dimensions, viz., length and breadth; Length only 
 one dimension ; and Number no dimensions. 
 
 Vulcanized India-Lubber, or Gutta-percha. These con- 
 tain sulphur, added chiefly for the purpose of diminishing the sticki- 
 ness of the surface. 
 
 A Washing Prints. The following mode of washing prints 
 appears to be the best that has yet been devised. The account 
 of it was given by the author in Photographic Notes, No. 21. 
 
 “ After a print has been fixed in hypo it is of the utmost impor- 
 tance to remove every trace of hypo from the paper, for if any be 
 left it will cause the print to fade. In order to remove the hypo, 
 the print is generally washed in water changed several times, and 
 allowed to remain in water several hours. But when a great number 
 of prints are to be washed at the same time, a separate dish cannot 
 be provided for each, so it is customary to put a number of prints 
 together in the same vessel of water. When this is done they soon 
 settle to the bottom, and lie there so closely packed that the water 
 cannot easily get between them and soak out the hypo. It is only 
 when the water is agitated by being changed at intervals, and for a 
 few minutes after each change that it gets fairly between them and 
 acts to advantage. It is desirable therefore to keep the water in a 
 state of constant agitation, so that the prints may not lie together ; 
 and this is more particularly desirable since it is found that damp 
 
WAT 
 
 407 
 
 favours the action of hypo in causing a print to fade. The ob- 
 ject is therefore to remove the hypo as quickly and effectually as 
 possible. 
 
 “ Yarious plans have been proposed for accomplishing this end; 
 and when a constant supply of water is obtainable, as it is in most large 
 towns, it has been a common practice to leave a tap constantly run- 
 ning into the vessel which contains the prints, the overflow passing 
 into a sink beneath. But the following plan is a great improvement 
 on this, because the prints are caused to circulate freely round the 
 vessel without sticking together, so long as the tap is kept run- 
 ning : — 
 
 “ The vessel is made as in the figure, with sides slightly inclined 
 outwards, like a washing tray. It should be square, and not too 
 deep, or too large for the supply which 
 is to run into it. The jet of water 
 from the tap is directed obliquely 
 against the further side of it near the 
 left-hand corner; it is then reflected 
 to the adjoining side, and thus a ro- 
 tatory motion of the fluid in the direc- 
 tion of the arrows is established and 
 maintained. The vessel is of course 
 always full, and the surplus runs over 
 the edge, as shown in the figure. The prints immersed are in this 
 way kept in a state of constant rotation, not en masse , but each 
 print following its own particular course, without adhering to its 
 neighbour. In order to insure the change of the water at the 
 bottom of the vessel it may be well to make a little hole about 
 the size of a cribbage peg, as shown in the figure. 
 
 “ This method of washing paper proofs is extremely simple and 
 effective.” 
 
 Water. HO = 9. Water may be considered as the protoxide 
 of hydrogen, being composed of one atom of oxygen and one of 
 hydrogen ; or, which is the same thing, of two volumes of hydrogen 
 and one of oxygen. 
 
 Water in its ordinary state exists either as rain, or river, or spring 
 water, 
 
 Bain- water always contains carbonic acid, ammonia, organic 
 matters, and sometimes nitric acid. It becomes putrid when kept. 
 If collected in leaden vessels oxide of lead is readily formed and a 
 small quantity dissolved by it. Bain- water is not suitable for the 
 nitrate bath or developer, although perhaps better than spring- 
 
408 
 
 WAX 
 
 water since it does not contain salts which form precipitates with 
 nitrate of silver. 
 
 Eiver and spring waters contain various salts and organic impuri- 
 ties ; the principal salts being chloride of sodium, sulphate of lime, 
 and carbonate of lime dissolved in carbonic acid ; these produce pre- 
 cipitates or turbidity in the water when nitrate of silver is added to 
 it. Carbonate of lime may be thrown down by boiling the water, 
 which expels the carbonic acid which keeps it in solution. Water 
 is said to be “ hard ” when it contains salts of lime. 
 
 Water may be purified by distillation and condensation of the 
 steam, provided it does not contain volatile oil or empyreumatic 
 matter, which impurities are generally present in the distilled water 
 obtained by condensing the steam from steam boilers. Perfectly 
 pure distilled water for delicate chemical operations is obtained by 
 the redistillation of ordinary distilled water in silver vessels at a low 
 temperature. It is then perfectly neutral to test paper, and a cubic 
 inch of it at 60° weighs 252*5 grains. 
 
 Water is very slightly compressible, and when submitted to 
 sudden and violent compression becomes luminous. It freezes at 
 82°, and boils at 212°. In freezing, or crystallizing, it expands in 
 bulk, therefore ice floats. Water, in freezing, rejects impurities, or 
 salts held in solution, therefore melted ice is very pure water. The 
 combustion of 2 volumes of hydrogen and 1 of oxygen produces 2 
 volumes of steam. At a mean atmospheric pressure, and tempera- 
 ture 212°, the bulk of steam is 1689 times that of water. 
 
 Wax. This substance is supposed to be produced by the bee 
 from the honey which it collects. Its composition is stated by 
 Lowig as C 34 H 34 0 2 ; but it varies, although in every case but one 
 there are as many equivalents of carbon as of hydrogen. 
 
 Bees’ wax in its ordinary state is yellow, but is bleached by being 
 exposed in thin ribands to the joint action of air, light, and moisture. 
 Sometimes, however, it is bleached more expeditiously by mixing 
 with it nitrate of soda and dilute sulphuric acid, and then traces of 
 sulphate of soda and nitric acid are generally retained. Wax melts 
 at 150°, and is almost entirely soluble in boiling alcohol, and ether; 
 but on cooling the alcohol only retains a small quantity, and the ether 
 none. When caustic potass is added to melted wax, a soapy gela- 
 tinous substance is formed which is soluble in a large quantity of 
 water. On adding an acid an oily liquid forms, which solidifies on 
 cooling and is soluble in hot alcohol; it is probably the “ Ceraine” 
 of Ettling. Succinic acid is obtained by digesting wax for some 
 days with nitric acid. 
 
WAX 
 
 WHI 
 
 409 
 
 Many vegetables secrete wax from their leaves and stems. In- 
 stances of this occur as myrtle-wax, palm-wax, Japaifr-wax, Ocuba- 
 wax, sugar-cane-wax, cork-tree-wax, &c. 
 
 White wax is commonly sold in round flat cakes, and is frequently 
 adulterated with tallow, stearine, &c. 
 
 Wax may be mixed in any proportions with oil of turpentine. 
 Positive prints, or sheets of paper rubbed with this mixture, acquire 
 a high polish on the evaporation of the turpentine. The ancient 
 fresco paintings upon the walls of houses were varnished and pre- 
 served with wax. See “ Encaustic Painting. 55 
 
 Waxed Paper Process. See “ Calotype Process, 55 Class 2. 
 
 Waxing Apparatus. In the paper negative processes the 
 paper is generally waxed either in the first or last stage of the opera- 
 tion. The best mode of doing this is to procure an oblong apparatus 
 of zinc or pewter, made exactly on the principle of a hot water 
 plate. This is filled with boiling water, and placed over the flame 
 of a spirit lamp to maintain the temperature. The sheet of paper, 
 or paper negative to be waxed is then laid upon the bottom of the 
 upper part of the apparatus, and a cake of wax rubbed over it, until 
 the paper has completely imbibed the wax. It is then removed, and 
 a fresh sheet introduced. When a sufficient number of sheets have 
 been treated in this way, they are held, one at a time, before the fire 
 and the superfluous wax which runs off is collected in a saucer. They 
 are afterwards ironed between sheets of blotting paper, with a 
 moderately hot iron, until no shining patches of wax appear on 
 either side of the paper. 
 
 Whey. See “ Serum of Milk. 55 
 
 White Enamel. Glass rendered white and opaque by the 
 addition of oxide of tin. When ground it forms a beautiful substance 
 to print upon, the negative being copied by a lens, and the wet 
 collodion process employed. 
 
 White Lead. Carbonate of lead. This substance should 
 never be ground by hand, but by machinery, as the minute particles 
 which escape into the air are fearfully injurious to the health. The 
 pallid, sickly appearance of painters is mainly attributable to the 
 extensive use of white lead in paint. Zinc paint is not open to the 
 same objection. 
 
410 
 
 WHI 
 
 WOO 
 
 White Fire. Mix together 
 
 ' 24 parts of saltpetre. 
 
 7 parts of sulphur. 
 
 2 parts of realgar (red sulphide of arsenic). 
 
 Wood Alcohol. This substance, which is the alcohol of the 
 methyl series, is sometimes called “ wood naphtha, 5 ’ and “ pyro- 
 xylic spirit. 55 It is one of the substances obtained from the 
 destructive distillation of wood, and is purified by redistillation over 
 quicklime. To obtain it pure, Kane adds to it chloride of calcium 
 in excess, and distils the mixture on a water bath as long as volatile 
 matter goes over. (This consists of pyro-acetic spirit and other 
 products.) The compound of wood alcohol and chloride of calcium 
 remains in the retort. To this he adds a quantity of water equal to 
 that of the original spirit, and continues the distillation. The pro- 
 duct which now passes over is pure wood alcohol diluted with water, 
 which may be separated by redistillation over quick lime. 
 
 Wood alcohol is a limpid liquid, having an aromatic taint, resem- 
 bling that of peppermint. Its S. G. at 60° is *8 00. It mixes in 
 all proportions with water, alcohol, and ether; is neutral to test 
 paper; permanent in the air; and boils at 150°. 
 
 This alcohol is extensively used as a solvent in making varnishes, 
 and also collodion. See “Methylated Spirits.” It closely resembles 
 common alcohol in most of its reactions, but is not liable to duty 
 on account of its hot and pungent taste. 
 
 Wood-Vinegar. See “Pyroligneous Acid. 55 
 
 Wood, Photographic Engraving upon. The wood engraver 
 would in many cases derive valuable help from photography, if 
 positive prints could be obtained upon the surface of wooden blocks 
 without injury to the texture of the .wood. The following process, 
 which appears to be the best at present known, is extracted from 
 Photographic Notes, No. 40, and is due to M. Lallemand, a well 
 known wood engraver in Paris. 
 
 “ The inventor of the process which we are about to describe, 
 M. Lallemand, is a skilful engraver. In consequence of his frequent 
 transactions with the editors of works, in the illustration of which 
 wood engraving is often employed, he endeavoured to solve the 
 problem stated above. But at first two difficulties presented them- 
 selves. In the first place it was necessary that the wood should 
 not be affected by the photographic chemicals ; and secondly, that 
 it should not be so coated or varnished with any substance as to 
 
XYL 
 
 YEL 
 
 411 
 
 interfere with the operations 'of the engraver. After more than a 
 year of fruitless experiments, M. Lallemand discovered a process 
 which is free from the above objections, and he has published it in 
 a communication made to the Academy of Sciences, in the following 
 terms : 
 
 “ ‘ The surface of the wood (and that only), is submitted to the 
 action of a solution of alum, and dried. The entire block is then 
 coated with a mixture of animal soap, gelatine, and alum. When 
 dry, the surface which is to receive the image is placed for some 
 minutes on a solution of hydrochlorate of ammonia, and allowed to 
 dry. It is next placed in a nitrate bath, containing twenty per 
 cent, of nitrate of silver, and dried in the dark. A negative, either 
 on glass or paper, is then applied to the sensitive surface of the 
 wood, in a pressure-frame made for the purpose, which allows the 
 progress of the printing to be watched. The image is fixed by a 
 saturated solution of hyposulphite of soda. A few minutes in this 
 solution will suffice. It is then washed for five minutes only' 
 
 “ The sizing protects the wood from any moisture, and eight 
 months’ experience has proved to the inventor that the employment 
 of alum and hyposulphite, instead of loosening the texture of the 
 wood, gives it a great toughness, which is favourable to engraving.” 
 
 Xyloidine, Nitramidine. This substance is obtained by the 
 action of nitric acid upon starch, or lignin. By acting on starch 
 with fuming nitric acid a transparent jelly is formed, and on adding 
 water xyloidine is precipitated as a white granular substance. A 
 portion of this is soluble in a dilute solution of caustic potass, and 
 yields a white flocculent precipitate on the addition of acetic acid. 
 
 Xyloidine is softened but not dissolved by boiling water, but is 
 soluble in concentrated sulphuric acid, and also in dilute nitric acid ; 
 concentrated acetic acid also dissolves xyloidine into a thick mucil- 
 lage, with which water forms a white coagulum. On evaporating 
 the acetic solution of xyloidine a transparent residue remains, which 
 forms a brilliant varnish when spread upon wood or paper, and 
 resists the action of water. 
 
 Xyloidine is nearly insoluble in alcohol. Its composition, accord- 
 ing to Ballot, is C 15 H 12 0 16 N. It is very inflammable, but not 
 explosive. Cold concentrated nitric acid converts lignin into a 
 substance very similar to the xyloidine obtained from starch ; the 
 protracted action of nitric acid on lignin, aided by heat, yields 
 oxalic acid. 
 
 Yellow Calico, Paper, &c. When used for photographic 
 
412 
 
 ZIjST 
 
 purposes the tint should verge on orange rather than green. Two 
 layers at least should always be employed, because white light passes 
 through the innumerable interstices between the fibres of the calico. 
 Yellow calico is generally bleached by light, and requires to be 
 renewed from time to time. 
 
 Zinc. Zn. = 32. A bluish-white metal of great utility in the 
 arts, and chiefly found either as an oxide, or sulphide. A sheet of 
 polished zinc is readily oxidized by exposure to damp air, but the 
 grey film of oxide formed protects the metal beneath from further 
 oxidation. Commercial zinc is generally contaminated with lead, 
 cadmium, iron, and carbon. Nearly all the metallic oxides are 
 reduced by zinc, which proves its high affinity for oxygen. 
 
 Zinc, Oxide of. Zn. 0=40. There is only one salifiable 
 oxide of zinc, viz. the protoxide, which has been called “nihil 
 album,” “ philosopher’s wool,” and “ flowers of zinc.” It is a 
 white, flocculent, tasteless substance, obtained by intensely heating 
 the metal in contact with air, when it burns with a bright flame and 
 produces the oxide. A pigment both in oil and water-colours is 
 made with it ; and it is found to combine with oil, and form a dryer 
 in the same way as litharge, being at the same time much less 
 injurious to the painter, and emitting less smell than white lead. 
 
 Oxide of zinc combines with water and forms a hydrate. It is 
 readily soluble in acids, and also in the caustic fixed alkalies, and in 
 pure and carbonated ammonia. 
 
413 
 
 TABLE of Elementary Bodies, with their Equivalents ; 
 
 ACCORDING TO BRANDE. 
 
 I 
 
 Symbol. 
 
 Name of Substance. 
 
 Equivalent. 
 
 i 
 
 Symbol. 
 
 Name of Substance. 
 
 Equivalent. 
 
 Al. 
 
 Aluminium . . . 
 
 14 
 
 Hg. 
 
 Mercury(Hydrargyrum) 
 
 100 
 
 Sb. 
 
 Antimony (Stibium) . 
 
 129 
 
 Mo. 
 
 Molybdenum . . . 
 
 48 
 
 As. 
 
 Arsenic . . . . ■ . 
 
 75 
 
 Ni. 
 
 Nickel 
 
 28 
 
 Ba. 
 
 Barium 
 
 69 
 
 N 
 
 Nitrogen .... 
 
 * 14 
 
 Bi. 
 
 Bismuth .... 
 
 213 
 
 Os. 
 
 Osmium , . 
 
 100 
 
 B 
 
 Boron 
 
 11 
 
 O 
 
 Oxygen 
 
 8 
 
 Br. 
 
 Bromine . . * . 
 
 78 
 
 Pd. 
 
 Palladium .... 
 
 54 
 
 Cd. 
 
 Cadmium .... 
 
 56 
 
 P 
 
 Phosphorus .... 
 
 32 
 
 Ca. 
 
 Calcium .... 
 
 20 
 
 Pt. 
 
 Platinum . 
 
 99 
 
 C 
 
 Carbon 
 
 6 
 
 K 
 
 Potassium (Kalium) . 
 
 40 
 
 Ce. 
 
 Cerium ..... 
 
 46 
 
 It 
 
 Rhodium .... 
 
 52 
 
 Cl. 
 
 Chlorine .... 
 
 36 
 
 Se. 
 
 Selenium * . 
 
 40 
 
 Cr. 
 
 Chromium .... 
 
 28 
 
 Si. 
 
 Silicium 
 
 15 
 
 Co. 
 
 Cobalt 
 
 30 
 
 Ag. 
 
 Silver (Argentum) . . 
 
 108 
 
 Ta. 
 
 Columbium (Tantalum) 
 
 185 
 
 Na. 
 
 Sodium (Natrium) . . 
 
 24 
 
 Cu. 
 
 Copper (Cuprum) 
 
 32 
 
 Sr. 
 
 Strontium .... 
 
 44 
 
 E 
 
 Fluorine .... 
 
 19 
 
 S 
 
 Sulphur 
 
 16 
 
 Gr 
 
 Grlucinum .... 
 
 5 
 
 Te. 
 
 Tellurium .... 
 
 64 
 
 Au. 
 
 G-old (Aurum) . . . 
 
 200 
 
 Th. 
 
 Thorium 
 
 60 
 
 H 
 
 Hydrogen .... 
 
 1 
 
 Sn. 
 
 Tin (Stannum) . . . 
 
 59 
 
 I 
 
 Iodine ..... 
 
 126 
 
 Ti. 
 
 Titanium .... 
 
 24 
 
 Tr. 
 
 Iridium 
 
 99 
 
 W 
 
 Tungsten (Wolfram) . 
 
 100 
 
 Ee. 
 
 Iron (Eerrum) . . . 
 
 28 
 
 U 
 
 Uranium .... 
 
 60 
 
 La. 
 
 Lantanum .... 
 
 44 
 
 Y 
 
 Yanadium .... 
 
 68 
 
 Pb. 
 
 Lead (Plumbum) . . 
 
 104 
 
 Y 
 
 Yttrium 
 
 32 
 
 Li. 
 
 Lithium . . '. 
 
 7 
 
 Zn. 
 
 Zinc 
 
 32 
 
 Mg. 
 
 Magnesium . . . 
 
 12 
 
 Zr. 
 
 Zirconium .... 
 
 23 
 
 Mn. 
 
 Manganese .... 
 
 28 
 
 
 
 
414 
 
 BEAUME’S HYDROMETER, OR AREOMETER. 
 
 TABLE SHOWING THE RELATION BETWEEN SPECIFIC GRAVITIES 
 and Degrees of Beaume’s Hydrometer, for Liquids 
 
 HEAVIER THAN WATER. 
 
 S. G. B. 
 
 S. G. B. 
 
 S. G. B. 
 
 S. G. B. 
 
 S. G. B. 
 
 1*000= 0 
 
 1*125 = 16 
 
 1*286 = 32 
 
 1*501 = 48 
 
 1-801 = 64 
 
 1*007= 1 
 
 1-134 = 17 
 
 1*298 = 33 
 
 1*526=49 
 
 1*823 = 65 
 
 1*014= 2 
 
 1*143 = 18 
 
 1-309 =34 # 
 
 1*532 = 50 
 
 1-847 = 66 
 
 1*022= 3 
 
 1*152 = 19 
 
 1*321 = 35 
 
 1-549 = 51 
 
 1*872 = 67 
 
 1*029= 4 
 
 1*161 = 20 
 
 r 1*334 = 36 
 
 1-566 = 52 
 
 1*897 = 68 
 
 1*036= 5 
 
 1*171 = 21 
 
 1-346 = 37 
 
 1-583 = 53 
 
 1*921 = 69 
 
 1*044= 6 
 
 1*180 = 22 
 
 1-359 = 38 
 
 1*601 = 54 
 
 1-946 = 70 
 
 1*052= 7 
 
 1*190 = 23 
 
 1-372 = 39 
 
 1*618 = 55 
 
 1*974 = 71 
 
 1*060= 8 
 
 1*199 = 24 
 
 1-384=40 
 
 1*637 = 56 
 
 03 
 
 c— 
 
 II 
 
 o 
 
 o 
 
 C3 
 
 1*067= 9 
 
 1*210 = 25 
 
 1-398 = 41 
 
 1*656 = 57 
 
 2*031 = 71 
 
 1-075 = 10 
 
 1*221=26 
 
 1*412=42 
 
 1*676 = 58 
 
 2*059 = 74 
 
 1*083 = 11 
 
 1*231 = 27 
 
 1*426=43 
 
 1-695 = 59 
 
 o 
 
 oo 
 
 ^3 
 
 II 
 
 ^3 
 
 1*091 = 12 
 
 1*242 = 28 
 
 1-440=44 
 
 1*714 = 60 
 
 
 1*100 = 13 
 
 1*252 = 29 
 
 1*454=45 
 
 1*736 = 61 
 
 
 1*108 = 14 
 
 1-264 = 30 
 
 1-470=46 
 
 1*758 = 62 
 
 
 1-116 = 15 
 
 1*275 = 31 
 
 1*485 = 47 
 
 1*779 = 63 
 
 
 In the above scale, 0 corresponds to pure water at 58° Eaht., 
 and the numbers 1, 2, 3, &c., correspond to water containing 1, 
 2, 3, &c., per cent, by weight of common salt. 
 
415 
 
 TABLE 
 
 Bor Liquids Lighter than Water. 
 
 S. G. B. 
 
 S. G. B. 
 
 S. G. B. 
 
 1-000=10 
 
 •896 = 27 
 
 •811 = 44 
 
 *993 = 11 
 
 •890 = 28 
 
 •807 = 45 
 
 *987 = 12 
 
 •885 = 29 
 
 * 802=46 
 
 *980 = 13 
 
 •880 = 30 
 
 • 798=47 
 
 •973 = 14 
 
 • 874=31 
 
 • 794=48 
 
 •967 = 15 
 
 • 870=32 
 
 •789 = 49 
 
 •960 = 16 
 
 • 864=33 
 
 •785 = 50 
 
 •954 = 17 
 
 • 859=34 
 
 •781 = 51 
 
 •948 = 18 
 
 • 854=35 
 
 •777 = 52 
 
 •942 = 19 
 
 •849 = 36 
 
 •772 = 53 
 
 •936 = 20 
 
 •844 = 37 
 
 •769 = 54 
 
 •930 = 21 
 
 •840 = 38 
 
 •764 = 55 
 
 •924 = 22 
 
 •834 = 39 
 
 •760 = 56 
 
 •918 = 23 
 
 • 830=40 
 
 •766 = 57 
 
 •913 = 24 
 
 • 825=41 
 
 •753 = 58 
 
 *907 = 25 
 
 •820 = 42 
 
 •749 = 59 
 
 •901 = 26 
 
 • 816=43 
 
 •745 = 60 
 
 The Areometer for liquids heavier than water is sometimes called 
 “pese-acide,” or “ pese-syrop for liquids lighter than water 
 “ pese-esprit.” 
 
416 
 
 ALCOHOL. 
 
 TABLE showing the Proportion by Weight of Absolute 
 Alcohol (S. G. *7938) contained in 100 parts of Spirits 
 OF DIFFERENT SPECIFIC GRAVITIES, AT 60° PaHRENHEIT. 
 
 S. G. 
 
 Per Cent, 
 of 
 
 Alcohol. 
 
 S. G. 
 
 Per Cent, 
 of 
 
 Alcohol. 
 
 S. G. 
 
 Per Cent, 
 of 
 
 Alcohol. 
 
 S. G. 
 
 Per Cent, 
 of 
 
 Alcohol. 
 
 •9991 
 
 0*5 
 
 •9638 
 
 26 
 
 •9160 
 
 51 
 
 •8581 
 
 76 
 
 •9981 
 
 1* 
 
 •9623 
 
 27 
 
 •9135 
 
 52 
 
 •8557 
 
 77 
 
 •9965 
 
 2 
 
 •9609 
 
 28 
 
 *9113 
 
 53 
 
 •8533 
 
 78 
 
 •9947 
 
 3 
 
 •9593 
 
 29 
 
 •9090 
 
 54 
 
 •8508 
 
 79 
 
 •9930 
 
 4 
 
 •9578 
 
 30 
 
 •9069 
 
 55 
 
 •8483 
 
 80 
 
 •9914 
 
 5 
 
 •9560 
 
 31 
 
 •9047 
 
 56 
 
 •8459 
 
 81 
 
 •9898 
 
 6 
 
 •9544 
 
 32 
 
 •9025 
 
 57 
 
 •8434 
 
 82 
 
 •9884 
 
 7 
 
 •9528 
 
 33 
 
 •9001 
 
 58 
 
 •8408 
 
 83 
 
 •9869 
 
 8 
 
 •9511 
 
 34 
 
 •8979 
 
 59 
 
 •8382 
 
 84 
 
 •9855 
 
 9 
 
 •9490 
 
 35 
 
 •8956 
 
 60 
 
 •8357 
 
 85 
 
 •9841 
 
 10 
 
 •9470 
 
 36 
 
 •8932 
 
 61 
 
 •8331 
 
 86 
 
 •9828 
 
 11 
 
 •9452 
 
 37 
 
 •8908 
 
 62 
 
 •8305 
 
 87 
 
 •9815 
 
 12 
 
 •9434 
 
 38 
 
 •8886 
 
 63 
 
 •8279 
 
 88 
 
 •9802 
 
 13 
 
 •9416 
 
 39 
 
 *8863 
 
 64 
 
 •8254 
 
 89 
 
 •9789 
 
 14 
 
 •9396 
 
 40 
 
 •8840 
 
 65 
 
 •8228 
 
 90 
 
 •9778 
 
 15 
 
 •9376 
 
 41 
 
 •8816 
 
 66 
 
 •8199 
 
 91 
 
 •9766 
 
 16 
 
 •9356 
 
 42 
 
 •8793 
 
 67 
 
 •8172 
 
 92 
 
 •9753 
 
 17 
 
 •9335 
 
 43 
 
 *8769 
 
 &S 
 
 •8145 
 
 93 
 
 •9741 
 
 18 
 
 •9314 
 
 44 
 
 •8745 
 
 69 
 
 •8118 
 
 94 
 
 •9728 
 
 19 
 
 •9292 
 
 45 
 
 •8721 
 
 70 
 
 •8089 
 
 95 
 
 •9716 
 
 20 
 
 •9270 
 
 46 
 
 •8696 
 
 71 
 
 •8061 
 
 96 
 
 •9704 
 
 21 
 
 •9249 
 
 47 
 
 •8672 
 
 72 
 
 •8031 
 
 97 
 
 •9691 
 
 22 
 
 •9228 
 
 48 
 
 •8649 
 
 73 
 
 •8001 
 
 98 
 
 •9678 
 
 23 
 
 •9206 
 
 49 
 
 •8625 
 
 74 
 
 •7969 
 
 99 
 
 •9665 
 
 24 
 
 •9184 
 
 50 
 
 •8603 
 
 75 
 
 •7938' 
 
 100 
 
 •9652 
 
 25 
 
 
 
 
 
 
 
417 
 
 ACETIC ACID. 
 
 TABLE Shewing the Specific Gravity of Acetic Acid at 
 Different Degrees of Dilution. {Thomson) 
 
 Atom of Acid. 
 
 Atoms of Water. 
 
 Specific Gravity at 60 °. 
 
 1 
 
 + 
 
 1 
 
 = 
 
 1-06296 
 
 1 
 
 + 
 
 2 
 
 
 1-07060 
 
 1 
 
 + 
 
 3 
 
 = 
 
 1 07084 
 
 1 
 
 + 
 
 4 
 
 = 
 
 1-07132 
 
 1 
 
 + 
 
 5 
 
 = 
 
 1-06820 
 
 1 
 
 + 
 
 6 
 
 = 
 
 1-06708 
 
 1 
 
 + 
 
 7 
 
 = 
 
 1-06349 
 
 1 
 
 + 
 
 8 
 
 = 
 
 1-05974 
 
 1 
 
 + 
 
 9 
 
 = 
 
 1-05794 
 
 1 
 
 + 
 
 10 
 
 = 
 
 1-05439 
 
 BAROMETER, SCALE IN MILLIMETRES AND 
 INCHES. 
 
 28 inches = 
 
 29 „ 
 
 30 „ 
 
 31 „ 
 
 1 millimetre = 
 1 inch — 
 
 711T87 millimetres 
 735-587 
 761-986 
 787-386 
 0-03937 inch 
 25-39954 millimetres 
 
 E E 
 
418 
 
 NITRIC ACID. 
 
 TABLE SHOWING THE QUANTITY OF NlTRIC ACID IN 100 
 parts or Dilute Acid of different Specific Gravities. 
 (Ure.) 
 
 Specific Gravity of 
 Dilute Acid. 
 
 No. of Parts 
 of 
 
 Liquid Acid, 
 
 S. G. 1-5 in 100. 
 
 Number of Parts 
 of 
 
 Anhydrous Acid in 100. 
 
 1-5000 
 
 100 
 
 79-7 
 
 1-4880 
 
 95 
 
 75-715 
 
 1-4730 
 
 90 
 
 71-730 
 
 1-4570 
 
 85 
 
 67*745 
 
 1-4385 
 
 80 
 
 63*760 
 
 1-4189 
 
 75 
 
 59-775 
 
 1-3978 
 
 70 
 
 55-790 
 
 1-3732 
 
 65 
 
 51*805 
 
 1-3477 
 
 60 
 
 47*820 
 
 1-3216 
 
 55 
 
 43-835 
 
 1-2947 
 
 50 
 
 39-850 
 
 1-2644 
 
 45 
 
 35-865 
 
 1-2341 
 
 40 
 
 31-880 
 
 1-2019 
 
 35 
 
 27*895 
 
 1-1709 
 
 30 
 
 23-900 
 
 1-1403 
 
 25 
 
 19-925 
 
 1-1109 
 
 20 
 
 15-940 
 
 1-0821 
 
 15 
 
 11-955 
 
 1-0540 
 
 10 
 
 7-970 
 
 1-0267 
 
 5 
 
 3-985 
 
 The intermediate parts may be found with sufficient accuracy by 
 simple proportion. 
 
TABLE OF THERMOMETRICAL EQUIVALENTS. 
 
 Faht. 
 
 Reaumur. 
 
 Centigrade. 
 
 
 212° 
 
 80° 
 
 100° 
 
 Water boils. 
 
 200 
 
 74*66 
 
 93*33 
 
 
 190 
 
 70*22 
 
 87*77 
 
 
 180 
 
 65*77 
 
 82*22 
 
 Alcohol boils. 
 
 170 
 
 61*33 
 
 76*66 
 
 
 160 
 
 56*88 
 
 71*11 
 
 
 150 
 
 52*44 
 
 65*55 
 
 
 140 
 
 48 
 
 60 
 
 Liquid Ammonia boils. 
 
 130 
 
 43*55 
 
 54*44 
 
 120 
 
 39*11 
 
 48*88 
 
 
 110 
 
 34*66 
 
 43*33 
 
 i 
 
 100 
 
 30*22 
 
 37*77 
 
 Blood heat ; Ether boils. 
 
 90 
 
 25*77 
 
 32*22 
 
 
 80 
 
 21*33 
 
 26*66 
 
 
 70 
 
 16*88 
 
 21*11 
 
 
 60 
 
 12*44 
 
 15*55 
 
 
 50 
 
 8 
 
 10 
 
 f Medium temperature of the 
 
 \ surface of the earth. 
 
 40 
 
 3*55 
 
 4*44 
 
 
 32 
 
 0 
 
 0 
 
 Water freezes. 
 
 20 
 
 -5*33 
 
 -6*66 
 
 
 10 
 
 -9*77 
 
 -12*22 
 
 
 0 
 
 -14*22 
 
 -17*77 
 
 Brine freezes. 
 
 -10 
 
 -18*66 
 
 -23*33 
 
 Brandy freezes. 
 
 &c. 
 
 &c. 
 
 &c. 
 
 To reduce Centigrade to Fahrenheit . Multiply by 9, divide by 5, 
 and add 32. 
 
 To reduce Fahrenheit to Centigrade. Subtract 32, multiply by 5, 
 and divide by 9. 
 
420 
 
 FREEZING MIXTURES. 
 
 Without Ice. 
 
 Muriate of Ammonia . 
 Nitrate of Potass 
 
 Water 
 
 , , 
 
 5 parts i 
 5 „ 
 
 16 „ J 
 
 • from + 50° to -*-10° 
 
 Nitrate of Ammonia . 
 Water 
 
 
 1 part 1 
 1 » J 
 
 j> from + 50° to +4° 
 
 Sulphate of Soda 
 
 Diluted Nitric Acid . 
 
 • 
 
 3 parts 1 
 2 » J 
 
 j> from+50° to —30° 
 
 
 With 
 
 Ice. 
 
 
 Snow . 
 
 Salt 
 
 . \ 1 
 
 2 parts ' 
 
 i » j 
 
 ^ to — 5 ° 
 
 Snow 
 
 Salt 
 
 Nitrate of Ammonia . 
 
 . » • 
 
 12 parts-) 
 5 „ 
 
 5 „ 3 
 
 1 to -25° 
 
 Snow 
 
 Chloride of Calcium . 
 
 
 4 parts 1 
 
 5 „ j 
 
 j> from 32° to -40° 
 
 Snow 
 
 Diluted Nitric Acid . 
 
 
 7 parts ] 
 4 » J 
 
 \ from 32° to -30° 
 
 Snow 
 
 Potassa . 
 
 
 3 parts ] 
 
 4 » J 
 
 j- from + 32® to -51° 
 
421 
 
 ENGLISH IMPERIAL MEASURES. 
 
 LONG MEASURE. 
 
 The imperial yard bears to the length of a pendulum vibrating 
 seconds of mean time in vacuo, at the sea level, in the latitude of 
 London, the ratio 36 : 39 T 3 93. 
 
 1 inch 
 
 12 = 1 foot 
 
 36 = 3 = 1 yard 
 
 72 = 6 = 2 = 1 fathom 
 
 198 = 16J = 5| = 1 pole, perch, or rod 
 
 7920 = 660 = 220 = 40 — 1 furlong 
 63360 = 5280 = 1760 = 320 = 8 = 1 mile 
 
 IMPERIAL MEASURE. 
 
 The imperial gallon is the volume occupied by 10 pounds avoir- 
 dupois weight of distilled water, weighed in air at 62° Eaht., the 
 barometer being at 30 inches. 
 
 Equivalents in Troy grains. 
 Distilled water 62° Faht. 
 
 1 pint 8750 
 
 2 = 1 quart 17500 
 
 8 = 4=1 gallon ..... 70000 
 
 16 = 8=2=1 peck .... 140000 
 
 64 = 32 = 8 = 4 = 1 bushel . . 560000 
 
 512 = 256 = 64 = 32 = 8 = 1 quarter . 4480000 
 
 APOTHECARIES MEASURE. 
 
 Troy grains. 
 
 Distilled water at 62° Faht. 
 
 1 minim ........ 0.91 
 
 60 = 1 fluid drachm . . . . . 54'7 
 
 480 = 8 = 1 fluid ounce . . . 437'5 
 
 9600 = 160 = 20 = 1 pint . . . 8750- 
 
 76800 = 1280 = 160 = 8 = 1 gallon . 70000- 
 
 IMPERIAL CUBIC MEASURE. 
 
 1 gallon 
 
 
 
 Equivalents in 
 cubic inches. 
 
 277-274 
 
 Equivalents in Troy grains. 
 Distilled water at 62° Faht. 
 = 70000 
 
 1 quart 
 
 = 
 
 693185 
 
 = 
 
 17500 
 
 1 pint 
 
 = 
 
 34-65925 
 
 = 
 
 8750 
 
 16 ounces 
 
 = 
 
 2772740 
 
 = 
 
 7000 
 
 1 ounce 
 
 = 
 
 1-73296 
 
 = 
 
 437-5 
 
422 
 
 ENGLISH IMPERIAL WEIGHTS. 
 
 The grain is the same in Troy, Apothecaries, and Avoirdupois 
 weights. 
 
 AVOIRDUPOIS WEIGHT. 
 
 Equivalent in Troy grains. 
 
 1 dram ..... 
 
 16= 1 ounce 
 
 256= 16= 1 pound 
 
 3584= 224= 14= 1 stone 
 
 28672= 1792= 112= 8= lcwt. 
 
 473440 = 35840 = 2240=160 = 20 = 1 ton 
 
 TROY WEIGHT. 
 
 1 grain. 
 
 24 = 1 pennyweight. 
 
 480 = 20 = 1 ounce.. 
 
 5760 = 240 = 12 = 1 pound. 
 
 APOTHECARIES WEIGHT. 
 
 Symbols. 
 
 1 grain ....... gr. 
 
 20 = 1 scruple ...... 3 
 
 60 = 3=1 drachm .... 3 
 
 480 = 24 = 8 = 1 ounce . . . % 
 
 5760 = 288 = 96 = 12 = 1 pound . . ife 
 
 27*34375 
 437-5 
 7000- 
 98000* 
 784000- 
 L5 680000* 
 
 Observe : 43 7 5 grains are considered equal to 1 ounce of nitrate 
 of silver ; that salt being sold according to the Dublin weights, in 
 which the scruple is only equal to 18 - 22 grains, and the ounce 
 equal in weight to a fluid ounce of distilled water at 62° Eaht. 
 
423 
 
 FRENCH WEIGHTS. 
 
 
 Troy Weight. 
 
 
 Troy Grains. 
 
 Lbs. 
 
 Ozs. 
 
 Dims. 
 
 Grains. 
 
 Millegramme 
 
 = -0154 
 
 
 
 
 
 Centigramme 
 
 = T543 
 
 
 
 
 
 Decigramme 
 
 = 1*543 =' 
 
 
 
 
 1-5 
 
 Gramme 
 
 = 15432 = 
 
 
 
 
 15-4 
 
 Decagramme 
 
 = 154*323 = 
 
 
 
 2 
 
 34 
 
 Hectograme 
 
 = 1543*234 = 
 
 
 3 
 
 1 
 
 43 
 
 , Kilogramme 
 
 = 15432*348 = 
 
 2 
 
 8 
 
 1 
 
 12 
 
 Myriagramme 
 
 = 154323*488 = 
 
 26 
 
 9 
 
 4 
 
 3 
 
 FRENCH MEASURE OF LENGTH. 
 
 
 English. 
 
 
 English Inches. 
 
 Miles. 
 
 Furlongs. 
 
 Yards. 
 
 Feet. 
 
 Inches. 
 
 Millimetre — 
 
 •03937 
 
 
 
 
 
 
 Centimetre = 
 
 •39371 
 
 
 
 
 
 
 Decimetre = 
 
 3-93708 
 
 
 
 
 
 
 Metre = 
 
 39-37079= 
 
 
 
 1 
 
 0 
 
 3-7 
 
 Decametre — 
 
 393-70790= 
 
 
 
 10 
 
 2 
 
 9*7 
 
 Hectometre= 
 
 : 3937079 = 
 
 
 
 109 
 
 1 
 
 1 
 
 Kilometre = 
 
 : 39370-79 = 
 
 
 4 
 
 213 
 
 1 
 
 10 
 
 Myriametre= 
 
 393707-9 = 
 
 6 
 
 1 
 
 156 
 
 0 
 
 0 
 
 The Metre is the length of a second's pendulum at Paris, 
 The Kilometre is rather more than halt a mile. 
 
 FRENCH MEASURE OF CAPACITY. 
 
 
 English 
 
 English Cubic Inches. 
 
 Gallons. 
 
 Pints. 
 
 Fluid Ozs. 
 
 Drms. 
 
 Minims. 
 
 Millitre = *0610= 
 
 
 
 
 
 16*9 
 
 Centilitre = *6103= 
 
 
 
 
 2 
 
 50 
 
 Decilitre = 6*1027= 
 
 
 
 3 
 
 4 
 
 13 
 
 Litre = 61*027 = 
 
 
 1 
 
 15 
 
 2 
 
 11 
 
 Decalitre = 610*27 = 
 
 2 
 
 1 
 
 12 
 
 5 
 
 51 
 
 Hectolitre = 6102*7 = 
 
 22 
 
 0 
 
 7 
 
 3 
 
 8 
 
 Kilolitre = 61027* = 
 
 220 
 
 3 
 
 13 
 
 7 
 
 30 
 
 Myrialitre =610270* = 
 
 2204 
 
 4 
 
 10 
 
 3 
 
 
 The French Litre is not quite an English Quart. 
 
• • 
 
Advertisements. 
 
 1 
 
 T. OTTEWILL & CO.’S 
 
 NEW TEAK CAMERAS FOR INDIA. 
 
 T. OTTEWILL and Co. beg to inform the Trade and Public generally, 
 that having made for the Government departments Capt. Fowke’s Cameras 
 with Teak, and having also made several improvements in the construction 
 of the same, they can highly recommend them as being the most portable 
 as well as the lightest cameras in use ; the 10 x 8, with single back, inner 
 frames, focussing screen, &c. is in the small compass, when closed, of 12J 
 inches x 10| x 3£, outside measurement. 
 
 T. OTTEWILL & Co’s new STEREOSCOPIC BOX CAMERA has 
 given general satisfaction, and they have already been favoured with very 
 great patronage for it; the box contains camera and lens six backs and 
 focussing screen, tripod head and board ; the board is so arranged that any 
 angle can be obtained. The box is 13£ inches long by 7 inches wide and 
 5 inches deep, outside measurement. 
 
 Their DARK BOX also continues to receive public favour for transferring 
 prepared plates from the box to the camera in sunshine, without injury. 
 
 Their REGISTERED DOUBLE-BODIED FOLDING CAMERA 
 continues to give, as it always has given, the greatest satisfaction, for its 
 simplicity and portability. 
 
 ILLUSTRATED PRICED CATALOGUE sent free on application. 
 
 T. OTTEWILL & Co.’s new PORTABLE TENT is the most efficient in 
 use, and is equally applicable to the Room as the Field. 
 
 BRASS BOUND CAMERAS, and every description of Photographic 
 Apparatus for India. 
 
 MANUFACTORY; 
 
 CHARLOTTE TERRACE, ISLINGTON, LONDON. 
 
2 
 
 Advertisements. 
 
 PHOTOGRAPHIC CHEMICALS, APPARATUS, & LENSES. 
 
 R. W. THOMAS, CHEMIST, &c., 
 
 MANUFACTURER OF PURE PHOTOGRAPHIC CHEMICALS, PREPARATIONS, 
 AND APPARATUS, AND SOLE MAKER OP THE 
 
 XYLO-IODIDG OF SILVER, 
 
 This well-known Collodion is used extensively at all the principal Photographic 
 Establishments, and by the most distinguished Photographers, both at home and 
 abroad. Its superiority is so well known that an enumeration of its qualities is now 
 hardly necessary. It may be as well however to state, that no preparation yet dis- 
 covered is so sensitive. It produces Negatives of great brilliancy and intensity, the 
 half tints in deepest shadow being at the same time clearly rendered, and works free 
 from spots, stains, or any other kind of blemish. 
 
 Testimonials from the best Photographers, and some of the principal scientific men 
 of the day, warrant the above assertions. 
 
 N ITRATE OF SILVER BATH for the above preparation can be procured of R. W. 
 THOMAS, ready made, at a cost little more than the price of ingredients used. 
 
 T HOMAS’S CRYSTAL VARNISH. — This valuable varnish for protecting negative 
 pictures, is applied without heat. It dries immediately ; the coating will be found 
 free from stickiness, hard, and transparent. 
 
 T HOMAS’S CYANOGEN SOAP — for removing all kinds of Photographic stains. 
 N.B. The genuine is made only by the inventor. 
 
 H YPO-COLOURING BATH— for permanently fixing and rendering the positives 
 on paper dark and rich in colour. 
 
 A LBUMINIZED PAPER carefully prepared. — Five- shilling sample quires of this 
 paper, put up in suitable cases for posting, can be had on receipt of 6s. 6d., 
 payable by Stamps or Post Office Order to RICHARD W. THOMAS. 
 
 I N the APPARATUS DEPARTMENT of this Establishment every kind of first-class 
 Photographic Apparatus may be seen, including : — 
 
 CAMERAS, folding and rigid, of superior construction. BRASS-BOUND CAMERAS 
 for India ; CAMERAS with RACKWORK Adjustment. 
 
 Jointed and other STANDS, of the best make. PRESSURE FRAMES. PHOTO- 
 GRAPHIC CHEMICAL CHESTS. 
 
 GLASS BATHS, arranged for carrying the Silver solution, thus dispensing with 
 the use of a Bottle and Funnel. STEWART’S IODIZING GLASS and Exhausting 
 SYRINGE. 
 
 GUTTA PERCHA BATHS, arranged for carrying the Silver solution for Plates up 
 to 15 by 12 inches. 
 
 Ditto ditto, for PLATES up to 15 by 12 inches, with moveable supports. 
 
 Vertical Porcelain BATHS, for Hyposulphite of Soda. GLAZING PRESSES, for 
 glazing Photographs when mounted on Card, size of steel plate, 13 by 10 inches. 
 
 Jointed LEVELLING- STANDS and SPIRIT-LEVELS. French and English 
 WEIGHTS, in sets. Camel-hair BRUSHES, round and flat. 
 
 COLLODION PLATE-HOLDERS, for preparing large Plates with facility. Pneu- 
 matic ditto. 
 
 PLATE-CLEANERS. COLLODION GLASSES, graduated for mixing the two 
 solutions, with wide mouth. 
 
 PLATE GLASS, of all sizes, bevelled edges. Deal and mahogany BOXES for 
 plate glass. 
 
 Graduated GLASS MEASURES, from 1 drachm to 1 quart; also, FRENCH 
 GRADUATED MEASURES, from 1 gramme to 1500 grammes. 
 
 A choice collection of PASSEPARTOUS, made expressly for this house, from origi- 
 nal patterns. Also, ELECTRO-GILT PASSEPARTOUS. 
 
 ALBUMINIZED and other PAPERS, French and English. A superior NEGATIVE 
 PAPER for CALOTYPE process. WAXP]D PAPER and IODIZED WAX PAPER. 
 FILTERING PAPER, for Silver and other Solutions. LITMUS PAPER, in boxes. 
 A great variety of Glass, Porcelain, and Gutta-Percha DISHES. Shallow and deep 
 Worcester China Dishes. 
 
 PORCELAIN DISHES for whole-sheet Canson Paper. Ditto, ditto, for half-sheet. 
 Also, a large assortment of ROSS’S PORTRAIT AND LANDSCAPE LENSES. 
 Caution. — Each Bottle of Xylo-Iodide of Silver is stamped with a Red Label bear- 
 ing my name and address, RICHARD W. THOMAS, Chemist, 10, Pall Mall — to 
 counterfeit which is felony. — Manufacturer of pure Photographic Chemicals, Prepara- 
 tions, and Apparatus. 
 
 Mr. Thomas’s ENLARGED PAPER OF INSTRUCTIONS, for the use of his 
 preparation of Collodion “ Xylo-Iodide of Silver,” sent free on receipt of Two 
 Stamps for postage, or may be had bound on receipt of Thirteen Stamps. Address, 
 RICHARD W. THOMAS, Chemist, &c., 10 , rail Mall. 
 
Advertisements. 
 
 3 
 
 NOVELTY IN PHOTOGRAPHY! 
 
 THE MINIATURE CAMERA. 
 
 THE LONDON STEREOSCOPIC COMPANY direct the attention of 
 Amateur and Professional Photographers to their 
 
 NEW POCKET STEREOSCOPIC CAMERA. 
 
 This Little Apparatus, with Focussing-glass, occupies a space of 4| ins. by 4 by 5. 
 With lens and Dark-slide, 8 by 43 by 6. 
 
 Ditto with six slides {or 12-groove box) and converging bars, 15 by 43 by 6. 
 
 Is equally applicable to the Wet or Dry Processes, and will be found the most compact 
 and complete piece of Apparatus yet contrived. 
 
 £ s. d. 
 
 Camera, Dark Slide, Focussing-glass and Parallel bars 2 2 0 
 
 Extra dark slides each 0 10 0 
 
 London Stereoscopic Company’s Double Achromatic Lens, with improved 
 
 stops 2 5 0 
 
 Stereoscopic View-Lens with stop 1 5 0 
 
 LONDON STEREOSCOPIC COMPANY, 54, CHEAPSIDE, E.C. 
 
 Just out , 
 
 ABC OF PHOTOGRAPHY. 
 
 Ninth Edition. Price Is., Free by Post. 
 
 This work is rendered as simple as it is possible, so that the amateur may not be 
 perplexed at the outset with any abstruse technicalties, but be enabled immediately, 
 after carefully perusing it, to manipulate for himself with success. 
 
 THE TRADE SUPPLIED. 
 
 Stereoscopes and Pictures in endless variety and at the lowest Wholesale 
 
 Prices. 
 
 BURGESS AND KEY, 
 
 95, NEWGATE STREET, LONDON, 
 
 MANUFACTURERS OF 
 
 GIITTA PERCHA PHOTOGRAPHIC BATHS, TRAYS, 
 
 &C. &C. &C. 
 
 B. & K., Inventors and Manufacturers of the Water-tight Baths, beg to caution the 
 Photographic profession against the many spurious imitations, which are not only 
 very imperfectly made, but of the most inferior Gutta Percha. 
 
 B. & K. warrant all theirs of pure Gutta Percha, and of the best workmanship, at 
 the following prices : — 
 
 Internal 
 
 
 
 
 
 With 
 
 Measurement. 
 
 
 
 Price. 
 
 tight top. 
 
 
 
 
 
 £ s. 
 
 d. 
 
 £ s. d. 
 
 2| X 
 
 33 . . 
 
 
 
 
 0 . 
 
 .066 
 
 43 x 
 
 53 . . . 
 
 
 
 . 0 2 
 
 6 . 
 
 ..076 
 
 53 x 
 
 73 . . 
 
 
 
 
 0 . 
 
 .096 
 
 7* X 
 
 103 . . 
 
 
 
 . O 6 
 
 0 . 
 
 . . 0 12 0 
 
 83 x 
 
 113 • • 
 
 
 
 
 6 . 
 
 . 0 13 6 
 
 93 x 
 
 123 . . • 
 
 
 
 . 0 8 
 
 6 . 
 
 . . 0 16 0 
 
 103 x 
 
 133 . 
 
 
 
 
 0 . 
 
 . 0 18 6 
 
 123 x 
 
 163 . . . 
 
 
 
 . 0 13 
 
 0 . 
 
 ..110 
 
 133 x 
 
 173 . . 
 
 
 
 
 0 , 
 
 .15 0 
 
 163 x 
 
 123 . . . 
 
 
 
 . 1 0 
 
 0 . 
 
 . . 1 10 0 
 
 Stereoscopic Bath, 33 
 
 X 9 
 
 
 . 0 4 
 
 6 . 
 
 .090 
 
 
 
 
 TRAYS. 
 
 
 
 
 
 £ 
 
 s. 
 
 d. 1 
 
 
 £ s. d. 
 
 43 x 
 
 6 . 
 
 . 0 
 
 2 
 
 6 12 X 14 . 
 
 .076 
 
 6 x 
 
 8 . . 
 
 , . 0 
 
 3 
 
 6 | 14 x 16 . 
 
 ..090 
 
 8 x 
 
 10 . . . 
 
 . 0 
 
 4 
 
 6 1 15 X 19 . 
 
 . 0 14 0 
 
 10 X 
 
 12 . . 
 
 . . 0 
 
 5 
 
 6 | 
 
 
 
 B. & K.’s New Collodion Plate Holder, the most perfect yet introduced, will take 
 plates from 5x 7, to 12 by 14, 5s. 
 
 Gutta Percha Work of every description, made to order. 
 
4 
 
 Advertisements. 
 
 1 
 
 PHOTOGRAPHIC CHEMICALS, APPARATUS, & LENSES, 
 
 WILLIAM BOLTON, 
 
 MANUFACTURER OF PURE PHOTOGRAPHIC CHEMICALS, PREPARATIONS, 
 AND APPARATUS, AND MAKER OF 
 
 COLLODIONS for POSITIVE and NEGATIVE PICTURES— These preparations 
 are highly sensitive and uniform in action, and are now most extensively used by 
 Photographers, producing the most satisfactory results. The Iodizing Solutions may 
 be had separately, in which state they keep good any length of time. 
 
 SOLUTION OF NITRATE OF SILVER FOR THE BATH. The above prepara- 
 tions can be procured of W. B., ready made. 
 
 CRYSTAL VARNISH— for protecting negatives, is applied without heat, and dries 
 immediately. 
 
 CYANOGEN PASTE— for removing all kinds of Photographic stains. 
 HYPO-COLOURING BATH— for permanently fixing and toning the positives on 
 paper. 
 
 ALBUMENIZED PAPER carefully prepared. — Three-shilling sample quires of this 
 paper, can be had by post on receipt of 4s. 6d. 
 
 IN THE APPARATUS DEPARTMENT maybe seen 
 Cameras, folding and rigid, of best construction. Brass-Bound Cameras for India ; 
 Cameras with screw Adjustment. New Portable Stereoscopic Cameras. Camera- 
 Stands of the best make. Pressure Frames. Photographic Chemical Chests. 
 Glass Baths (Water-tight), arranged for carrying the Silver solution. GuttaPercha 
 Baths, Water-tight. Glass, Porcelain and Gutta Percha Dishes in all sizes. 
 Vertical Porcelain Baths. Glazing Presses for glazing Photographs. Levelling 
 Stands and Spirit-Levels. French and English Weights, in sets. Camel-hair 
 Brushes. Pneumatic Plate -Holders. Plate-Cleaners. Collodion Bottles, 
 graduated. Plates, Glass, of all sizes. Deal and Mahogany Boxes for glass plates. 
 Graduated Glass Measures. A choice collection of Passepartous of superior quality. 
 Also, Electro-Gilt Passepartous. Photographic Papers, French and English. 
 Negative Paper for Calotype process. Waxed Paper and Iodized Wax Paper. 
 Genuine Papier Saxe. Filtering Paper. Litmus Paper. Boss’s Portrait and 
 Landscape Lenses. Lerebour’s Lenses. New German Lenses. 
 
 Illustrated Catalogue free on application. 
 
 WILLIAM BOLTON, (late BUTTON, and formerly DYMOND & Co.,) 
 
 0perattbe anti Manufacturing ^fjcnust, 
 
 146, HOLBORN BARS, & No. 1, HOLBORN BUILDINGS, LONDON. 
 
 PHOTOGRAPHIC CLASS. 
 
 JAMES HETLEY AND CO., 
 
 Supply every description of Glass used by Photographers, and of the very 
 best quality. 
 
 Their new List of Prices will be sent free on application to 
 
 JAMES HETLEY & CO., 
 
 6lass Merchants, 
 
 35, SOHO SQUARE, LONDON, W. 
 
 WHEN FOR BUSINESS PURPOSES, PLEASE ENCLOSE CARD, 
 
Advertisements. 
 
 5 
 
 PHOTOGRAPHIC INSTITUTION, 
 
 168, NEW BOND STREET, W. 
 
 Messes. Cundall, Howlett, and Downes desire to draw the especial 
 attention of Architects, Engineers, and Artists, and of all those who wish to 
 possess Photographs of places or objects of interest, to their present arrange- 
 ments for the production of any number of First-class Pictures at the 
 shortest possible notice. 
 
 The Negatives are taken in every instance by the members of the firm, who 
 also devote the greatest attention to the proper Printing of all Photographs 
 entrusted to their care. 
 
 They likewise supply Apparatus of the best description only; and give 
 Lessons in the various branches of the Photographic Art at moderate terms. 
 
 COUNTRY HOUSES, CHURCHES, WORKS in PROGRESS, VIEWS, 
 WEDDING GROUPS, PAINTINGS, &c.. Photographed. Each day’s 
 attendance. Three Guineas and Travelling expenses. 
 
 A SERIES of SIX LESSONS in the COLLODION and the PAPER 
 PROCESSES, including the PRINTING of POSITIVES. Five Guineas. 
 
 THREE LESSONS in the COLLODION PROCESS and PRINTING. 
 Each One Guinea. 
 
 A COMPLETE SET of APPARATUS for PICTURES, 8 in. x 6 in., 
 including a Ross’ Lens and Instruction, £20. 
 
 Larger complete Sets for Pictures from 10 in. x 8 in. to 15 in. x 12 in., 
 with a Portrait Lens and a Landscape Lens by Ross, including Instruction 
 in the Collodion and the Paper Processes, from £40 to £80. 
 
 Dark Box and Slide for the Oxymel Process, for holding prepared plates 
 and transferring them to the Camera without exposure to Light, adapted to 
 any Camera. 
 
 HOWLETT’ S PORTABLE TENT, which gives ample accommodation, 
 is easily put up and taken down, stands well against the wind, is waterproof, 
 and without stick in the centre, weighs only 16 lbs. Price £3 105.; or 
 with table £4 45. 
 
 THE CALOTYPE PROCESS. A Handbook to Photography on Paper, 
 by Thomas Sutton, b.a. Second Edition, carefully revised, with new 
 Chapters on Positive Printing. Price 25. 6d.; per post, 25. 8 d. 
 
 THE PHOTOGRAPHIC PRIMER; for the Use of Beginners in the 
 Collodion Process, by Joseph Cun dale. Second Edition. Price 15. post 
 free. 
 
 ON THE VARIOUS METHODS OF PRINTING PHOTOGRAPHIC 
 PICTURES UPON PAPER. With suggestions for their Preservation. 
 By Robert Howlett. Price 15. post free. 
 
 PHOTOGRAPHIC INSTITUTION, 
 
 168, NEW BOND STREET, and at Me. HOWLETT’S RESIDENCE, 
 10, BEDFORD PLACE, KENSINGTON. 
 
6 
 
 Advertisements. 
 
 BURFIELD AND ROUGH, 
 
 MANUFACTUBEBS OF 
 
 PURE PHOTOGRAPHIC CHEMICALS, 
 PREPARATIONS, AND APPARATUS, 
 
 AND SOLE WHOLESALE AGENTS FOE 
 
 MR. HARDWICH’S 
 
 ISTEQ-.A.TI'VE COLLODION. 
 
 180, STRAND (corner of Norfolk Street), LONDON, W.C. 
 
 BURFIELD & ROUCH respectfully solicit an Inspection of their Exten- 
 sive Stock of FIRST-CLASS PHOTOGRAPHIC APPARATUS and 
 LENSES. 
 
 COMPLETE SETS OF PHOTOGRAPHIC APPARATUS, from £3 
 to £30. 
 
 ALBUMINIZED PAPER of the Finest Quality, size 11^ x 91, 3s. 6 d. 
 per quire. A Sample quire sent free per post, on receipt of 3s. 6 d. in stamps. 
 
 MR. HARDWICH’S 
 
 PHOTOGRAPHIC PREPARATIONS. 
 
 NEW NEGATIVE COLLODION 
 
 May be obtained in two solutions, at 16s. per pint, including Bottles ; or, for 
 the convenience of Amateurs, in quantities not less than 8 ounces in two 
 solutions at 6$. 10c?., with Bottles. Full directions for use. 
 
 CADMIUM IODIZER, for occasional use with the above, 9c?. per ounce. 
 NEGATIVE NITRATE BATH, for the above. 
 
 MR. HARDWICH’S NEW POSITIVE COLLODION, in two solu- 
 tions, 16^. per pint, including Bottles, or 8 ounces in two Bottles, 65. 10c?. 
 
 POSITIVE NITRATE BATH (40-grain solution). 
 
 BURFIELD & ROUCH also direct the attention of Photographers to 
 their Preparation of 
 
 PURE RE-CRY STALL I ZED AND DRIED NITRATE 
 OF SILVER, 
 
 Which is unequalled as a Photographic Agent. 
 
Advertisements. 
 
 7 
 
 Now Ready , 
 
 ROUCH’S REGISTERED PORTABLE DARK OPERATING CHAMBER. 
 
 FOR WORKING IN THE OPEN COUNTRY. 
 
 This contrivance answers every purpose of a dark room, in place of which it may 
 be used either at home or in the open country. 
 
 It will be found preferable to any of the numerous forms of dark tent, all of which 
 are complicated and cumbersome, and very uncomfortable to work in. With the above, 
 the operator manipulates on the outside and not in, whilst the greatest steadiness is 
 ensured by the hands resting on a ledge when coating or developing a picture. One 
 of its principal recommendations is great simplicity, consisting only of a rectangular 
 box, in which the whole of the apparatus required on an excursion may be conveniently 
 packed. It may also be used as a stand on which the Camera can be placed. Size 
 No. 1 is especially recommended for taking Stereoscopic pictures, and is at present 
 extensively used for that purpose. Size No. 3, adapted for pictures 12 x 10 and under 
 may be seen at the above address. 
 
 For Opinions of the Press, see — “ Illustrated London News,” May 22, “Liverpool 
 and Manchester Photographic Journal,” July 1st., &c., &c. 
 
 Full description sent free by post. 
 
 Price of simple Stereoscopic Camera , with sliding front fitted on lathy with screw and 
 index, and which may he used on any box or stand, £2 5s. ; Ditto, fitted with Stereoscopic 
 Lens for Landscapes and Groups, £3. 
 
 Manufactured only by 
 
 BURFIELD & ROUCH, 180, STRAND (corner of Norfolk Street), LONDON. 
 
8 
 
 Advertisements. 
 
 Published on the 1st and 15 th of every Month , price 4id., 
 
 PHOTOGRAPHIC NOTES, 
 
 EDITED BY 
 
 THOMAS SUTTON, , 
 
 st.'brelade, jersey. 
 
 This Journal will be forwarded by the Editor , post free , on the 
 following terms : — 
 
 s. d. 
 
 ANY SINGLE NUMBER . . .04 
 
 QUARTERLY SUBSCRIPTION . ..20 
 
 YEARLY DITTO . . . .80 
 
 Payable in advance by Postage Stamps or Post Office Order. 
 
 LONDON. 
 
 Mr. A. ROSS, 2, FEATHERSTONE BUILDINGS, HOLBORN. 
 Mr. F. COX, 22, SKINNER STREET, SNOW HILL. 
 
 Messrs. HORNE & CO., 121, NEWGATE STREET. 
 
 Messrs. KNIGHT, 2, FOSTER LANE, CHEAPSIDE. 
 
 Mr. J. SOLOMON, RED LION SQUARE, HOLBORN. 
 
 NEW YORK. 
 
 Messrs. ANTHONY, 308, BROADWAY. 
 
 PARIS. 
 
 Mr. G. FOWLER, 6, RUE MONTPEN SIER. 
 
 AND THE PRINCIPAL PHOTOGRAPHIC FIRMS IN THE PROVINCIAL TOWNS 
 OP ENGLAND, SCOTLAND, AND IRELAND. 
 
 Vol. I., for 1856, 6s.; Vol. II., for 1857, 8s.