BROMIDE PAPER INSTRUCTIONS FOR CONTACT PRINTING AND ENLARGING. 1 Digitized by the Internet Archive in 2014 https://archive.org/details/bronnidepaperinstOOjust BROMIDE PAPER Instructions for Contact Printing and Enlarging BY DR. E. A. JUST FOURTH EDITION. Percy Lund, Humphries & Co., Ltd., The Country Press, Bradford; AND Amen Corner, Paternoster Row, London. 1896. PhRCY LL'KD, HUMPHRIES AND CO., LTD., THK COUNTRY PRESS, BRADFORD; AMEN CORNER, PATERNOSTER ROW, LONDON. BROMIDE PAPER: INSTRUCTIONS FOR CONTACT PRINTING AND ENLARGING. The Preservation of the Paper. Effecfl of Dust and Moisture. — The paper should be kept dry and cool in a light-tight and dust-proof box. Warmth and moisture greatly facilitate the development and growth of mildew. Without their assistance the germs, hovering about in the atmos- phere, and of which dust is largely composed, are unable to exercise any influence even upon such sensitive material as gelatine. The preservative box should be quite light-tight to give additional security in case the packets be insecurely wrapped. Black Wrapping Paper not Light-Tight. — If the box is not kept in the dark-room it will be necessary to wrap the sensitive paper in several thicknesses of black paper. A single wrapper will not suffice. Black paper, especially that which is enamelled, contains many small holes (produced by grains of sand in the paper-pulp) which allow light to pass through. By holding the paper up to the sun or bright sky, fine holes will be visible. But how many of the finest holes are present in the paper, and how far from light-tight it is, will be best seen by printing upon bromide paper through single, double and treble thicknesses of it. On development the emulsion paper exposed under one sheet of the black paper will usually be found more black than white ; under two sheets a few black spots will still be visible ; but if exposed under three thicknesses no acflion of the light should be noticeable. The author, in his experiments, has discovered that the strong enamelled black paper used for the packing of plates is not the best for the purpose, but that a non-enamelled paper, which he obtained, was decidedly preferable on account of its freedom from pin-holes. Pure Air in the Preserving Box. — The box for preserving the paper must not be used for keeping volatile chemicals, albumen paper, etc. It is a matter of necessity that the bromide paper should not be kept in a place where it is likely to be exposed to the 6 BROMIDE PAPER. destru(5live influences of damp, or an impure atmosphere impreg- nated with the fumes of ammonia, sulphuretted h3^drogen, essential oils, ordinary house gas, or acids. Photographers will often place bromide emulsion and albumen paper together in the same box, without taking into consideration the important fa(5\ that the sulphuretted hydrogen given off by the albumen paper will destroy the emulsion. If it is necessary to keep the two together, the bromide paper should be placed in a tin box or well wrapped up in tin foil or waxed paper for additional protedliion. Dwelling- rooms are unfit for the preservation of emulsion papers. The Advantage of Labels with Dates of Exposures. — Much time will be saved by attaching to each packet a label, upon which are noted the dates of exposure. The date of receiving should also be noted for the following reasons : — The permanency of bromide emulsion paper is very great in comparison with other sensitive papers, but it is not unlimited, as its sensitiveness gradually decreases. After a few months the paper will be found to work clearer and harder, and after one or two years it will become very insensitive. Paper should be rolled face inwards. — It is, of course, possible to roll the bromide paper either way, but owing to its sensitiveness to damp lingers, it is preferable to roll it with the film inwards, and it should be laid face downwards when cutting to size. The best arrangement for keeping the paper when cut to sizes is a light-tight box, construdled in such a manner that a second inner lid can be placed in loose and pressed down with screws. By this means the paper will always be kept perfecStly flat and well protecl:ed from atmospheric influences. Cutting and Handling the Paper. All operations should be condu(5led in a Photographic Dark-room, illuminated only by non-a(5i:inic light. The paper must not be exposed to even the dark-room light for a longer period than is absolutely necessary ; for both yellow and red rays of light will produce a latent eflecl: upon the sensitive paper by a lengthened exposure. It must also be remembered that many kmds of red glass also allow of the transmission of some blue rays, and in such cases the possibility of fog is considerably increased. For this reason, therefore, only red glass which has been examined by the specffroscope should be used for dark-room lighting. The film is very tender and sensitive. — Touching it with the damp hand, or mechanically hurting it by scratching it with a jagged knife or the finger nails must, therefore, be carefully avoided. Chloride of silver emulsion is peculiarly sensitive to finger- marks (perspiration), so with this paper the use of gloves is CUTTING AND HANDLING THE PAPER. 7 indispensable. The danger of damaging the paper by touching it with the finger is not, however, equally great with every person. With some the least touch is sufficient to damage the comparatively less sensitive bromide paper, whilst others may touch the paper with the dry finger tips without affecfting it in the least. This sensitive- ness of the film to the perspiration of the hands, is very consider- able if an acid or a neutral developer be employed. The ferrous oxalate developer especially, rapidly develops the least impression caused by it. With the alkaline developers — pyrogallol, hydro- kinone, etc. — this is not the case. It is evident that the fatty acids of perspiration become neutralised at once by the alkali. The cutting up of large sheets of emulsion paper into smaller sizes should not be done by folding and dividing with a paper-knife as with albumenized paper, as the least damage to the film becomes noticeable on developing, and every scratch becomes a black line. Rough edges are also easily produced by the use of a paper knife, and often cause considerable damage in the developing and washing operations, by rubbing against the films of the other pidl:ures. There are persons who, in their habitual carelessness, even cut paper on a table upon which they have been previously working with developers or fixing salts, and then the paper is, of course, " sent out full of imperfecftions." The film side of the paper may be recognised by its bright look in comparison to the non-prepared side, which has a granular appearance. Further, if there should be any doubt, by touching it (at one corner) with a moistened finger, the prepared side may be dete(51:ed by its slight stickiness. It is very important that the sheets should be laid film-side downwards in cutting, because it is thus easy to control the tendency of the paper to roll inwards. By adopting this method, the trouble of looking at the paper is saved when placing it in the frame. If, however, it is desired to examine the prepared side, it is advisable to hold the red lamp at one side, or as low down as possible, as by this means the shiny surface of the film is more readily detecfied, as is also the porous or non-prepared side, by the shadow of the grain. Exposing. — The purpose of exposing is the produ(5tion of a latent image from the negative. This latent image is not visible until after the subsequent chemical development. The acffion of the light is very rapid — it usually occupies but a few seconds. The more care must be taken, therefore, with regard to the necessary observations in exposing. These are : — I St. Exposing within the limits, so that the latent image produced by the light be neither too weak nor too strong. To do this we have to consider the transparency of the negative, the 8 BROMIDE PAPER. sensitiveness of the emulsion, and the choice and intensity of the ilhiminant. 2nd. The diffusion of the hght evenly over the whole surface of the negative and the even regulation of each of a series of exposures. Limits of Exposure. — As the image produced by the light is invisible, no method of judging the corre(51: exposure is possible except the experimental development of the image. No photometer sufficiently sensitive for the purpose has as yet been invented. In experimental development the intensity of the adfion of the light is perceived by the rapidity v^ith which the image appears, and by the details of the lights and shadows of the pidfure. Signs of Under-exposure. — The exposure has been too short if slow development, in the manner hereafter described, fails to produce an image containing all the detail visible in the negative in both high-lights and shadows. Signs of Correcft Exposure. — The exposure has been corredl if the image gives all the fine details observable in the negative, with perfecT: high-lights, sufficient depth of shadow, and everything in harmonious gradation. Signs of Over-exposure. — The exposure has been too long if, on developing, the image suddenly flashes up and the high-lights appear fogged. These remarks, however, need further explanation, because the relation of the effecft produced by the light to the method of developing can, within certain limits, undergo various modifications. The requisite intensity of the latent image produced by the light is governed by the strength of the developer to be used, in the same manner as a workman drawing up a heavy weight would take into consideration the muscular strength of a second workman, whose duty it is to help him by getting the weight to a certain height. In all parts where the light reaches the emulsion film, a chemical a(5fion takes place in the silver haloids. The negative, through which the light passes, varying considerably in opacity at different parts, offers various resistances to the passage of the light. Some portions, being nearly transparent, the light passes easily through, \^hilst with others, it experiences a difficulty owing to the opacity of the negative at those parts. The thinner portions of the negative will be so easily penetrated by the adlinic rays as to rapidly give rise to a strong chemical adfion. The denser parts, on the contrary, will obstru(5t the passage of the rays, weakening their influence upon the silver salts. In the one case the acftion of the light is easily made visible by a weak developer, whilst in the other case the light has adfed upon the silver in such a slight manner that a weak developer is insufficient to upset the chemical affinity of the silver haloid, but a more powerful developer will attain this end. EXPOSING. 9 Solarisation. — As soon as the acftion of the Hght reaches a certain point, solarisation takes place. By solarisation is meant the change in the haloid salts, by which after prolonged exposure they cease to develop. In order that this term, solarisation, be better understood, a short, theoretical illustration is required. I am of opinion that the effedl of the light, so long as it causes an invisible latent image, must be looked upon as a work mtrodu(5lory to chemical decomposition, but by no means producing the same. This view gives a clearer understanding of the other operations and readfions in the emulsion process than any other theory. This theory may be further described as follows : — The freshly produced, undigested emulsion contains, we know', the most powerfully resisting haloid salts, which require a consider- able light and a powerful developing agent for their redu(51:ion. The digestion of the emulsion is that warming process, which has for its objecft the loosening of the haloid salts, so that the work of the light is diminished. The latent image produced by the light may be caused merely by a continual loosening of the decomposed haloid from the more resistant qiiasi silver sub-haloid, which is kept together by greater chemical affinity. The greater the extent to which this loosening is carried the less becomes the work necessary for the developer. Solarisation cannot, therefore, be said to be due to over- exposure, but more corredfly to the completion of decomposition or separation of the haloids. The solarised silver haloid is easily recognised by the change in colour. Strongly solarised chloride of silver emulsion becomes a reddish tint, and bromide of silver emulsion assumes a faint blue colour. According to my view the various stages of solarisation perceptible in the image during development are as follow ; — Different Stages of Solarisation. — First Stage: The latent image is invisible previous to development. At this stage the molecules on the surface having received the greatest amount of exposure to the light, a separation or splitting up of the haloid takes place at these parts. After development, the image comes up strongly, except the shadow details, which are either veiled over or have disappeared. The deeper shadows of the pidlure no longer differ from the half-tones, the depth of tint being pretty even all over, for the reason that the haloid silver lying deep in the film of emulsion has succumbed to development, wdiilst the slightly solarised part already consists of sub-haloid which is either developed with difficulty or not at all. Second Stage : At this stage the image produced by the light is already visible to the eye previous to development ; at least, more corredlly speaking, the deeper shadows only are visible, having a reddish colour with chloride of silver emulsion. With bromide of silver an extremely faint bluish-grey image is barely visible. The lO BROMIDE PAPER. decomposition has advanced still further, the underlying molecules of the emulsion film in the deeper shadows have already been attacked, and on developing they, therefore, appear considerably lighter, whilst the previously invisible half-tones appear of the deepest colour, and the high-lights appear considerably shaded. Advanced Stage: The image produced by the light has long since ceased to be a "latent" or invisible one, as the picfture is distincflly perceptible before the acftion of the developer, although, perhaps, wanting detail in the high-lights. With chloride of silver emulsion this image appears in a strong, blue-red colour, and bromide of silver in a faint blue-grey tint. The decomposition has now advanced to the high-lights of the pi(5lure, causing them to be rapidly developed to the deepest shade. In the half-tones con- siderable separation or splitting up of the haloid has taken place, and only the deeper portions will develop, whilst the deep shadows, which have long^ since been converted into silver sub-haloid, recommence to loosen and again become susceptible to the acffion of the developer. The result of the whole process is usually to produce a negative image, although far from a perfedf one. It is generally what may be termed a mixed image still retaining some of the shades which belong to the positive, as by the decomposition which takes place the molecules at first lose their property of becoming developed, and afterwards approach a new state of separation and to a certain degree regain their property of developing. The image is, therefore, devoid of any clear or white portions, and is of no use in photographic pra(5\ice. By a further greatly prolonged and intense exposure, the image darkens visibly even in the high-lights, and gradually loses its negative characTter on development, and finally becomes a complete positive again. The further study of these curious changes is, however, only of theoretical value, several authors (Moser, Jansen, Eder and others) have dealt further with the subjecft, and given accurate information regarding it.* It must, however, be noted that the commencement of the effe(51: of solarisation often takes place in pracftical photography, both in the negative and positive processes. In many positives upon albumen, platinotype paper, etc., the solarisation of the negative from which they were printed is observable. Many prints upon emulsion paper have also this effe(5l produced by a too mtense light or by incorrecft exposure. The effecfl of solarisation is, however, too little recognised and seldom noticed. The Influence of Light on Solarisation. — The appearance of solarisation depends, to a considerable extent, upon the intensity of the rays of light. The author has proved that exposures with daylight produce solarisation much more readily than if weak, * See Eder's " Handbuch der Photographie," part vi., page 42 ; part ix., page 77. EXPOSING. II artificial light be employed. With the latter, even if exposed for a great length of time, solarisation will rarely take place." Experimenting in this direcflion, the following were compared : I St. The light of a petroleum lamp with refle(5lor, placed at a distance of about 1 5 inches. 2nd. Diffused daylight. 3rd. Dire(5t sunlight. With equal development the following lengths of time were required to give equal exposures, as tested by the development of sensitometer numbers. 300 seconds with petroleum light. 5 seconds with diffused daylight, about ^ second with direcff sunlight. The following results were also achieved. With the exposure to the petroleum lamp no solarisation was effedfed at all during this period, but only after an exposure of 500 seconds, and then only in its first stage. The diffused daylight effecfted solarisation in the photometer numbers i and 2, and with diredl sunlight in the numbers i to 7. Further experiments were made by the author (in consequence of a discussion which took place in Photographische Wochenblatt, 1889, page 130), with several developers, varying in strength and chemical composition (oxalate, hydrokinone), also with oxalate developer to which potassium bromide was added in quantities up to 10 per cent. Without any exception the same degree of solarisation was visible in each case. With weak developers the same degree of solarisation became visible only after prolonged development. From these results the following conclusions are derived : — I St. Solarisation is dependent entirely upon the exposure. 2nd. Solarisation increases with the intensity of the illuminant. 3rd. Development has no influence whatever on the degree of solarisation. Solarisation requires a certain intensity of acflinic light, wdiich with weak illuminants is difficult to obtain, and which will certainly not be obtained within the space of time necessary for the exposure of a perfe(ft positive. From this we get the following rules for photographic pradfice. Rules for Printing from Dense Negatives. — With dense negatives the danger of solarisation is greater, owing to the prolonged influence of light necessary. Emulsion paper must not be printed with too intense an illuminant, if solarisation is to be avoided. But as dense negatives require strong illumination to bring out the detail in the high-lights, it will often be necessary to put up with a small amount of solarisation. * Eder's "Jahrbuch," i88g, page 50. 12 BROMIDE PAPER. In the choice of the intensity of the illuminant, the trans- parency of the negative and the sensitiveness of the paper must be taken into consideration. The resuhs of experimental development will show the direcftion in which the exposure must be varied. Exposure, the Produ(ft of the Power of Light, and Length of Exposure. — As exposure may be considered to be the product of the a(51:inicity of the light during the space of time it is allowed to acft, it is possible to form either a strong or a weak latent image by varying the intensity of the light, as well as by varying the length of exposure. Usually it is best to vary the exposure, and keep the intensity of the light as constant as possible. Although not always the scientifically correcSl method, this is no doubt the most convenient, for besides the duration and the intensity of the exposure we must also take into consideration the sensitive- ness of the emulsion and the transparency of the negative. But these produce the same effe(5t thus — The greater the sensibility of \ the emulsion, [ the less the ac^inicity of the The more transparent the nega- i light necessary, tive, ) The less sensitive the emul- \ sion, [ the greater the acffinicity of the The less transparent the nega- I light necessary, tive, J to produce a corredlly exposed latent image. Everyone will at once perceive the correcftness of these statements a priori, but it is only by experience that one becomes acquainted with the closer relation of the transparency of the negative, and the sensitiveness of the emulsion to the intensity of the illuminant than to the duration of the exposure. It does not always follow, therefore, that the same effedl is obtained with an exposure of say 5 seconds, and with an exposure of 10 seconds, reducing the intensity of the illuminant by one half. It becomes necessary to take into consideration what degree of transparency the negative possesses, and the sensibility of the emulsion. By experience we find — The more transparent the nega- \ the less the intensity of the tive, I light required in proportion The more sensitive the emul- to the duration of the expo- sion, J sure. The less transparent the nega- \ the greater must be the inten- tive [ sity of the illuminant in a The less sensitive the emul- ( relatively shorter duration of sion, J time, to obtain an image possessing perfe(5f gradation and with correcft detail in all parts. EXPOSING. 13 On examining a negative through a powerful microscope we find that the film is built up of small transparent and dark opaque granules unevenly distributed over the surface. In the deep shadows ("clear glass") the opaque granules are few and far between, while in the half-tones and high-lights they are more numerous and closer together. It appears at first sight, and has been stated, that the amount of light passing through the negative depends entirely on the number and consequent closeness of the dense particles in the film, but this idea is somewhat misleading, since it looks upon the film as a perfedfly plane surface over which a greater or lesser number of perfectly dense particles are spread in a layer of one uniform thickness. A closer microscopic examin- ation will show that long before the densest parts of the negative are reached there is a sufficient number of the dark granules to absolutely cover the surface of the glass if they were all placed in one plane. But as a matter of facft, suspended in the gelatine, in a layer which increases in thickness as the denser parts are reached, the granules do not touch edge to edge, but are suspended indis- criminately, leaving tortuous passages between them through which rays of light which have not a dire(51: passage may pass by refledlion from particle to particle. A strong light acfting for a short time will succeed in passing through many of these tortuous passages through which a weaker light, however long continued, would not pass at all. In pradliice, it is best, with a thin negative, to reduce the number of light passages penetrated by employing a weak i]luminant, whilst with a denser negative it will be necessary to endeavour to employ as great a number as possible of the com- paratively smaller number of light passages by the use of an illuminant of high intensity. In printing-out processes the intensity of the light can easily be controlled, but in development processes it is not so simple a matter. The subjedl is so inexhaustible that the preceding and following considerations deal only with a small portion of it. Weak in comparison with Strong Light. — In the same manner that a weak blow affe(fls only the parts of the body struck, and a strong one will affec5l deeper lying portions, so will a wave of light of high adlinic power affedt the sensitive emulsion film even in the deeper portions, and produce that action of splitting up or loosening which precedes decomposition, the latter being effecfted by the chemical adfion of the developer, while a wave of light of weak adlinic power will exhaust its strength as soon as it has reached the surface of the emulsion film. In like manner as a number of weak blows following each other in succession cause no effe(5t in the underlying parts, this efFe(5l only being obtained by a few strong blows, so it will be seen that light of low intensity will not cause the same loosening a(5lion in the deeper portions as would be caused by employing a light of high power, but is confined to the film surface, loss of vigour in the print being the result of long \ 14 BROMIDE PAPER. exposure. The surface of the film wiU always receive the image, and parts, especially the finer hig^h-light details, will be exclusively on the surface. For this reason images upon the surface are always very delicate, but are, however, wanting in life and vigour. It is for this cause that the albumen image, which may be chiefly considered as a surface image, requires a film rich in silver, which is obtained by strongly salting the albumen paper. Its Acftion in the Film. — It is true that the chemical a(5fion of light also affe61:s the albumen paper in its deeper parts, but at the same time this adlion is not only a loosening, but also a decomposing one. The molecules of sub-chloride formed during printing obstrudl the light, so that from the commencement of the exposure obstacles will be found in the surface of the film which, during the printing process, steadily increase, finally entirely preventing the light from adling in the deeper portions. The developed image, on the contrary, will in most cases be a deep one, and although this penetration of the light-image cannot be said to be a chief characfteristic of the emulsion picfture, nevertheless its remarkable " plastique " and vigour distinguish it from the albumen picfture. Considering that the delicate light acftion necessary to form the latent image permits of the use of artificial light of very low intensity being used in connecftion with daylight, these differences will be easily observable. They exist in the variation of the intensity as well as in the more penetrating effecSt of the light. Experience fully confirms this. Whilst with chloride of silver emulsion exposed to strong daylight, deep images rich in colour may be obtained ; weak, parafBn light only gives weak, flat surface images, and if from dense negatives, they will be without a trace of detail in the high-lights. Arbitrary Variation in the Light Intensity. — In order to comply with the rules given on pages 7 and 12, it will be necessary and of importance, to know the best means of obtaining a variation in the intensity of the exposure. Methods of varying the Intensity of the Exposure. — These are the foUowinj^ : — 1. Changing the position of the printing frame or copying arrangement to lesser or greater distances from the source of light- . . . . , , 2. Using shades of tissue paper or light curtams in front of the printing frame. The method of laying papier mineral over the emulsion paper, as often adopted in albumen paper printing, is not to be recommended in contact printing, as the strucfture or grain of the paper will always be printed, no matter how short the exposure. 3. Varymg the illuminant by turning the wick up or down, or by regulating the gas tap. EXPOSING. 15 4. Coating the glass side of the negative with a film ot coloured varnish or collodion. Thin negatives require a light yellow, and dense negatives a blue-coloured varnish or collodion. Uniformity of the Exposure. — The principles of optics teach us how to keep the intensity of the light -acflion uniform for exposures following each other, and also how to diffuse the light equally over the w^hole negative. The important rules derived from these principles can, however, with natural light be brought only partly into consideration, because of the immense distance of the light-reflecfting sky. With artificial light they are, however, under all circumstances of the greatest importance. The illuminant in this case can be considered constant. Principles of Exposure. — Optical science teaches us that the value of the exposure is dependent upon the distance of the exposed obje(5t, and upon its inclination to the light rays. These principles are better explained by the following two rules : — 1. The intensity of the exposure is reduced in proportion as the square of the distance from the illuminant increases. 2. The exposure intensity of an even plane is proportional to the cosine of the angle at which the rays of light fall upon it. A few examples will be given to show how errors are made by exposing without consideration, or without a knowledge of these two principles. Let us suppose we make several exposures, placing the printing frame at different distances from a gas flame. Let these distances be |-, i, 2, 2| and 3 metres from the flame. The images produced by the light would, on development, prove to be of very different values, for by the first rule given above the intensities of the exposures of equal length would be in the following proportions : — 100 : 25 : ii'i : 6*25 : 4 : 2-77. These figures give a better idea of this law than verbal explanation, and we get the first chief principle of photographic exposure. Distance from the Light Source. — Exposures with the same negative and with the same illuminant must be made at exactly the same distance from the light, if the latent images are required to be exadlly the same. By the figures given above, we note further that the difference in the intensity diminishes as the distance from the illuminant is increased. This shows, firstly, that errors made in the position of the printing frame are lessened the greater the distance from the light ; and secondly, that owing to the immense distance of the sky refledfing the daylight, the difference of intensity pracftically disappears. The rule above is, therefore, only observed when using artificial light at short distances ; thirdly, that a position close to the illuminant is prefer- able, if a high intensity and a saving of time and cost of illuminant is desired. l6 BROMIDE PAPER. For two reasons, however, a position too near to the hght is to be avoided. 1. Because the heat given out by most artificial illuminants when in close proximity is very noticeable. 2. Because the divergence of the rays of light in exposing negatives causes a double source of errors. These are in a. the distance L N (fig. i) from the illuminant L, and b. the angle of incidence a, of the divergent rays, varying with the different portions of the exposed plate. An example will show the importance of these sources of error. With a plate lo in. by 8 in., the difference of the exposures on different parts of the plate at a distance of half a metre would be very marked. The proportionate intensities of the centre, sides and corners would be : — At a distance of ^ metre loo : 94*1 : 90*7 At a distance of i metre 100 : 99*24 : 9876 With regard to the different angles of incidence, rule 2 above Fig. I. must be observed, that the intensity of the exposure of a plane is proportional to the cosine of the angles of incidence of the rays of light. Accordingly the intensities would be in the following proportions : — At a distance of ^ metre 100 : 97 : 95-3 At a distance of i metre 100 : 99*3 : 98-8 Errors of Close Proximity. — Both these sources of error multiply each other, and by acfting together result in the following proportions of the intensities of the centre, sides and corners of a 10 in. by 8 in. plate : — At a distance of | metre 100 : 91-2 : 85-8 At a distance of i metre 100 : 98*6 : 97*6 It will be evident that these errors increase with the dimensions of the plate, but decrease when the distance from the source of light is increased. Rule 2 regarding the importance of the angle of incidence of the light, or better still, the figure example, gives the second chief rule for photographic exposure. EXPOSING. 17 Inclination of the Printing Frame. — Exposures which are intended to produce equal latent images, must be made with the printing frame placed accurately perpendicular to the axis of the light rays. From this rule, also, several conclusions can at once be made. Firstly : That it not only concerns artificial light, but every exposure in which pradtically parallel rays of light are employed. Therefore, sunlight may be included. With diffused daylight it is not necessary to be so particular, because the rays of light fall almost equally from all parts of the sky, and the angles of incidence are the same for every position. Only when the available sky is narrowly bounded by buildings, or by the refle(5fed light of a white wall or white cloud, will it be necessary to consider the direc5fion of the rays. Secondly : It will be at once seen that by exposing close to an artificial light, every movement out of the perpendicular (to the axis of the cone of light) will be an offence against the first law, for the reason that one portion of the negative becomes placed nearer the source of light than another. Further, the Fig. 2. oblique position will in itself cause a further decrease in the intensity of the exposure over the whole plate. How important this rule is when working close to an artificial light, and how easily small and apparently trifling errors become enlarged to greater and more serious ones, the following concrete example will serve to show. Let us suppose a negative, B C (fig. 2) having a diameter of 20 centimetres, be placed at a distance of one metre from the light, and receives, when in a position perpendicular to the axis of the light cone, an exposure intensity equalling 100. According to the law that the intensity varies in inverse proportion to the square of the distance, if the negative is placed at the angles of 15°, 30°, and 45° round the axis B, the intensity at C would decrease from 100 to 90*4, 82-6, and 76-8, while at B it would remain at 100. Further, as the exposure intensity of a plane surface is in proportion to the cosine of the angle of incidence of the rays of light, a further gradual decrease in the intensity for the w^hole negative would take place, from 100 to 96-6, 86*6, 707. B i8 BROMIDE PAPER. These decreases in the intensity from the two causes united, would give the relative intensities of the exposure as follows : — • At B. At C. In a perpendicular position loo loo At an angle of 15° 96-6 87-3 At an angle of 30° 86-6 71-6 At an angle of 45° 707 54-3 These errors decrease with the increase of the distance from the light. If, for instance, the distance be increased to 2 metres, the following would be the relative intensities: — At B. At C. In a perpendicular position 100 100 At an angle of 15° 96-6 91-8 At an angle of 30° 86*6 78-5 At an angle of 45° 707 617 Use of ReflecTtors. — For the sake of economy, reflecflors have long been used when the light is only needed on one side of the lamp. These reflecftors not only give an increased amount of intensity, but by their use the two sources of errors already spoken of are considerably lessened. Parabolic Reflecllor. — This is especially observable if the reflecStor employed be a parabolic one, which, as is generally known, possesses the property of refledfing in parallel diredlions all those rays which fall upon it from a light placed in its focus. Such parallel rays of light are much more valuable for photographic exposure than the divergent rays falling direcff from the illuminant upon the negative, for the reason that with the increasing distance from the light source, the only loss that they will sustain will be in the absorption of the intermediate stratum of air, whereas the other rays sustain a further loss in intensity proportional to the square of the distance. If, however, the source of light is not placed in the focus of the parabolic reflector, we no longer obtain this valuable parallel refled^ion, but the rays of light are refledled with either divergence or convergence. This being the case, we obtain the following rule : — In the employment of a parabolic reflecStor the light must be placed in its focus as accurately as possible. Fig. 3 shows the exposure with a parabolic reflecftor. The illuminant L throws a cone of rays upon the negative TV P at an angle expressed by N L P, and upon the negative n at double the distance, the angle being expressed by n L p. The reflecflor R F receives a cone of light from the illuminant, the angle being expressed by R L F, which reflecfls with equal strength on N P, and on n p. The exposure that N P and n p receive respedlively will therefore consist of a portion of the light from the illuminant, and EXPOSING. 19 a portion of the indire(5l rays from the refledlor. The latter is a good deal greater than the former, and they are diredtly proportional to the square of the degrees of angle N L F and RLE. Supposing L F to be about three times larger than N L P, and therefore about six times larger than n L p, the portion of the exposure which N P receives from the refledfor would be nine times greater than that received direct from the illuminant, and the portion of the exposure received hy n p would be 36 times greater than the amount of the direcft exposure. If we take the diredt exposure of the light upon the negative to be equal to i, the exposure of the refle(5for is 36, the total exposure oi n p would therefore be 37. As, however, N P \s situated at one half the distance of n p, the exposure falling diredl from the iUuminant upon it is equal to 4, the part from the refledlor is nine times greater, and is therefore equal to 36, and the total exposure at iV P being 40. Placing these results together, we get : — , — Exposure — n of P oinp Without reflecflor 4 i With reflecftor 40 37 Advantages of ReflecTtor. — We see, therefore : — I St. The great advantages to be gained by employing a refledfor. 2nd. To what a trifling amount the defecfl: is reduced which is caused by inequality of exposure over the whole plate due to a too close position. 3rd. How considerably the defecSl caused by alteration in the position diminishes. A parabohc refledlor does not very greatly counteracft the defe(51:s caused by departures from the second chief rule. A concrete example will make this clearer. Supposing the angle R L F (fig. 4) was four times greater than the angle CLE, then the amount of light from the refled^or and the illuminant would be as 16 : i, the total exposure would therefore be 17. 20 BROMIDE PAPER. If the position B C perpendicular to the axis of the rays was changed to angles of 15°, 30°, and 45° respe(5tively, the defeat would again be a double one, as an error against the two principal rules laid down. The fadf, therefore, that C by the alteration in position becomes removed from the light L, would only relate to a 17th part of the exposure. If we take, for instance, the intensity of the illumination of the exposed negative B C, placed at a distance of i metre, to be equal to 100, the amount of direct illumination from the light would be about 6, and the amount from the reflecStor about 94. The law that the intensity decreases in proportion to the square of the distance, would only affe(5t the small share of light (6) coming diredfly from the illuminant source, and therefore by altering the angle of the negative to 15°, 30°, and 45° respe(5fively round the axis B, the intensity at C would only decrease 6% of the amounts given on page 18, that is to say, from 100 to 99*56, 98-96, 98-61. Fig. 4. The second law that the intensity of the illumination of a plane is in proportion to the cosine of the angle of incidence of the rays of light, would be followed by both the diredl and the refleifted light. Consequently, with the above methods, angles of inclination of the whole surface of the plane would suffer an equal decrease in illumination from 100 to 96*6, 86*6, 70*7. And as this diminution in intensity from the two causes combine, the relative intensities of the illumination of the plate B C would be as follows : — At B. At C. In a corre(5l perpendicular position 100 100 At an angle of 15° 96-6 96-2 At an angle of 30° 86-6 85-7 At an angle of 45° 70-7 69*7 These figures sufficiently prove that although the exposure for the different parts of the negative remains almost equal when the latter is removed from a perpendicular position, yet if a number of successive images are made with the same exposure it will be necessary that the same angle of inclination towards the axis of EXPOSING. 21 the rays of li^ht should be maintained, or better still, the perpen- dicular position, if the latent images are all required to be equal. It is not, however, necessary that the refledlor should be absolutely parabolic, for even nearly plain spherical reflecftors, provided they only constitute a small portion of the spherical surface, will give good refledlion. Spherical reflectors having a depth exceeding J^th of their diameter, as well as cone-shaped refle(5tors, are unsatisfacftory, partly because they produce an unevenly illuminated surface afiedled by focal curves, and partly because they diffuse the light so much as to considerably reduce its intensity. Evenness of the Illuminating Surface. — From this we derive another rule : — In using a reflecftor, it will be necessary to previously ascertain if the same gives a perfe(5tly even illuminating surface. A pra(5lical experiment to ascertain whether a refle(ftor is satisfadfory for photographic printing is easily made by stretching a large sheet of paper over a board, and placing it at different distances from the light source in a position perpendicular to the axis of the rays. Those reflecflors, which, at a distance of 3 to 6 feet from the illuminant, according to the size of the negative, give an even illumination without focal curves, will be at once recognised by their greater brilliancy on the paper. Those which illuminate a plane without spreading out too much light over the surrounding objec$l:s are to be recommended for pradfical and economical purposes. In cases where a reflecT:or gives a strong illuminating surface, but affedled by curves of cross-reflecftion, the only thing to be done is to insert a piece of tissue paper, rubbed over with vaseline, between the refledlor and the printing frame, but near to the former. This will not, however, be necessary if the refledlor gives a good, even illuminating surface. Especially good are those reflectors which can be bought, pressed out of Pakflong,* or oat of sheet brass, and well plated with nickel silver or platinum. White enamelled or lacquered mirrors are also serviceable. Diffused Daylight. Diffused daylight is not only the cheapest illuminant but is also the best (especially for chloride paper), because it produces strong images and brilliant tones, and permits of short exposure. An artificial light, to be of the same adfinic value as diffused daylight, would have to be very powerful and very expensive. Unfortunately the extreme variations of the power of daylight, especially in and near towns, excepting on the brightest and clearest * A mixture of copper nickel and zinc. — Trans. 22 BROMIDE PAPER. days in summer, render its use so very uncertain that in the great majority of cases it is better to use artificial light ; for even if the best possible results obtainable with artificial hght are slightly inferior to the best possible results with dayhght, the uncertainty of the latter prevents these results being obtained with any regularity. The variations in daylight depend upon 1. The season of the year. 2. The time of the day. 3. The clearness of the atmosphere. 4. The passing clouds ; either natural, or artificial clouds of smoke, steam, etc. The normal power of the light for any given hour of the day on any day in the year may be calculated with the greatest exacftitude from tables of adfinicity compiled and published by some of the leading meteorologists. But Enghsh daylight so seldom is normal, while the influence of the clearness of the atmosphere is very difiicult, and the influence of the changing clouds impossible to reckon. Therefore, excepting for enlarging, when the exposure is necessarily much longer and the effecff of variation less important than in contacfl printing, the use of daylight will generally be found inadvisable. For those who do use daylight a few considerations deduced from the tables of normal dayhght may be valuable. Variations from Hour to Hour. — The normal variations of light from hour to hour are greatest in the morning and evening, and least in the middle of the day. In fadl, from ii-o to i-o o'clock, the light may be said to be constant, even in winter. In the summer months, when the sun is higher, the length of time during which the light is constant is greater, so that in April, May, June and July, the light may be said to be constant from 9-0 a.m. to 3 p.m. Therefore, if all the printing or enlarging is done between these hours the variation caused by the altitude of the sun need hardly be considered, and after a satisfadl:ory test exposure has been made, any negative of similar density will require the same exposure at any time during the next few hours, if the clearness of the air and the clouds remain the same. More than this: if the hours are between 9-0 a.m. and 3-0 p.m. in the months above named, between lo-o a.m. and 2-0 p.m. in March, August and September, and between ii-o a.m. and i-o p.m. during the rest of the year, it will be possible to calculate the necessary normal exposure with a negative of given density for any day in the year by means of a very simple table of adlinicity. This eliminates the consideration of the variation that would be caused by the altitude of the sun if the exposures were made earlier or later in the day. To use such a table it is necessary that the proper exposure for thin, medium and dense negatives should be ascertained by experiment on a clear, cloudless day. Then the exposure necessary on any other clear day can be reckoned by a simple rule-of-three sum. DIFFUSED DAYLIGHT. 23 THE NORMAL ACTINIC POWER OF DAYLIGHT at noon for each month in the year. From observations made at the Royal Observatory, Kew. Jan. Feb. Mar. April. May. June. 145 225 245 460 680 710 Jnly. Aug. Sept. Oct. Nov. Dec. 875 750 700 300 190 100 For instance : If the exposure necessary with a given negative in June is 10 seconds when the a(5Iinic power is represented by 710, and it is required to find the exposure for the same, or an equally dense negative in Ocftober, the sum will be stated thus : — As 300 (the a(5linic number for 0(51ober) is to 710 (the a(ffinic number for June) so is (the exposure in June) to the answer, which works out : as 300 : 710 : : 10 : 23*66 (seconds). Or again : If the February exposure is known to be 20 seconds and the June is required, then : — as 710 : 225 : : 20 (seconds). The acftinic numbers given in the little table above are calculated from observations made throughout the month, and may therefore be taken to apply most corredtly to the 15th. For days between, the numbers must be proportionately altered. Thus : — The a(51inic number for the ist of June, midway between May 15th and June 15th, would be 695, and the acftinic number for February ist would be 195. Therefore, if the exposure for June ist is known to be 12 seconds and the exposure for February ist required, the sum will be stated : — as 195 : 695 : : 12 (seconds). Reducing the Ac5linicity in a Room. — In a room the adlinicity of the diffused daylight from the window decreases as one recedes from the window or as the size of the latter is reduced. For instance, it may be mentioned that in a room about 16 feet wide, looking towards the north-west, the acflinicity of the light at the wall opposite the windows was only ^^Xh of that at the window. It will be seen, therefore, that the acflinicity of the light can very easily be regulated by ere(5ting the printing-board a few feet farther from, or nearer to, the window. Artificial Lights. Of the numerous artificial methods of producing light which we possess, only those are treated of which are cheap enough for use in the positive printing processes. 24 BROMIDE PAPER. Magnesium light, lime-light, and zirconium light can only be considered when required for enlarging, and then only when a very strong acSlinic light is required. For the simple process of printing we will first consider. a. Paraffin light, which may be obtained everywhere ; and b. Gaslight, obtainable in most places, whilst c. ElecStric light can only be recommended in places where it is already produced by dynamo-eledlric machines, and where it is possible to economically obtain current. Under these circumstances it would be recommended before any of the others. Illumination with Paraffin. — For the printing of small negatives (up to cabinet size) upon bromide emulsion paper, any ordinary table lamp will suffice, and it is immaterial whether it have a flat or a round burner, or if it be provided with a refle(5tor or not. A simple burner is sufficient. If a large series of photo- graphs are not required, any convenient lamp in the dark room can be used. It will be necessary, however, to consider the important rules laid down on pages 15 and 16, relating to artificial lights used at short distances. In prmting upon chloride emulsion paper it is a difficult matter, as this requires more adfinic light. Also when arrange- ments are to be made for large and continual produdf ions of the same photograph. For this purpose a stronger light must be used, as, for instance, a good duplex burner, fitted with a refledfor if time is an obje(il:, or if the printing is to be on chloride paper. The advantages of using a reflecffor are explained on page 18 and following pages, although every lamp is not suitable for one. The special lamp illustrated and recommended for this purpose (fig. 5), is a sun-burner, with a compac5f cylindrical flame and suitable special refledlor which is a combination of parabolic with other curves. The oil-reservoir is placed well away from the flame, so that it avoids variation of light from super-heating, and also allows the refledfor to completely surround the flame. Treatment of Paraffin Lamps. — Paraffin lamps (especially large ones), to burn evenly and properly, require 1. Proper attention to the wick. 2. Careful cleaning of the apparatus. 3. Use of good and safe paraffin. 4. Attentive regulation, more particularly during the first 10 minutes after lighting. 5. Regular filling of the oil reservoir. Rubbing, not Cutting the Wick. — The wick of the lamp should be rubbed with the finger before lighting. Cutting should only be necessary when one side of the wick protrudes very much, and cannot be regulated by pressing it down with the finger or •The lamp here ilhistrated is praftically identical in construftion with Ditmar's Gigant- Brenner, strongly recommended by Dr. Just, but unobtainable in England. ARTIFICIAL LIGHTS. 25 raising up the other side. The edges of the the burner round the wick and also the inside and outside air holes should be kept perfectly clean, as the intensity of the light will be considerably diminished if insufficient air be admitted. The tubes which carry away the combustion gases must also be cleaned of the soot which often accumulates on the sides. It is necessary that everyone be warned against using the Fig. 5- cheap kinds of paraffin sold, which are only made combustible by mixing heavy oils with lighter kinds. These inferior qualities of paraffin not only cause rapid choking of the wick, but also increase the liability of dangerous explosions. In lighting it is necessary to avoid any smoking points in the flame. These can be removed either by regulating or pressing down the wick, or by removing the protruding bits of cotton. 26 BROMIDE PAPER. When the lamp has become heated to the proper degree the danger of smoking is removed, and the size of the flame then remains constant. One of the chief defecfls in paraffin lamps will often be found in the oil reservoir being too deep. When such is the case, the flame sinks when the oil is burned down to a certain level, for the reason that the absorption height of the wick becomes too great, and this will happen the sooner with dense and impure paraffin. It will, therefore, be found the safer plan to see that the oil reservoir is well filled before use, and that the level of the oil in faulty reservoirs does not, in burning, sink below a certain point determined by experiment. Arrangement of the Lamp. — In fitting up a lamp it is necessary to consider the following points : — 1. That large lamps generate considerable heat. 2. That the air becomes vitiated, by the consumption of oxygen and the produdlion of combustion gases. 3. That the lamps should be capable of regulation even during the exposure. Generation of Heat. — The first and second points are hardly brought into consideration when only small lamps are lighted for a few hours in the evening. With the use of larger lamps, how- ever, they are of special importance. The generation of heat is not only exceedingly troublesome to the operator, but has also a detrimental effedl: upon the printing arrangements, especially with varnished negatives, and more particularly if a number of prints are to be made in succession. Consumption of Oxygen. — The consumption of oxygen is an important consideration, especially when the combustion gases return to the same space. The latter (as is well known) are com- posed chiefly of water and carbonic acid (with traces of carbonic oxide). Carbonic acid gas is unfit for respiration. Escape of the Combustion Gases. — The importance, therefore, of fitting up the lamp in a separate chamber, or with an efficient chimney leading out of the workroom, is at once apparent. When sufficient space for this arrangement is not at one's disposal in the work-room, it is easy to fit up a conveniently roomy tin box outside the window, with a tin chimney at the top, and a number of air holes at the bottom. An arrangement of this kind is shown in fig. 6. The regulating of the lamp is permitted by the window in front. For this purpose the aperture admitting the light should have a movable sheet of red glass, so that it can be covered at will, and for enlarging purposes by a piece of ground glass. Such an arrangement can be fitted in a room adjoining the printing-room, which may be used for other purposes. In this case, however, it will be necessary that the chimney of the box be lengthened so as to carry the fumes to the chimney of the room or out into the open air. ARTIFICIAL LIGHTS. 27 The regulation of the lamp must be attended to as often as possible, as the flames of large lamps (especially when enclosed in the tin box) will begin to smoke when the lamp is kept burning for some time, in consequence of the heating of the burner. Escape of the Heat. — The oftener the air in the box is changed and the cooler the lamp will keep. If the tin chimney of the lamp chamber be connecfted with a good free-drawing chimney, and if air holes be made between the lamp and the glass of the light opening, the change of air will be sufficient. Fig. C. Gas Illumination. — The comfort, convenience and cleanli- ness of gas are well known. It will be interesting to learn a few data regarding different kinds of gas burners, as they show the intensity of the light and the amount of gas consumed per hour by each flame when burning London gas. Candle power. 1. Slit Burner* 12 2. Ordinary Argand burner (24 holes) 17 3. Intensive Argand burner (48 holes) 30 4. Sugg's Argand (2 rows of holes) 88 Special rule for height of room. * Technically known as bat's-wing burners. 28 BROMIDE PAPER. Candle power. Special rule for height of room. 5. Sugg's Argand (3 rows of holes) over 12 feet 6. Intensive Flat Burner 25 7. / The Incandescent Burner 20 8. Albo-ca rbon Gas Light 22-24 9. Siemen's Regenerative, OOO boo at least 20 feet 10. DO. do. 1. 340 ,, 15 II. Do. do. III. 00 ,, 9)5 12. Do. Horizontal Regenerative Slit Burner DO ,, 9 )) Do. Inverted Regenerative Bur- ner, No. III. 14. Do. Inverted Regenerative Bur- ner. No. IV. at least 10 feet Do. Inverted Regenerative Bur- ner, No. VII. 250 )) ^2 ,, 16. Do. Inverted Regenerative Bur- ner, No. XI. 457 15 Of these burners, Nos. i to 8 can be attached to any gas fitting allowing sufficient gas to pass through, but Nos. 9 to ii require a special arrangement for condudling away the combustion gases. In proportion as the size of the gas burner increases, the generation of heat also increases, which fa(5f gives sufficient reason to be satisfied with burners of medium adlinicity, as by their use the heat of the work-room remains tolerable, and the heat rays have no detrimental effe6\ upon the printing apparatus or on the negative. With regard to this, the employment of the incan- descent or the albo-carbon light will be found of no little advantage. The Incandescent Gas Light. — The incandescent light burns with a Bunsen burner, giving a colourless, but very hot flame, which causes a cylindrical "mantle" to glow and emit rays of very even and soft light. The light is about equal to that given by a 40-hole Argand burner without mantle, the advantage it has over the latter is in the smaller amount of gas consumed, about 70 litres as compared with about 210 litres per hour, and also in the smaller amount of heat generated, which is not half as great as with the Argand burner. According to the experiments of Dr. Eder and Captain Von Reisinger, the incandescent light takes a stand between the ordinary paraffin lamp and the zirconium or lime-light. Experimental exposures on bromide paper showed to these gentlemen that with this lamp one-third to one-fifth of the exposure was required that was necessary with a sciopticon paraffin lamp. The Albo-Carbon Gas Light (fig. 7) is a method of illumin- ation reintroduced within the last few years. Ordinary gas mixed with so-called albo-caibon (napthaline), is burnt in such a manner that the flame itself heats the reservoir containing the albo-carbon. ARTIFICIAL LIGHTS. 29 and this, when brought to its temperature of subhmation, is drawn along with the gas flowing through it to the burner, where it is consumed. The albo-carbon Hght can be used with any existing gas fitting, all that is required being the albo-carbon container with burner, and the albo-carbon (costing 3d. per lb. retail). The form of container or generator which we illustrate costs 7s. 6d. There is a cheaper form at 6s., and others more expensive in various sizes. They can be obtained to order with two or more burners imme- diately behind one another, and in this form, with three flames close together and parallel one behind the other, the albo-carbon light was strongly recommended for enlarging purposes by the late Mr. J. Traill Taylor. As the generator fits to any gas fitting provided for an ordinary burner, it can, if necessary, be used in the shop window or elsewhere when not required for enlarging. When using it, some attention is required, but it effedfs a considerable saving in gas and gives a steady white flame of relatively great intensity. That the intensity of the albo-carbon gas flame not only refers to its optical brilliancy, but also to its chemical a(5finicity was proved by experiments on emulsion paper with a scale-photometer. With the same consumption of gas the albo-carbon flame gave a chemical intensity nearly three times as great as that produced by an ordinary burner, whilst no increase in heat was noticeable. The special three-burner light referred to above is far more useful than the ordinary stock form, as it gives a great power of light without the great heat generated by intensive burners. A Fig. 7. 30 BROMIDE PAPER. Siemen's Inverted Regenerative Gas Lamps. — Of special value for photographic copying purposes are Seimen's regenerative gas burners (fig. 8), partly on account of their extremely favourable arrangement, and partly on account of their intense and absolutely steady light. With these lamps the flame is of a spherical shape, and burns downwards in an inverted hemispherical glass cover. It therefore has its maximum intensity vertically downwards. Fig. 8. This arrangement enables exposures to be made on an ordinary table. By this means the rules regarding the position in relation to the axis of the rays and to the distance from the source of light can be easily and corre(5fly carried out. The intensity of this light,, even with the smaller burner numbers, is sufficient to enable one tO' make both conta(5t prints upon chloride of silver, and enlargements. ARTIFICIAL LIGHTS. 31 upon bromide paper l)y means of an enlarging apparatus erec51ed vertically. If the enlarging apparatus be set up horizontally, a mirror placed at an angle of 45° towards the vertical must be employed. Siemen's Horizontal Regenerative Lamp. — Siemen's horizontal regenerative flat-flame lamp (fig. 9.) possesses an open horizontal gas flame of about 60 candle power. It does not give the absolutely steady light of the regenerative burner, but it is distinguished by its high intensity and by its cheapness. This lamp also gives the greatest light intensity vertically downwards. Fig. 9. The KlecSlric Light, with its enormously high intensity and its richness in chemical effedf in positive printing would be the ne plus ultra, if ist, it were not so expensive both in the installation and in its maintenance ; and 2nd, if it would work equally and steadily. With regard to the first point, only those can be expecfted to make use of it who have some cheap motive power at hand, or can hire a share of eledfric current from some existing installation. The light of the ele(51:ric arc lamp has been proved to be occasionally 32 BROMIDE PAPER. irregular ; and owing to the very short exposure required with the high intensity of the hght, it is absolutely necessary that it be perfecftly free from any irregularities. Slipping of the belt connecfling the motor with the dynamo, delay in the automatic regulation of the carbon points, and many other circumstances will cause considerable irregularity in the light intensity which, although of little importance when enlarging with long exposures, yet will cause considerable error in conta(5l printing with exposures lasting but a few seconds.* Advantages of the Elecflric Light. — For enlarging the irregularities are of but little importance, and therefore, the eledf ric to any other method of illumination for the following reasons : — I St. On account of its exceedingly high light intensity. 2nd. Because the light emanates from a point, permitting parabolic refledfors to be used, and is also suitable for con- centration by condensers. *The best lamp for this purpose, giving great regularity of feeding and overcoming all difficulties except that of a possible irregularity of current, is made by Messrs. Gwynne & Co., under the Pilsen patents. It is in two forms : one intended for printing (which will fully print ordinary albumenized sensitized paper in fifteen minutes) and the other for enlarging (see later). Trans. ARTIFICIAL LIGHTS. 33 3rd. On account of its convenience and safety in com- parison with lime-light or the zirconium light with oxy-hydrogen Elecftric light of low^ intensity, such as the incandescent lamp, has too little advantage over gas or paraffin as an illuminant to allow of an expensive installation. Magnesium Light. — Since the price of magnesium was considerably reduced, it has often been employed as a light source for photographic operations. In the positive process, however, magnesium can hardly be recommended except, perhaps, in enlarging when not required for continual work. Even then it would be necessary to employ a lamp with several magnesium ribbons, so that the exposure would not be affec1:ed by occasional irregularity in the burning of the ribbon. Such a lamp may be obtained from several London houses. Printing Arrangements. Contacfl printing requires but a short exposure. Exposures of over 10 seconds are quite exceptional, and are only possible when using artificial light for chloride emulsion paper. With these short exposures the clumsiness of the ordinary printing frames used for albumen prints is at once apparent. The opening and closing of the printing frame, the changing of the paper, and the exposure are all so very difBcult and cause so much loss of time, that one wonders why photographers continue to use the old-fashioned frame for emulsion paper. Simpler and handier copying arrangements are desirable, and we w^ll consider the qualities necessary in a printing arrangement for emulsion paper. Requirements for Printing Apparatus. 1. Convenience in handling. 2. Perfecft protecflion of the negative from breaking, and of the films of the negative and the paper from mechanical injury. 3. Complete and even contacl of the paper with the negative in all parts. 4. Perfe(5l protec5i:ion against foreign exposure, as well as against shifting of any of the different parts during exposure. 5. An arrangement to permit of the same position towards the axis of the rays of light, and the same distance from the light being maintained when printing successive copies. The short exposure and the high sensibility of the material to extraneous light, mechanical injury, and the production of spots by c 34 BROMIDE PAPER. touching with the fingers, led to the idea of having an arrangement in which copies could be printed on endless paper. This idea was realised in Schlotterho^s's rapid printing apparatus, and in John Urie's automatic printing machine. It will easily be seen that complex and "expensive machines of this kind can only be of value for printing on a large scale, even in large businesses. They have not been largely introduced, partly, perhaps, because professionals have doubts as to their perfection, and partly because they would rather continue to work in the old way to which they are accustomed than allow their employes to experiment. But, after all, there can be no doubt that these machines will be more generally used when a thorough knowledge of the development process is more universal. The following resume of the existing arrangements for printing should be carefully studied from the different points of view given above. These are — a. The plain printing pad. h. The printing frame. c. The printing window. d. Hand and automatic machines. The Plain Printing Board can very well be employed for exposures with small negatives (c.-de-v. and cabinet). It consists of a flat, non-transparent plate of the same size as the negative to be copied. It may be made of wood, glass, hard rubber or metal, as its only purpose is to adt as a b^d for the sensitive paper, and to distribute the pressure of the clamps which press the negative paper and plate tightly together during the exposure. The Printing Frame is too well known for any description of its arrangement to be necessary. Working with it is inconvenient, and much time, as compared with the short exposure, is wasted in changing the paper. It, however, gives relatively greater safety against breakage, and may, therefore, be advantageously used for printing from large negatives. Many attempts have been made to improve upon the old form of printing frame, and to replace it by a new arrangement, but without much success. Limitation of the Exposure. — For the perfecft limitation of the exposure, the use of a black cloth or a stiff cardboard cover is recommended, which can be quickly withdrawn and replaced. To retain the correct position of the frame, and its distance from the light, special marks must be made or special contrivances used. The Printing Window. — This was specially construdfed by the author for printing upon emulsion paper, and unites the advantages of the printing frame with the simplicity of the printing board. It is especially used for exposing with diffused daylight direct from the dark room. With slight alteration, however, it can be advantageously employed with artificial light. PRINTING ARRANGEMENTS. 35 Fig. II shows the arrangement of the printing window, and its simpHcity in working will be easily understood. The small apparatus is fitted in the window in place of one of the panes of glass, and closed perfedfly air tight all round. The light falls as shown in the sketch, if the flap K be open, upon the horizontal mirror S, which reflecfts it upon the emulsion paper through the negative lying upon a stout piece of plate glass. The mirror S can be raised by screws from the outside when the light falls more vertically. The method of working is as follows : — The operator has on one side of him the exposed and on the other the unexposed paper pressed flat. The outside flap K being closed, the inside flap Fig. II. is opened and the sensitive paper changed. The inside flap is then closed and pressed down. The circular lever H is then pressed down, which opens the outside flap during the exposure. The lever H is then released and the flap falls down of its own weight. Instead of a flap opening upwards, two hinged flaps opening sideways may be used to advantage. This little apparatus possesses the advantage of great simphcity in arrangement and management, of meeting all the requirements stated on page 34, and of being approximately perfecfl and very cheap. But one of the principal advantages gained by using this apparatus is that the negatives can be instantly changed at will, no matter what size they are, provided, of course, that they are smaller than the sheet of plate glass. 36 BROMIDE PAPER. When a number of pictures of the same size are required, an endless band of paper can be used, a roll of it being fixed at the side. The apparatus can also be employed as a retouching desk, as it is construdted after the same principle. Further, the outside flap is provided Avith a piece of red glass, so that the operator can view the negative when changing, and can also make prints from any part with exadlitude. Rapid Printing Machines. — Only three kinds of machine for continuous printing by hand or automatically are known to the author. 1. The so-called continuous printing frame of Tronel and Koch. 2. The rapid printing frame of Schlotterhoss, and 3. John Urie's printing machine. There is a considerable difference between the principles of the two first machines and Urie's printing apparatus. In the former the negative lies fast and immovable, pressed against the glass of the printing window, and the endless band of sensitized paper is pressed against the negative during the exposure by a movable pressure board. Urie's machine works by clockwork, gas flames a6f as illumin- ants, and any influence of the heat from them is prevented by the interposition of a water layer through which the light passes. Two weights set the clockwork in motion. When this is started the different movements are as follows : — First the gas flames are turned down quite low, and a heavy frame holding the negative is lifted up. The band of paper is then drawn along for a certain length, the frame drops down again, the gas flames are turned up again, and the regulated exposure takes place. After the exposure is made the operations are again repeated. The apparatus is also provided with the necessary arrangements for marking and numbering. In the continuous printing frame of Tronel and Koch, all the necessary manipulations are performed separately by hand. The window during the changing operation is covered over with a sheet of yellow glass placed in front of it. The continuous printing frame is consequently surpassed by the rapid printing apparatus of R. Schlotterhoss (fig. 12), with which, by a single turn of the crank, all the necessary operations are successively executed. Schlotterhoss also attaches the crank to a weight which is released by an elecftric current connedled with a clock, and in the same manner stopped, so that the machine is rendered automatic (fig. 13). With this attachment the apparatus is termed the " Exposing Automaton." When employing this machine all that is necessary is to wind up the weight from time to time, so as to keep the apparatus continually in motion. The duration of the exposure is regulated by small contadf plates attached to a metallic clock dial, so that the time is always equal, and consequently latent images of equal PRINTING ARRANGEMENTS. 37 value are produced. This " Exposing Automaton " was exhibited in going order at the International Ele(51ric Exhibition, but is not now on the market. However, the cheaper rapid printing apparatus (fig. 12), with which as already stated, one turn of the crank performs all the different operations of changing and exposing in succession, the same being effe(5\ed by hand, and regulated by the second hand of a clock, or by a second pendulum, is so very handy and reliable that the automatic attachment can easily be dispensed with. The operator, Avhen printing, has only to note the duration of the exposure from one turn of the crank to the other. Supposing the exposure requisite to be as much as eight seconds, and allowing two seconds for the time occupied in turning the crank each time, it will be possible to make six exposures per minute, or 360 per hour. If the required exposure is only two seconds, the number possible to be completed in an hour increases to 900. Fig. 12. A detailed description of Schlotterhoss's apparatus must be dispensed with here, suffice it to describe the different movements made by each turn of the crank. First, the flap which covers the sheet of plate glass upon which the negative is firmly fixed, closes. Next, the pressure board is lowered and lifts up the endless band of paper, which is drawn along the requisite length. The pressure board again presses the paper into close contadl:, and at the same time a pencil mark is made at the back to denote the margins of the pictures. Finally the flap re-opens and the exposure again takes place. At the present time these rapid printing machines are made in two sizes, 8^ in. x 5^ in. and 8Jin. xyjin., these dimensions being the largest size of plate that can be used. By inserting smaller pressure boards and negative rests, smaller negatives can be printed from, or particular portions of a large negative. Double carte-de-visite negatives can, for instance, be exposed. The prints obtained are plainly indicated and attached to each other on the 38 BROMIDE PAPER. roll, from which they may be separated at any moment. This apparatus is sufficient, therefore, to satisfy all demands such as convenience, accuracy, economy in time, safety of the negative, and cleanliness of the copies. The great and insufficiently appreciated advantages of Schlotterhoss's machine, particularly for printing upon chloride emulsion paper, are the following : — I St. Perfedl economy of time, as the changing requires the least amount of time imaginable, and can be done in the space of two seconds at the most. This point is of considerable value, especially in printing large numbers of prints by diffused day- light, as it then becomes possible during the brief time of regular light (about noonday) to make in rapid succession several hundred prints of equal value. Fig- 13- 2nd. Perfedl safety of the negative from breakage or mechanical injury by scratching, etc, as with this arrangement it is in a perfectly immovable and safe position. The sensitive paper is always pressed against it in a vertical diredlion. 3rd. Perfedl protedlion of the sensitive paper from con- tamination with the fingers, etc., which is a very important point, especially when the iron developer is employed. 4th. The ease with which it can be set up, and its port- ability. It is not necessary to use the light at the window, but any desired light intensity can be used by removing it further from the window. This is an advantage that is not possessed by any printing window. 5th. Its cheapness ; as notwithstanding the numerous arrangements required for the different operations which auto- matically follow the turning of the crank, it only costs ninety shillings. REGULATION OF THE EXPOSURE. 39 Regulation of the Exposure. To obtain prints equal in value from the same negative and with the same illuminant, it is necessary that the duration of the exposure be corredtly maintained. This is of especial importance when pidlures of the same tone as albumen prints are to be pro- duced on chloride papers, as in this case a slight under or over exposure, at least when strong diffused daylight is used, will cause a noticeable change in the tint in developing. With bromide of silver, and also when long exposures are made with weak light, a little error is of less importance. Nevertheless, even with a perfecft printing arrangement, permitting of an equal commencement and finishing of the exposure, the operator will require to attain a certain amount of regularity in his operations and in the manage- ment of the printing apparatus. Aid of the Chronometer in Printing. — When exposing with artificial light, the operator should direct his attention to the distance from the illuminant, the position with regard to the axis of the rays, and should observe the light itself. If without other means, the exposure must be counted, although it is difficult to count evenly, and mistakes may be easily made. For this, there- fore, a watch with second hand, a metronome, seconds pendulum, or clock provided with elec5lrical bell signal is to be recommended. The bell signal is the best as it only affects the sense of hearing, leaving the eyes free for other observations. The Seconds Watch must be brought close to the printing arrangement, so that the hands remain free, and one is able to look simultaneously at the printing frame and the timepiece. The Seconds Pendulum is rather inconvenient on account of its length as well as for the reason that it requires to be frequently set in motion, unless very perfectly construcfled, in which case it is a rather expensive instrument. In using it, it is necessary to observe that the beat of the pendulum be not too slow. For occasional use, and in cases where no other resource for ascertaining the time is at hand, the improvisation of a seconds pendulum is recommended. A piece of fine string, exacSlly two metres long (ySf inches), is drawn through a perforated leaden ball, or any other round weight, and the two ends are suspended to the ceiling at a distance of exa(5lly 22 centmieters (8f inches) from each other. The leaden ball hung in the centre of the string forms a gently swinging seconds pendulum measuring vertically 944 milimetres (39i inches). The Metronome* is a much smaller contrivance, and pos- sesses many advantages. In its ordinary form it may be employed by marking the position of the running weight for a half-second or * An instruiuent used for beating time for pianoforte players. 46 BROMIDE PAPER. a second. It would probably be better if the manufadturers were to accurately adjust it for these time-interval swingings. The metronome consists of a clockwork, which keeps a short pendulum in motion, having its support not at the upper end but below the middle, so that the upper part operates retardingly. The pendu- lum is fitted with a heavy weight at the lower end and a movable running weight on the upper part, by which the centre of gravity is, of course, lessened ; by pushing it downwards the speed is increased. More perfccfl metronomes have, besides, a small signal bell, which sounds after a certain number of oscillations, this number being regulated by a lever. The price of these instruments ranges between iis. and 17s. Thesignal bell attachment facilitates the work, as no doubt occurs regarding the number of oscillations that have been made. Signal Clock, with Elecftric ContacTts. — An excellent arrangement is a clock fitted with eledlrical contac5fs, as used by Schlotterhoss for his automatic exposer, if the same be provided with signal bells. A piece of clockwork is connecfled in the elecftric circuit of a galvanic battery in such a way that the current passes through the hand of the clock and through the metallic dial, which is insulated from the hand. Over the dial is placed a piece of paper cut out in parts, as shown in hg. 14. Over this the hand moves, and when passing over the parts which are cut out an ele(51ric contadf is made, and in passing over the paper the circuit is broken and the ele(5fric flow stopped. Further, if a small elecSfro magnet is connedled in the circuit, the anker of which is provided with a hammer or spring, to strike one bell every time the circuit is made and the other bell every time it is broken the apparatus is ready (see fig. 14.) The only thing necessary is to cut the paper discs accurately and in doing this we have to take into consideration the speed of the revolving hand, and the length of the contac5f circle. Supposing, for instance, the time occupied in one revolution of the hand is 3 minutes or 180 seconds, and the circular line of contacfl is 270 REGULATION OF THE EXPOSURE. 41 millimetres long, it Avould, therefore, be necessary to cut out portions of the paper 1-5 mm. for every second measuring in the periphery made by the hand of the clock. For the spaces between, sufficient paper should be left to give ample time for changing. For this purpose it will be well to have paper discs for every exposure time. For instance, for an exposure of 6 seconds and an interval of 5 seconds for changing, it will be necessary to make the cut out sections (always measure in the circle made by the hand) 9 millimetres broad, and the cogs or paper intervals 7*5 millimetres broad. This would give a total length of 16-5 milli- metres ; divided into the length of the periphery, 270 millimetres gives 16 exposures. As however, 16 x 16*6=264 or 6 less than 270, one cog would have to be broader by 6 millimetres. Purpose of Development.— The purpose of development is to transform the invisible latent image into a visible one. Development is a chemical process, and as such is dependent upon the degrees of chemical affinity which cause it, and further, can be influenced by change of temperature as well as by other circum- stances or by additions which either favour or delay the process. This chemical process is based on the power of the developer to ;il)Sorb oxygen and on the inclination of the haloid (combined with the silver in the emulsion) to unite itself with the hydrogen to form a haloid hydracid. Both the acflions united are more powerful than the sum of the affinities, which, in the exposed silver haloid, binds haloid to silver, and on the other hand hydrogen to oxygen. They are, therefore, able to overcome these, and to effedf a mutual decomposition and mutual exchange. Chemical AcTtion in Development. — Silver haloid, water and developer give as a result of development, oxidized developer incapable of further a(5lion, haloid hydracid, and first silver sub- haloid" and afterwards silver. This adlion, which is so important in photography, is a mutual process of transposition, and may be exemplified in a few remarks. If we place unexposed emulsion paper in the developer it will at first undergo no change, but after some length of time the emulsion film becomes slightly tinted, and only after a number of hours is the chemical process so far advanced that the film becomes vividly coloured. If, however, we previously hold the paper to the light for a moment, development is much more rapidly accomphshed, for by the influence of the same developer for a minute or so which previously gave no alteration on the unexposed film, an intense colouring is observable. Development. * Silver sub-haloid is hypothetical. 42 BROMIDE PAPER. Relation of the Light AcTtion to the Chemical Work. — By the above we see that the light exercises considerable influence over the chemical adlion. It is this co-operation of the hght that enables us to obtain an image by the chemical acftion by locally confining it in accordance with the negative. If the chemical development process is interrupted at that stage when the exposed parts only, and not the non-exposed parts, have been coloured, we obtain a coloured image on a white ground. If the developer is allowed to 3.61 for a longer time the unexposed parts would also become tinted, and finally the difference would be almost equalised, and we obtain a monotonous dark-coloured image. When Development should be terminated. — It follows, therefore, that development should be stopped at the latest when it arrives at that stage when the unexposed parts of the emulsion commence to darken. If one piece of the emulsion paper had been previously exposed for a short time, and another piece exposed for a longer period to the same source of light, we should find that in the same developing bath if the papers be taken out at the same time, how much more rapidly the longer exposed portion darkens than the one less exposed. If these exposures have been made under a negative, and if we develop the two latent images at the same time in the same developer until that stage arrives when the details in the high-lights have been brought out and the development is accomplished, we shall require to remove the longer exposed picture much sooner from the developer ; we shall also find that in the pic5lure which received the shorter exposure and required the longer time in development, much darker shades are obtained than in the picSIure obtained by longer exposure and shorter development. Influence of Time on Development. — It will be seen, therefore, that the depth of the shadows is much dependent on the time the development is allowed to adl according to the details in the high-lights. If, however, the shorter exposure had been under- taken with a relatively stronger light, the development would be accomplished in the same or a shorter time than required after a longer exposure to a weaker light, but the image would prove to be much stronger. The depth of the shadow tones increases with the depth and energy of the light ac5Iion. Influence of the Developer upon the Shadow Depth. — If we develop two pieces of emulsion paper both having received an exposure equally intense, in developers varying in strength, the weaker developer is not only slower in adfion but with the same time of development produces a lighter tone. But if we limit the development by the appearance of the details in the high-lights and not by time, the more concentrated developer gives the weaker tone. THE DEVELOPMENT. 43 Influence of the Transparency of the Negative upon the Shadow Depth. — If we expose two pieces of similar emulsion paper through negatives varying in density we find that the latent image obtained with the denser negative requires considerable time for development, but for all that gives great depth of shadow-tone, while on the other hand the latent image obtained with the thinner negative quickly develops in the high-li'^ht details, but gives much less depth in the shadow-tones. By this we observe that the depth of the shadow-tones is essentially dependent on the transparency of the negative, because with the amount of density of the negative the exposure and development must be proportionally increased. Taking these considerations together, the following may be deduced : — The result of development is influenced I St. By the proportion of the preceding exposure and by the intensity of the light employed. 2nd. By the duration of development. 3rd. By the strength of the developer. 4th. By the quality or transparency of the negative. An image giving the greatest depth of shadow would, there- fore, be obtained with as intense an exposure as possible, in conjuncflion with long development with as strong a developer as possible, and with very strong, dense negatives. But these con- ditions are not always possible. In some cases the above-mentioned factors cannot be made use of to the desired extent, for more often it is necessary to partially waive one consideration for another since the total results are required to possess, besides the greatest depth of tone possible, a rich detail throughout, without either any trace of fog in the high-lights or loss of detail in the shadow. On page 8 is given an example of two workmen labouring together successively to drag a certain weight to a certain height. The produc5f of their labours united gives the final result required, consequently the labour of the second workman is dependent on that of the first. In precisely the same manner are development and exposure dependent on each other, not only with regard to amount, but also with reference to the quality of the work and the time occupied. Thus a weak exposure requires a strong developer or long develop- ment. There thus exists a possibility of mutually substituting in part these two chief facftors of photographic development to obtain the proper results. 44 BROMIDE PAPER. The chief difficulty, especially with the negative process, is that the one agent, the exposure, always leaves us in a state of uncertainty and requires all deficiencies or excesses to be righted in the development. We are confined within certain limits, and in the positive process these are even narrower because the requirements with regard to deficiencies or excesses are not only stricter but have other points in addition, which in the negative process are of less importance, viz., retention of the colour of the pidfure and the absolute avoidance of fog. If these limits be overstepped certain defedls m the positive are produced, causing it to become more or less imperfect. Of these we may mention — Want of detail in the high-lights, absence of depth of shadow, deficiency of gradation in the shadows, merging of the shadow details, veiling, solarisation of the deeper shades, uneven tone of the image, 'i^hese defe(5fs will be best treated by a few pracffical examples. Over and Under-Development. The guide for the completion of development is the appearance of the high-light details. We withdraw our print from the developer as soon as the high-lights appear, and the image is completely developed. We then endeavour to free it from the adherent developer as rapidly and as thoroughly as possible by means of acid baths, and frequent washings to prevent any after development. Over-development a Technical Defecft. — If we did not at once stop the development the high-lights would become fogged. Under-development a Technical Defecft. — If, however, we remove the picfture before the details in the high-lights have sufficiently appeared, the result is a hard, imperfedl: pidture. Incorredl Relations between Exposure and Develop- ment. — In the majority of cases these easily avoidable defedts are due to incorredl: relations between the amount of exposure and the strength of the developer, and very often occur both at the same time. The deeper shadows may be over-developed, while the light details are missing, or it may be that the shadows are under- developed, that is to say, of insufficient depth, while the detail in the lights is already over-developed, and covered with a veil. Such cases as these are produced by both working facSfors, exposure and development being relatively too strong or too weak. Strong Developer with Over-exposure. — For if we develop an over-exposed latent image with a relatively too strong developer, the adlion will commence very rapidly, and will become completed so quickly over all the tones of the shading that the pidlure appears to flash up almost instantaneously. The necessary consequence will be that the shadows have no time to obtain any depth, and OVER AND UNDER-EXPOSURE. 45 they remain without strength, and therefore under-developed. With the hght details coming up so rapidly, they will also colour and become fogged or over-developed. The combined results, therefore, of both fa(5tors being too strong, will be flat and veiled images, over-developed in the lights, with little gradations and little depth of shadow, but to under-development in those parts. Weak Developer with Under-exposure. — When an under-exposed latent image is treated with a relatively weak developer, the development is very slow in commencing, and still slower in completing. In consequence, the half-tones appear at the same time as the deep shadows, and of about equal density; and, also, as development progresses in a lateral direcflion, the fine light parts in the shadow details, become gradually smaller, and finally, disappear altogether. By this the shadows appear cloggy, without detail, gradation or stru(5lure, and are, therefore, con- siderably over-developed ; whereas, the light details, owing to insufficient exposure, do not make their appearance at all, and remain under-developed. In the endeavour to obtain the desired detail in the lights, we have to develop for too long a time, and until, perhaps, completely unexposed parts of the film become accessible to development. When this occurs the film becomes tinted all over, and covered by what is termed total fog. The combined results, therefore, of both facitors, when employed too weak, are generally images far too blotchy in the shadows, owing to their being over-done, and with a hard appearance, due to the absence of detail in the high-lights. Very different, however, will be the results obtained if one or both fadlors employed be very much too weak. Whether the exposure was in the first place too short, or too weak an illuminant was employed ; or whether fog on the negative film opposed too much resistance to the light, a frequent occurrence in enlarging ; or whether an over-diluted developer was employed; in all these cases the overlying molecules of emulsion only will be developed, and this quantity being so small the pidfure has a superficial appearance without any depth of shadow or detail in the lights. Corredl Relations between Exposure and Development. As exposure and development can, within certain limits, be substituted for each other, a certain variation in the relative pro- portions is possible. The instances of incorre(5f relations discussed up to the present lie outside this, but although we speak of correcft relative proportions it must be borne in mind that the image necessarily varies somewhat with every variation in the relative proportions of the two facftors. From the possible relations three 46 BROMIDE PAPER. typical cases may be taken for discussion. We will suppose, in the first place, absolutely corre(5l proportions yielding perfedlly satisfadlory results. As the developer must be of a strength corresponding to the exposure, and as the exposure is always the basis and preliminary consideration, we will take three cases in which the exposure is short, medium and long respedlively. The results of these three cases will differ as a necessary consequence of the different methods of development. In the subordinate cases * given the results of incorrect relations produced by variations will be discussed. Apparent repetition in the discussions regarding these secondary cases, showing a certain uniformity of appearance must not mislead, as the results with regard to tone, especially with silver chloride emulsion, are charadleristically different. I. EXPOSURE SHORT. DEVELOPER STRONG. Development will commence rapidly and energetically, but will slacken as it proceeds, especially if at the end the delicate half-tones are to come out. Thereby the deep shadow-tones at first distincftly graduated, will strengthen considerably and become darker, the final result being a richly shaded dark image with delicate light detail. The colour of the bromide of silver pidfure will, in this case, be of a good black. The same with the chloride image, which is usually of a weak, yellowish brown, and will appear of a strong blue black tone. Subordinate Case (a). If the development, which at the commencement sets in with great vigour, slackens in a strikingly quick manner, so that the deep shadows strengthen to excess, and the light parts in the shadows clog before the more delicate high-light detail comes out, it will be easy to recognise the fad that either under-exposure was the cause or that the developer was relatively too weak. The black tone would, in this case, become somewhat deeper still in both the bromide and the chloride pidures. Subordinate Case (b). If, on the contrary, the development does not slacken considerably, and the details in the high-lights appear quickly and vigorously, so that the high-lights become weak while the shadows remain only half tinted and the colour instead of being black becomes grey, a circumstance especially notice- able with the chloride pidures, we may conclude that either over-exposure was the cause, or that the developer was too strong. The colour, especially with chloride pidures, will in the former instance be of a warm, or in the latter of a colder grey tone. 2. MEDIUM EXPOSURE. (IDEAL CASE.) The development commences moderately quickly, and con- tinues with the same speed until the moment the light details appear, when it is concluded. The shadows will have deepened without clogging, the result being a harmonious pidfure reproducing all the quality of the negative, well graduated in the shadows and with plenty of the most delicate light details. The colour with bromide of silver pidfures will, in this instance, be likewise a pure black, and upon chloride paper a rich sepia brown. CORRECT RELATIONS BETWEEN EXPOSURE AND DEVELOPMKNT. 47 Subordinate Case (a). If the development had set in late and had slackened considerably, or the pidture appeared somewhat cloggy, with little gradation and with undeveloped light details, then either the exposure was insufficient or the developer was somewhat weak. The bromide pidure will in both instances become only a trifle deeper black, while the chloride pidures will, in the former case, turn into brown, green, or even black, and in the latter will appear of a somewhat darker and colder sepia tone. Subordinate Case (b). If, on the other hand, the development had begun very rapidly and speedily become completed right up to the light detail, and if the high-lights appeared inclined to colour before the shadows have had time to strengthen, so that the picture had adopted a softer and feebler charafter, the error could only be in too strong an exposure or developer. The bromide pidture will again show in both instances a grey instead of black tone, while with chloride the pidture will, in the former case, be of a warmer and lighter brown, and in the latter a rich sepia brown, but of a darker tinge than in the principal case. 3. EXPOSURE LONG. DEVELOPER WEAK. Development sets in late and gradually brings up the shadow tints one after the other, and after protracfted development the light details also. The rate of development will not, however, slacken, but maintain a uniform speed. Notwithstanding this slow develop- ment in comparison with the previous case, the strength of the deep shadows will not be so great, because the effe(5t of the weak developer is less energetic and more superficial. Great vigour in the shadows can then only be obtained by the addition of restraining substances to the developer. Without these the result remains as an image wanting in vigour, indistindlly graduated, and a soft, thin charadfer, and with a slight over-development, will show inclination to fog. The tone of the bromide picfture inclines towards a warmer brownish black, and if much potassium bromide has been added it becomes a noticeable green tinge. The chloride pi(51:ure, on the other hand, gives a violet red colour which, after fixing, turns to a brick red. By gold toning, however, it assumes a beautiful rich violet colour. Subordinate Case (a). If development commences still later and slowly produces only a hard pidture, the fault must be looked for either in insufficient exposure or in the developer being too weak. The bromide pidture will then have a blacker image than in the principal case, and the colour of the chloride pidture will appear of a darker and much colder tone than the above. Subordinate Case (b). If, however, the image after a little while makes its appearance suddenly, the shadows and lights simultaneously, the latter being perhaps covered with a veil, either the exposure was too great or the developer too strong. The bromide pidture then appears slightly brownish black in colour, and if potassium bromide has been added, it has a conspicuous greenish tint. The chloride pidture will, in this case, be of a light tone which by fixing becomes of a light yellowish red. Moreover, such pidtures lying more on the surface lose much of their intensity in fixing. 48 BROMIDE PAPER. DIFFERENCE BETWEEN BROMIDE AND CHLORIDE EMULSIONS. The most important influence upon the combined e{fe6i of the other fadlors in the positive emulsion process is exercised by the kind of haloid contained in the emulsion. In the positive process bromide of silver and chloride of silver are exclusively used. As already mentioned, bromide of silver emulsion develops much more slowly than chloride of silver. The difference is so considerable that whereas the latter produces an image almost instantaneously with full vigour, bromide of silver, with a developer of equal intensity, and ample exposure, requires hours to yield weak indica- tions only of the image. On the other hand, the bromide of silver is, as is well known, considerably more sensitive to the process of exposure than chloride of silver. Chloride of silver closely approaches the conditions of decom- position in development, and allows of but a very weak developer. A strong developer would have a much too rapid acflion to allow the operator time to work with certainty. Bromide of silver, however, in chemical development has a feebler tendency towards decomposition, and therefore requires a very strong developer. Its small property of development neces- sitates the image produced by the light to be of very delicate proportions only. And this is best obtained by short exposure. Here the great sensitiveness of the silver bromide to the exposure acfts favourably, because this sensibility permits of the use of a very weak source of light and a short exposure. By special preparation of the silver bromide emulsion the sensitiveness may be increased considerably, by which the amount of exposure requisite is reduced to a minimum, while yet producing a sufficiently detailed image. DIFFERENT DEGREES OF REDUCTION IN DEVELOPMENT. The great difference in the strength of the developer to be employed with silver bromide on the one hand, and silver chloride on the other, implies also a considerable difference in the results, for besides the great rapidity and intensity of the development of silver chloride emulsion, the fac51 that we are able to make use of a strong light and a weak developer allows us to use that intermediate degree of reduction which gives a warm tone. Two Degrees of Reducflion. — The chemical development of the silver haloid produces two different degrees of reducffion, the first one having a reddish and the second one a black product. The latter is produced from the former by further prof^ression of the developing process. The more powerfully the developer is allowed to adf the sooner will the redu(5tion be complete, and the red produdl: change to the black. This explains the reason why, by using bromide of silver, and in consequence employing a developer of high intensity, we do not obtain red picftures. The existence of CORRECT RELATIONS BETWEEN EXPOSURE AND DEVELOPMENT. 49 the red intermediate produdl: can, however, with silver bromide, be shown experimentally. Further on we shall see why, in using exceptionally weak developers on chloride of silver, the development cannot be carried beyond the red stage, for although the black produ(51: of reducftion will eventually take the place of the red, it is necessary to terminate the development not according to these stages of reducftion but according to the appearance of the light detail. The first stage of Reducftion is not always noticeable. — It will naturally be inferred that in development the first visible producft of redu(5fion will invariably show the same red colour. That this is not the case the attentive worker cannot fail to discover. This can be explained, but it is perhaps unnecessary to give the explanation, which is somewhat involved. VARIATIONS IN THE TONE OF SILVER CHLORIDE IMAGES. It would be unjustifiable to conclude from the above statements as to the existence of these two different degrees of redudlion, that we are bound to produce picflures of these colours only, red and black. For the redudfion from the red or middle stage into the black is far from being carried out suddenly over the whole of the exposed particles of emulsion. This secondary redudlion is likewise efTedfed later in the deeper lying particles of emulsion than in the superficial parts, the latter being more affec5fed by the light. Change of Colour during Development. — During the process of further reducffion, progressing more or less rapidly according to the strength of the developer, the tone gradually changes according to the number of the red and black molecules produced, into reddish brown, sepia brown, brown green, and lastly black, and as it is in the power of the operator to have the development complete at any stage by proportionally changing the value of exposure, he can produce the image in any one of these different stages of colour. Therefore, the scale of colour must be richer in intermediate tones with slow than with quick development. Choice of Tone. — If we require to choose from amongst them, it is only a matter of terminating the exposure so that the light detail* (on the appearance of which the development must be concluded) becomes visible exadlly at that moment when development has reached the colour that is desired. The tones mostly demanded are the violet tone of the gold toned albumen print, and the pure black crayon colour. Both of these can be produced with certainty on silver chloride emulsion, if one has a clear idea of the above described relations of exposure and development. *By light detail is meant the delicate gradation between half-tones and pure high-lights, D 50 BROMIDE PAPER. Albumen Tone. — The albumen tone, only obtainable by gold toning, requires development carried on to a brownish red colour of the image, which can easily be managed by regulating exposure and developer. If at the first attempt the tone should be a little too brick-reddish, or otherwise too sepia coloured, it is possible to know how, by a little more or less exposure, we can correct it in future. Brick-red pictures show, when toned, an intense violet colour ; the more the original colour changes into a sepia tone, the darker and at the same time the duller becomes the violet tone obtained with gold. Black Tone. — To obtain a pure black on chloride paper it is necessary to use a very short exposure and strong developer. This proportion must be so arranged that the formation of the visible image in the red intermediate stage is delayed as much as possible, that is to say that the red tone only becomes visible at farthest at the commencement of development and in deep tones only. By holding fast to this rule the black tone will be devoid of brown or greenish shades which point to the presence of a producft of the middle stage ; the colour will then be pure black, equal to the most beautiful platinum print. Very concentrated neutral citrate, oxalate, and, above all, acetate developer, will produce this pure black colour, and weak sources of light will suffice. Chloride of silver development (especially with the assistance of gold or other methods of toning) affords an endless variety of tones, giving in some cases most brilliant effedls. Unfortunately, however, the matter has not been properly examined, and still less introduced into practice. This will only take place when photo- graphy frees itself from the absurd bonds of a fashion which only admits the tone of the albumen print. Scale of Colours with Bromide Emulsion cannot be utilized. — With silver bromide the author has succeeded not only in obtaining the red produ(5l experimentally, but also in producing images of a light reddish colour. Unfortunately, however, the red produdf is of no pradlical use with bromide of silver for the reason that the image in this red stage of development is quite superficial, flat, and usually only obtainable in the deepest shadows. Every attempt at increasing the value of the exposure or using weaker developer has resulted in solarisation. Influence of the Developer on the Tone. — Now, although the results of development are, in the aggregate, the same with all kinds of developers, yet in images produced by various developers differences exist in the shade as well as in the depth of tone. The number of different developers is exceedingly large, and in the nature of things they are necessarily oxygen absorbing substances. We can here only treat explicitly upon the few that are generally used, and that yield really good results. CORRECT RELATIONS BETWEEN EXPOSURE AND DEVELOPMENT. 5I Every Developer requires careful study. — Every developer, provided it is required to give other than incidental results, requires careful study ; the mere copying of a formula is insufficient. The student will obtain many merely negative results, as has already occurred to many an experimenter with hydroxy- lamine, acetate, and other novel developers. To the worker who disregards these difficulties and who intends to study a developer closely, we will briefly recommend a method which leads fairly quickly to the objedf in view, at the same time avoiding extrava- gant waste of material. Take a number of test-tubes and paste behind each one a scale (strips of closely ruled or squared note-paper will do very well) for the purpose of measuring proportions of mixtures placed in them. In these test-tubes mix the developer in systematic proportions in respe(5l of concentration, ingredients, etc., according to the diredlion in which you wish to study its adfion. First choose a cabinet or double c.-de-v. negative with an even gradation of light and shade, expose under it a sheet of emulsion paper, giving a different exposure to every half inch or so, from under- exposure at one end to over-exposure at the other. Cut the positive thus produced into narrow strips in such a way that each strip has all the different exposures on it, and write on the backs the proportions of the respecftive mixed developers, taking two slips for each test-tube. Each pair of these slips is placed simultaneously in the test- tube for development, one to be withdrawn after a brief develop- ment, the other to be left until development is complete. By this means we are enabled to see the course of development and the continual changes of colour of the image. After fixing and washing, the strips should be laid upon glass so that they can be more easily examined and handled. It is also desirable to describe the colour tints in their wet state, as many of them have characSferistic changes when drying. Finally, the strips should be placed in order after drying and mounted beside each other. They then form a plain illustration of the different degrees of develop- ment. Such experiments as these cannot be dispensed with by the investigating student. They will be found to give more satis- fadlory data than any other work. Variation m the Strength of Developer. The effe(5f of the developer depends on its inclination to oxidize. It, therefore, follows that I. A freshly prepared developer always possesses greater strength than one already used. 52 BROMIDE PAPER. 2. During development the power decreases more and more, and the smaller the quantity the more rapid is the decrease. 3. As the developer's property of absorbing oxygen is satisfied by contadl: with the air, the developing power of the developer must necessarily decrease by exposure to the air. Alteration of the Strength in the Dish. — The smaller the quantity of solution in proportion to its surface, or the larger the dish in proportion to the quantity of developer, and the quicker is the decrease in strength. It is more noticeable with strong developers than with weak ones. Rocking the Dish. — The principle of development requires continual substitution of the developer adhering to the emulsion by fresh. This we endeavour to obtain by rocking the dish : but the oxidation by the air is also accelerated. As this oxidation is greater than that resulting from development of the image, some operators are opposed to the frequent movement of the dish, and content themselves by only slightly moving the liquid or shifting the pi(5fures from time to time. The change in the intensity of the developer during the development requires compensation when a long series of pidlures is to be developed. Compensation for decrease in Intensity. — Besides this, owing to the withdrawal of the prints, some of the liquid is always removed, so that not only does the quality decrease, but the quantity also. If, for the purpose of compensating for the loss of quantity we add fresh developer without a restrainer, its great strength will also partly compensate for the loss of quality. Soaking of the Prints. — Air bubbles which adhere to the emulsion and prevent development beneath them, cause no small number of spoilt pi(5lures. Although the adhesion of these bubbles may be easily prevented, or they may be removed by properly placing the paper in the developer or by moving it to and fro ; yet some operators recommend wetting the prints with distilled water before putting them in the developer, so as to cause the developer to a(5f equally all over. Disadvantages of Wetting. — The author cannot recommend this as a general rule, as it weakens the developer, so that uniform a(51:ion can no longer be guaranteed, and the developer soon becomes useless. Besides, with iron developers, especially with the oxalate developer rich in iron, and in this state no longer very acid, dilution with water causes the separation of the basic salts which, in the shape of a yellowish precipitate, settle on the film, colouring it and being very difficult of removal. Advantages of Wetting. — With even the alkahne developers the preliminary wetting can only be recommended under special conditions, namely — I St. If economy and regularity of results are no objedl:. VARIATION IN THE STRENGTH OF DEVELOPER. 53 2nd. If development is accomplished by pouring the solution over. This is often necessary with enlargements when sufficiently large dishes are not at the operator's disposal. In this case preliminary wetting is necessary for those who have not had pradf ice in flowing over solutions. Well-water must not be used, however, but distilled water, or better still, an old and almost perfedfly oxidized developer applied by means of a soft brush without allowing the solution to become dry at any place. 3rd. If the development is done by means of a sponge as described in another place. Discretionary Variation of Strength. During the process of development it may become necessary to increase the strength of the developer to bring up missing light details, or to decrease the strength to correcff over-exposure, or finally to add a "restrainer" to obtain greater depth of shadows by slower progress in development. Such discretionary alterations may be obtained ist. By change of temperature. 2nd. By the addition of developer either more concentrated or in a more diluted or strongly oxidized condition. 3rd. By the addition of accelerating or retarding substances. Change of Temperature — The course of every chemical process, including development, is considerably accelerated by an increase of temperature and delayed by decrease. Warmth may be applied by pouring the solution into a dish which has been previously warmed, or by direcSt heating of the vessel. The withdrawal of warmth is accomplished by inserting the developing dish in a larger one filled with cold water or ice. The variation must never be excessive, a few degrees of temperature more or less may have an important influence upon the strength of the action, too great cold almost entirely stops development, and a few degrees over-heating (highest 75° Fahr.) will considerably accelerate it. But with the latter the film becomes soft, sensitive to scratches, etc., and liable to frill. Over 75° Fahr. the emulsion film becomes dissolved. Concentration of the Developer. — The strengthening of the developer by the addition of more concentrated solution is the simplest method. This with hydrokinone, pyrogallol, etc., is easily accomplished, but with the iron developers it will be necessary to provide a reserve solution especially made up for this purpose. The most concentrated form of iron developer is obtained by placing in the alkali solution (the potassium oxalate for instance) the iron sulphate in solid form and not in solution, and by shaking until all is dissolved. 54 BROMIDE PAPER. Dilution. — We may add water diredlly to the alkaline developers and dilute them with any amount of it without fear of causing decomposition or turbidity. Turbidity caused by Dilution with Water. — Of the iron developers, citrate, acetate and tartrate can be diluted with water for immediate use, but this cannot be done with oxalate, as it becomes turbid on the addition of water, and the basic salts are separated, especially if the solution be rich in iron. The presence of acid retards and decreases the separation. More suitable than aqueous dilution is the addition of an old oxidized developer, because the oxide contained in it also a6ls as a restrainer. There are accelerators and restrainers for the development process. Acflion of the Accelerator. — Accelerators assist the process of redudlion, and the light detail which determines the conclusion of development appears in the same manner as it would do without their addition, but much more quickly. The image becomes finished before the developer can adl upon the deeper portions of the shadows. These must consequently become weaker by the addition of much accelerator. Addition of Hypo. — For the iron developers sodium hypo- sulphite is the most powerful accelerator. It should be used for bromide development, dissolved in distilled water in the proportion of I in 200, and for chloride development in the proportion of i in 1000 only. Even in this proportion the addition of only a few drops will cause considerable acceleration. The use of the hypo accelerator has a considerable effedl: upon the resulting pidlure as well as upon the speed of development. With bromide paper it gives blacker tones than would be otherwise obtained after short exposure. With prolonged development it produces an almost certain trace of general fog. One must, therefore, be prepared to clear up the high-lights on the finished print with a piece of india- rubber eraser. With chloride paper the accelerator causes a rapid transition to the second degree of redudlion and, therefore, produces duller, greyer tones than would otherwise result. The employment of an accelerator need only be exceptional, especially with silver chloride, as the considerable under-exposures so frequent in the negative process need not take place in positive printing. Hypo preliminary Bath. — Hyposulphite of soda has been strongly recommended as an accelerator in the form of a prelimin- ary bath. Audra* recommended a preliminary bath of this kind with water in the proportion of i to 10,000 for gelatino-bromide, and recently Dr. Kriigner advocated the following formula for a prehminary bath in the development of silver bromide with eikonogen. I gramme sodium hyposulphite. 2000 ,, spring water. 15 ,, chloride of mercury solution (i in 100). * Photographic News, March 7th, 1884. DISCRETIONARY VARIATION OF STRENGTH. 55 The bath is used for about a minute, and then without any washing development is proceeded with. For chloride paper the author found a hypo solution in the proportion i to 40,000 used for 15 seconds only, to be sufficiently strong, as greater concentration produced fog. This solution is very rapidly used up, and must therefore be repeatedly renewed and employed as fresh as possible. The change in the colour caused by such a preliminary bath is only a very slight darkening in shade, while the acceleration of the development after 15 seconds in a I to 40,000 hypo solution is equivalent to a times increase of sensitiveness. Caustic Alkaline Accelerators. — The caustic and carbonic alkalis are especially important accelerators with alkaline develop- ment, as an increase in the addition to the developer will consider- ably increase its strength. Hydroxylamine developer, which has been quite used up and which has no further adtion upon the latent image, will become quite newly revived by the addition of caustic alkalis. The amount of the addition is however limited, as if too great a proportion be used it will cause frilling and blistering. Acftion of Restrainers. — The adlionof the restraining agents is probably an indire(5l dissolving of the smallest particles of the reduc5fion producft during the process of their formation (i.e. separa- tion). As the dissolving adlion is greatest in the high-light detail where the smallest particles of the reducftion produdl are more completely isolated, and therefore offer more points to the attack of the chemical adfion, it is decidedly more visible there than in the shadows where separation takes place in greater quantities. On the whole the development of the image is much slower, and it will be seen that the shadow tones have more time to develop deeply, and consequently obtain more vigour. If the addition be too great, hardness and absence of the finer high-light details will result. The above statement explains the a(5lion of restrainers upon the formation of fog, which they considerably retard if they do not entirely prevent. Potassium Bromide as a Restrainer. — The best known restrainers are mentioned below : — Potassium bromide acls as a most energetic restrainer and fog preventer. It is often used in development with pyro, oxalate and eikonogen, but rarely with hydrokinone. For silver bromide development a solution with distilled water is usually kept in stock, proportion i to 10; and for silver chloride in the proportion of i to 50, and a portion is added to the developing bath before commencing development, and a portion during the progress, in drops by means of dropping bottles. Acftion on the Colour. — For the development of bromide positives the author has abandoned potassium bromide as a restrainer because it turns the tone greenish, and he prefers an old, already much-used developer. The acflion of potassium bromide 56 BROMIDE PAPER. upon silver chloride images is very striking. It makes them slow to change their colour, the red tone remaining much more brilliant and vivid than if developed without the bromide. The blue-black colour of the silver chloride images in the black stage of develop- ment becomes of a pure black by the addition of a little potassium bromide, but by a large addition it becomes greenish. Ammonium bromide has an analogous a(5lion, but is seldom used. With pyro development especially it adfs as a much stronger restrainer than potassium bromide. According to Messrs. Spiller and Young the retarding strength of potassium bromide to ammonium bromide is only as 3 to 5, and according to Eder as about I to 10. Potassium iodide and tin(5lure of iodine have been often recommended for bromide dry plates, but are rarely used nowadays. More important for positive development may be considered the chlorides, namely, sodium chloride, ammonium chloride, mercuric chloride, and iron chloride. Alkaline Chlorides. — Sodium chloride and ammonium chloride have a much weaker action than bromides, they are consequently only employed in silver chloride development. The best pradfice is to keep in stock for this purpose a solution in distilled water made up in the proportion of i to 10. The addition to every 3 fl. oz. of developer may amount to as much as 45 drops (minims). The influence upon the colour is far less with these salts than with potassium bromide, and they also keep the developer clear. Sodium chloride is employed with the various iron developers as well as with hydrokinone. Mercuric chloride is rarely used. According to Eder it only results in effe(5ling greater strength of reduction, that is to say, a greater thickness; but according to others it also effecfls a blacker colour and clearness. Kriigner uses mercuric chloride in developing Avith eikonogen as an addition to the hypo preliminary bath for under-exposed pi(5lures. Iron Chloride. — The addition of iron chloride was recom- mended some time ago for negative development by Prof. Eder. The liquor ferri-perchloridi of the chemist is diluted with an equal volume of water, and of this solution up to 25 drops are used for every 3 fl. oz. oxalate developer for bromide paper. Iron chloride has a strong restraining and fog-destroying adlion. The colour of the image is not so strongly influenced as with potassium bromide, but still its effecSl is distincffly noticeable. If a large addition be made the half-tones of the image will appear very coarse grained and as if they were strewn over with black spots, and the whites will be yellowish. Every trace of the developer with which it is used must be thoroughly removed by very careful and repeated washing before fixing. Acids. — Strong mineral and organic acids, as well as in a lesser degree, their salts, have generally a retarding influence. Some, however, are only rarely used as restrainers, but usually DISCRETIONARY VARIATION OF STRENGTH. 57 added for other reasons. Thus, sulphuric acid, citric acid and others are added to the pyro and other alkahne developers for the purpose of neutralising the alkalinity of the sodium sulphite. In the same manner an acid (citric, tartaric, or sulphuric) is added to the iron sul- phate solution to prevent turbidity caused by separation of basic salts. In the majority of these cases their restraining a(5lion is not considered, and often their presence is the cause of hardness in the developed image, which is attributed to other causes. With iron acetate especially, a very small excess of acid suffices to cause exceeding hardness. The addition of acid to the ferrous sulphate solution must be undertaken with care, as with this developer perfecft neutrality will alone guarantee a good, regular and soft development. With other iron developers, however, a slight acidity is advisable ; for these are acid developers originally, and a slight increase in the acidity is of little moment, or positively advantageous. It is by the formation of salts on their addition to the developer that the acids adl as restrainers, and some workers prefer to use ready prepared salts, as, for instance, potassium citrate with pyrogallol. No doubt such salts have a favourable influence upon the clearness of the image. In adding acid to the alkaline developer we must not forget that by so doing a certain quantity of acffive alkali is withdrawn from the developer, and it necessarily becomes weakened. An excess of iron sulphate has a restraining acSlion, but it is seldom employed. With the oxalate developer the addition of sulphate of iron is limited. If the quantity given be exceeded, turbidity is produced, by the separation of ferric salts in the form of a yellow precipitate. With citrate, acetate, etc., no such turbidity takes place with excess of iron sulphate, but the strength of the developer is diminished and the development of the image is considerably retarded. Oxide Salts as the best Restrainers. — The oxide salts of the different developers are, in general, excellent restrainers. Therefore, a much used developer w^orks clearer and gives greater shadow depth than a fresh solution. The salt must not be employed in the form of an addition, but either a quantity of freshly made developer is exposed to the air for some time, or one works with an old developer, strengthened from time to time with fresh solution. The use of the oxidized developer is advisable both with chloride and bromide paper. In the development of enlargements especially, to obtain great depth of shadow and black tone, all other restrainers, especially potassium bromide, must be avoided, and a partly oxidized developer always used. In the oxalate developer the oxide salt partly precipitates to the bottom in the shape of emerald green crystals, and partly adheres to the sides of the stock bottle. Citrate and tartrate, however, keep the oxide salt in solution, acetate only partly so. All developers except hydroxylamine become consider- ably darker coloured by the presence of oxide salt, and one can see by the colour whether they contain sufficient or not. 58 BROMIDE PAPER. Used Alkaline Developers. — Much used pyrogallol, hydro- kinone and eikonogen solutions work decidedly clearer than new, so that with these developers also it is best to commence with a partially oxidized developer, and afterwards add fresh, strong solution to bring out the fine high-light details. The restraining and fog removing action of the oxide salts is not so strong as that of potassium bromide, although the oxide salt has a double adlion, at least with iron developers, as it has not only the dissolving property analogous to potassium bromide, but also a retarding influence upon the chemical adfion owing to the slimy, thick consistency which it gives to the developer. Viscous Substances. — Sugar, gum, dextrine, glycerine, gelatine, etc., adf as restrainers by the same process of rendering the solution dense, and thus retarding the chemical ac5fion. But these substances, producing viscid developers, give rise to air bubbles and streaks, and consequently cause much waste of paper. ILLUMINATION DURING DEVELOPMENT. The light must, of course, be non-aclinic, but it may be much brighter than is used in cutting and changing the paper. Bromide paper may be developed in yellow light, provided the source be an artificial one. Daylight will require red glass. White Light for Chloride Development.— Chloride emul- sion may be best developed by the diredf light of a stearine candle if its light falls only upon the developing dish, and the paper is exposed to it only when in the developer. Development by candle light without coloured media possesses the specially important advantage that one can judge the tone better, which is the chief point with chloride images. MANIPULATION IN DEVELOPING. Developing is best accomplished in flat, open dishes as used for dry plates. Developing Dishes. — It is immaterial whether the developing dishes be made of porcelain, glass, papier mache, varnished tin, or wood. It is of more importance that the same dish be always used for the same purpose, and not first for the fixing solution and then for the developer, as with such indiscriminate use the solutions are sure to be contaminated. For large picT:ures, especially for enlarge- ments, the author uses paraffined wood dishes with great success. These dishes are made of dry, polished boards, coated on the outside with asphaltnm varnish and on the inside with hot paraffin wax. The paraffin film may easily be renewed. Quantity of the Developer. — It is always well to have a good quantity of developer in the positive process, as by this means the decrease in intensity is not so considerable (see page 52). An exception is when single over-exposed images are to be developed. DISCRETIONARY VARIATION OF STRENGTH. 59 In this case it is better to develop with as Httle as possible, so that the strength is rapidly decreased and the development of hght details is retarded. Placing the Prints in the Bath. — The prints should be inserted fihii upwards and gradually lowered into it edgeways to prevent as far as possible the formation of bubbles. At the same time the prints should be reversed to see that the surface is well covered with the solution and thoroughly soaked. Air bells which remain adhering to the pidfure surface must be removed as soon as possible. These air bells are easily formed if the prints be placed in the bath film downwards ; they appear more frequently if the developer has already been once used, and, therefore, contains a large percentage of oxide, or if a viscous substance such as glycerine, gum, etc., be employed. The solution does not then adhere so easily as if freshly made and thin. The whole operations of inserting, pushing down, and afterwards reversing should be done quickly, especially if the developer is strong. Flowing the Developer over. — With enlargements where one has to simultaneously cover a large piece of paper, one cannot always use sufficient developer to evenly cover the whole of the pi(5fure when inserted. The flowing-over method is then adopted (see page 52). Many Prints in one Bath. — When a great many pid:ures are to be developed, several prints can always be in the bath at the same time, but they must be inserted at equal intervals and care must be taken that there is sufficient quantity of developer to prevent them from lying too closely upon each other and adhering together, or irregular development will result. Of course, we here suppose that the development is a slow one, for with rapid develop- ment (for instance, in strong black chloride images with acetate) it is a difficult matter to keep watch over several different develop- ments. Finger-marks. — Considering the great reducing adlion of the fatty acids, salts of perspiration, it is necessary, at least with neutral and acid development, to touch the prints as little as pos- sible with the bare finger. Flat horn or ebonite forceps are suit- able for taking hold of the paper positive, though some people prefer india-rubber finger-stalls. In case of necessity the fingers maybe dipped in and thoroughly rubbed with the developer, and then an instrument for laying hold of the paper is unnecessary. With alkaline development there is no chance of staining the picffure by the perspiration. If unhealed wounds in the hands are repeatedly wetted with oxalate, hydrokinone, pyrogallol, etc., extensive sup- puration will be caused. Rocking during Development. — If only a small quantity of developer is used, constant rocking is indispensable, because the 6o BROMIDE PAPER. paper does not lie flat, and owing to its wavy position it may be covered unevenly with the liquid, in fadl, some parts may not be covered at all, and these would appear as dry spots (see also page 52). How far is Development to be carried ? — The rule, to develop until the high-lights begin to shade, although correcft with diapositives on glass, does not hold good for paper positives. This moment would be too late. The paper should be removed from the developer the instant the detail in the high-lights is distinguish- able, which may be taken as a kind of guide. The high-lights themselves should remain entirely white. The little excess in development which has no injurious effecSl on the transparency of the negative or diapositive and gives a guarantee of perfedl: develop- ment, would produce on the white paper a shade of the lights. This must, of course, be absolutely avoided if brilliant positives are desired, or the only course left is to clear up the high-lights with a piece of india-rubber. There is, however, an exception to this rule. This is chloride emulsion paper which is to be subsequently toned. The toning operation has a dissolving adtion which first becomes noticeable in the lights. If these be slightly shaded by over development they will become clear in the toning. Developers for Bromide of Silver Emulsion Paper. All developers used for ordinary bromide dry plates can also be used for bromide paper. As, however, with positive printing an agreeable tone is desired, they cannot all be used with equally good effe(5fs. Only those agents which give a rich deep black, but at the same time, soft tone, even without the use of bromide restrainers and which have no influence upon the gelatine or upon the paper substance, can be satisfactorily used. To systematically classify the different developers it would be well to divide them into three classes, i.e., the alkaline, neutral and acid. The Alkaline Developers are those with which the power of developing the latent image depends more or less upon the nature or quality of the alkali. These are pyrogallic acid, hydro- kinone, hydroxylamine, eikonogen and others. All these are organic substances which reduce the silver salts. The reducing power is entirely caused or increased by the presence of an alkali. The Neutral and Acid Developers are the salts of iron (oxalate, citrate and others). These have a neutral or more or less acid nature, as if made alkaline they would not only mostly separate, as insoluble basic salts, but their reducing energy would also be too powerful, and the unexposed as well as the exposed portions of the emulsion film would become black. THE PYROGALLIC ACID DEVELOPER. 6l The alkaline developers decidedly surpass the iron developers in developing power, but have, however, in comparison, a great disadvantage owing to rapid oxidation that colours them brown (especially in warm weather), and gives a yellow stain to the gelatine film (except with hydroxylamine) if development is pro- longed. Almost as soon as prepared the developer shows signs of discolouration, if precautions are not taken to free the distilled water, which is used for dissolving the different chemicals from air, by boiling it and cooling down in a closed flask. As, however, the presence of an already oxidized developer has a favourable influence upon the brilliancy of development, the slight colouration of the developer by the oxygen contained in the water need not be heeded. THE PYROGALLIC ACID DEVELOPER. Pyrogallic acid is a white substance usually crystalhzing in bright flakes or needles. It dissolves readily in water (2^ parts) as well as in alcohol and ether. Solutions have a neutral readlion, but become alkaline by the addition of a very small quantity of an alkali. They may be pre- served if protedfed from the air, but if exposed to its influence they rapidly absorb oxygen and become of a brown or black colour. This property of alDsorbing oxygen is increased by the presence of an alkali. Addition of Sodium Sulphite. — In order to preserve pyro- gallic acid and render it capable of being used as a developer for a lengthened period, and also to preserve its effedf within prescribed limits, it is dissolved''' in an aqueous solution of neutral sodium sulphite or in a solution of ammonium sulphite.! Acids and Bromides are Restrainers. — In pra(51:ice the sodium sulphite is the only one usually employed. This salt gives a slight alkaline reacftion, and is very easily decomposed. For the latter reason a few drops of sulphuric, citric or formic acid are usually added, but not too much, however, as such acids, as well as their salts, usually have a considerable restraining influence upon the adliion of the developer (see page 55). Bromides have also a similar effedl. Alkalis are Accelerators. — Caustic potash or simple alkaline carbonates, on the contrary, acft as accelerators. As a substitute for sodium sulphite, the metabisulphite is recommended. This adfs as a more powerful preserver of pyro solution than the sodium sulphite ; it is, however, a strong restrainer, and must therefore be used with caution. Power of Pyro. — Next to Eikonogen, pyro is the strongest known developer.]: Its a(5tion is very rapid, and it produces * Berkeley. + Eder. I Its reaction cannot always be corre(5tly ascertained, as it is always alkaline. 62 BROMIDE PAPER. extremely delicate details. Its disadvantages are that it stains the film, especially during warm seasons, that it stains the hands, and that it is poisonous. If the hands have open wounds or imperfecfily healed cuts on them it is necessary to be especially careful. The ordinary Pyro and Soda Developer.* Solution I. loo grammes sodium sulphite (crystallized) are dissolved in 500 grammes of distilled water (free from air), Then add 14 grammes of pyrogallic acid, and 5-10 drops of sulphuric acid. Solution II. 50 grammes of carbonate of soda crystals (chemically pure). 500 grammes of distilled water. For development take equal parts of sol. I., sol. II. and distilled water. The developer should be perfecfily colourless and odourless ; if it is to be used several times it should be kept in well-corked bottles filled as full as possible. When repeatedly used it works similarly to other old developer — harder and giving stronger con- trasts. The best restrainer is potassium bromide, 3 to 10 drops (i in 10) to each 3^ fl. oz. of developer. (h) The Pyro and Potash Developer.! — The potash developer has a stronger adlion than the soda, but produces picftures of a browner colour. Solution I. 25 grammes of sodium sulphite (crystallized) dissolved in 100 grammes of distilled water (free from air). Then add 3 to 8 drops of concentrated sulphuric acid, \ and 10 grammes of pyrogallic acid. The filtered solution should keep well for several months. Solution II. 90 grammes potassium carbonate, and 25 grammes of sodium sulphite dissolved in 200 grammes distilled water and filtered. For development mix 100 c. c. m. distilled water, 3 solution I., and 3 ,, solution II. The potash developer gives a slight yellowish tint to the gelatine film. This discolouration can, however, be mostly removed by bathing in a concentrated solution of alum or of aluminium sulphate (Eder), or in Lainer's acid fixing bath. It will sometimes be necessary with paper, however, to clear the lights with a piece of india-rubber. As restrainer a few drops of potassium bromide (i in 10), or from 30 to 50 drops of potassium citrate (i in 10) are added to every 3^ fl. oz. of developer. * Cooper, from Eder's "Jahrbuch der Photographie." + Stolze, from Eder's "Jahrbuch der Photographie." X Or from i to i| grammes of Citric Acid. THE HYDROKINONE DEVELOPER. 63 (c) The Pyro and Soda Metabisulphite Developer." Solution I. 4 grammes pyrogallic acid and grammes potassium metabisulphite are dissolved in 100 c. c. m. distilled water. This solution keeps several weeks if stored in well-corked bottles. Solution II. ID grammes carbonate of soda crystals and 15 grammes sodium sulphite dissolved in 100 c. c. m. distilled water For use take equal parts of I., II. and distilled water. This developer adts times slower than the ordinary pyro and soda developer. By increasing the proportion of soda and lessening the amount of water the vigour of the image may be increased. Dr. Eder obtained with this developer some very beautiful results, with perfecftly clear high-lights, upon the author's bromide of silver emulsion paper. THE HYDROKINONE DEVELOPER.! Hydrokinone takes the form of colourless crystals soluble in warm water, alcohol, or ether. The hydrokinone of commerce is mostly in fine greyish white floury crystals, or in small monoclinic tablets, resembling benzoic acid, in which shape it may be pre- served by careful sublimation. Besides the white or colourless hydrokinone there is also sold, commercially, a so-called permanent hydrokinone (Dr. Heinrich Byk's) which has a yellow colour, and consists of a combination of hydrokinone and sulphurous acid. The acid gives it a greater permanency than the ordinary produ(fl:. Hydrokinone, in the solid form, resists the adlion of the atmos- phere, l3ut its solutions gradually oxidize and become brown. Its reducing acftion, which is based on the easy separation of two atoms of hydrogen, may be considerably increased by the addition of alkalis, especially ammonia. Mixed with the latter, the hydro- kinone solution, when exposed to the air, rapidly becomes of a reddish-brown colour. Additions. — Abney's first suggestion was a mixture of hydrokinone and ammonia. Since then, however, formulae have been given for hydrokinone, w^ith potash, soda, caustic potash, caustic soda, mostly with the addition of sodium sulphite or potassium metabisulphite. These two salts play the same important part with the hydrokinone developer as with pyro. Dissolving. — In preparing the hydrokinone developer the sulphite of soda solution should be made first, and heated to about 160° Fahr., and the hydrokinone dissolved in it, as it requires a heated solvent. The hydrokinone and the potash, soda, or other * Eder, in Eder's " Jahrbuch der Photographie." i First suggested by Abney in 1880. 64 BROMIDE PAPER. solution can be kept separately, and only mixed immediately before developing, or the whole developer may be mixed up, but separate keeping is better. Preserving. — As a means of preserving the readily-oxidizing hydrokinone, either alcohol or sodium sulphite, or potassium metabisulphite may be used. They are given in the order of their inferiority. Alcohol only preserves the developer for a short while, as will be noticeable by the brown colouring of the solution. Metabisulphite is the strongest preservative, its adlion lasts several months ; but, unfortunately, it also adfs as a very powerful restrainer, and has not therefore found much favour. On the other hand, the use of alcohol has been discarded. Restrainers. — Besides potassium metabisulphite, acids also adf as restrainers, especially acetic and citric acid and their salts ; also bromides, and old, used, and consequently oxidized developers. Accelerators. — The accelerator is " an increased addition of the alkali." Hypo is not, however, an accelerator. The hydro- kinone developer is not quite equal to the pyrogallic developer in power, but it has the advantage of better keeping qualities. Further, it does not stain the film or the hands. Yellow Stain. — A slight yellow stain on the film will, how- ever, be produced with hydrokinone if the time necessary for development is very much prolonged. In this case Lainer's acid fixing bath is to be recommended. Hydrokinone, prepared with sodium sulphite or potassium metabisulphite, may be preserved for some time. It has a light-brown colour, and may be repeatedly used ; it gradually works clearer and harder. When much used it discolours the film in case of prolonged development. Staining in Hot Weather. — ^But if it becomes of a dark- brown colour, which in hot weather it will do, it loses its strength and stains the film badly. Temperature and Duration of Development. — With hydrokinone the duration of development is somewhat dependent upon the temperature. Dr. Bannow found that with the same exposure 2 to 3 minutes were required at 70° Fahr., and at 60° Fahr. as much as 7 or 8 minutes. Hydrokinone solution is very sensitive to the smallest particles of iron salts, which are sometimes to be found in the cracks of dishes previously used for the oxalate developer. A fixing bath in which prints developed by iron developers have been previously fixed must on no account be used. Hydrokinone, according to Mr. G. Robens, possesses poisonous qualities, and must on no account be allowed to get into flesh wounds. Various authors have given different formulae for the hydrokinone developer. The author has sele(fted these from Eder's Jahrbuch, i88g, and many German, French and English publications, and tabulated them. To simplify comparison, they have all been calculated to i part of hydrokinone. HYDROKINONE DEVELOPERS. 65 S 2 ° a; O o • V ( — - P JQ > M OOOOOOiOO oi oc c^ O ccvj 000000 ■ I II O >-i w Hydro- kinoiie Sodium Sulphite Potassium Meta- b^sulphite A'cohol. Caustic Potash Caustic Soda Potash Soda Lime Water Citric Acid Acetic Acid Potassium Bromide * Bouillard employs either of these developers alone, or mixed in the proportions 4 to 1 or 2 to 1, according to the exposure. •* Mathet prepares the sugar of lime as follows:— 100 parts of slaked lime, 160 parts of sugar, 160 parts of glycerine, and 1000 parts of water are mixed up together, allowed to htard 24 hours, and then filtered. t Isiering prepares sugar of lime solution by placing in a bottle about one-third of quiclilime, and filling up with a sugar solulion (6 parts of sugar to lOO parts of water). After thoroughly shaking, it is allowed to precipitate and then decanted. It can be filled up wiih ihe sugar solution several times. ft Himly gives as an accelerator about three drops of a caustic soda solution (1 In 8) to every 100 c c m (3i fl. oz.) of developer, and recommends an acetic acid bath after devtlopment for positives. 66 BROMIDE PAPER. THE EIKONOGEN DEVELOPER. " Eikonogen " (pidlure producer) is the name given to a developer discovered by Dr. Andresen. Its chemical name or formula will not be of any interest to the non-chemist. Com- mercially it is supplied in the form of fine, bright, colourless, floury crystals. It is readily soluble in v^ater, and its solution (which is of a pale, greenish-yellow colour) has the valuable quahty of not turning brown if exposed to the air, as is the case with pyrogallol and hydrokinone. With the ready-made developer a discolouration takes place after it has been used several times, and has been standing some weeks. It then changes from a light greenish-yellow to a darker shade, and then to brownish-green. Its permanency surpasses most of the other developing agents. The developing power of eikonogen is at least equal to, if not surpassing, pyrogallol. The author has proved the possibility of obtaining a strength five or six times greater than with the oxalate developer. Finally, eikonogen is said to be absolutely harmless, and free from poison. Restrainers. — The restrainers are bromides and acids, especially acetic acid and its salts. With this developer the tone of the print is not so unfavourably influenced by the addition of potassium bromide, as when employing oxalate. The use of an acid bath (acetic acid) immediately after development is indis- pensable ; the use of Lainer's combined alum and fixing bath is also to be recommended. Krugner's Two-solution Formula. — Dr. Kriigner's formula for developing is as follows : Solution No. I. 200 grammes (6| oz.) sodium sulphite are dissolved in 3 litres (5J pints) distilled water, and add 50 grammes (13 drs.) eikonogen, which soon dissolves. Solution No. II. 150 grammes (4! oz.) soda crystals are dissolved in I litre (if pints) distilled water. For plates take 3 parts of solution I. to i part of solution II. For positives upon bromide paper the addition of i to 2 parts of water is necessary. Solution I is not acidified. Krugner's One-solution Formula. 200 grammes (6^ oz.) sodium sulphite and 150 grammes (4! oz.) carbonate of soda crystals are dissolved cold in 4 litres (7 pints) distilled water, and finally add 50 grammes (13 drs.) eikonogen in dry powder. For plates this solution is used without the addition of water. For positives on bromide paper 2 to 4 pints of water are added. With these formulae half the time of exposure is necessary com- EIKONOGEN DEVELOPER. 67 pared with the ferrous oxalate. For shorter exposures a preHminary bath is used. For longer exposures the developer must either be diluted with half its bulk in water, or potassium bromide added. It is not necessary that the soda crystals should be chemically pure, but they may be the same as used for domestic purposes. Instantaneous Developer. — For Very Rapid Instantaneous Exposures. Dissolve 10 grammes (2J dr.) sulphite of soda and 5 grammes (ij dr.) carbonate of potash in 150 grammes (5^ fi. oz.) distilled water, and add 5 grammes (i| dr.) eikonogen. With this developer instantaneous picffures may be obtained that are impossible with any other developing agent. The potash should not be used in general work, because its acflion is too energetic. For the same reason ammonia should be avoided. Potash development should not be employed for positives, as it is too powerful, and almost invariably causes fog. The distilled water can be substituted by pure rain or soft well water, or well water purified with soda by adding fifteen to thirty grains of soda to every pint of water, and allowing it to become clear. To obtain the best results it is necessary to use perfecftly fresh sodium sulphite. Preliminary Bath for Short Exposures. 1 gramme (15^ gr.) sodium hyposulphite. 2 litres (3J pints) well water. 15 drops bichloride of mercury solution (i in 100). The prints are bathed for about i minute, and developed without any previous washing. Dr. Vogel's Formula. — According to the experiments of Dr. E. Vogel, jun.,^' equally good results may be obtained by using less eikonogen and more alkali, which the author can confirm, for his emulsion paper. Dr. E. Vogel, jun., gives the following formula : Solution No. I. 40 grammes (loj dr.) sodium sulphite (crystallized), (or 20 grammes anhydrous). 5 grammes dr.) eikonogen. 500 grammes fl. oz.) distilled water. Solution No. II. 60 to 75 grammes dr. to 18 dr.) potash. 500 grammes fl. oz.) distilled water. For use mix equal volumes of solutions I. and II. This developer produces, as the author has proved, equally fine images on emulsion paper as Kriigner's formula. Potassium bromide was not added. In using this developer it will be found that old, used developer * " Photographische Mitteilungen," 1889, page 94. 68 BROMIDE PAPER. works clearer and gives greater depth in the shadows than freshly- made solution. There is no doubt that eikonogen will take a prominent place, especially for the development of enlargements. ALUM BATH. After development, the prints are washed and placed in the alum bath (i oz. of alum to i pint of water) for one minute. They are then washed for a short time and fixed. The alum bath must not be overlooked, and must be employed previous to fixing. It can be used many times. A very good effedl: can be produced by the use of an acid-alum fixing bath, according to Dr. Kriigner, as follows : — 200 grammes (6| oz.) hypo. j 200 grammes (6J oz.) hypo. 100 grammes (3 oz. i dr.) sodium sulphite. j 50 grammes (i J oz.) sodium sulphite. 50 grammes (ij oz.) alum. 1000 c. c. m. fl. oz.) water. 1000 c. c. m. (35^ fl. oz.) water. | 6 c. c. m. (if dr.) sulphuric acid. THE FERROUS-OXALATE DEVELOPER is a developer which the author, notwithstanding the advantages of hydrokinone, still prefers for bromide paper. ADVANTAGES. Non-Staining. — Although it is a yellowish-red colour direcftly after mixing, and becomes darker on oxidation, it does not stain the prints. Regeneration. — It also possesses the advantage that it may be partly regenerated if iron wire (or turnings) be added to it, and the developer exposed to the light. It is therefore possible to use the same solution for some time. It will in this way work clear and give great depth, and only requires renewing occasionally with fresh concentrated oxalate solution. Preservation and Permanency.— In narrow-necked bottles, filled up to the neck, it will keep for many months without any alteration. When required for use it is decanted. The author has abandoned the use of preserving bottles with mineral oil ; any narrow-necked bottle will answer the purpose, the only thing necessary is to keep it well filled up to the neck. Cheapness. — The oxalate developer, owing to the number of times it may be used, is probably the cheapest, and produces the richest black tones in the shadows, and the purest tints if properly used. A green-black reduc51:ion is produced by faulty exposure and development, with potassium bromide as a restrainer. To obtain the pure black tone the potassium bromide should be entirely discarded, and only old developer employed as a restrainer. THE FERROUS-OXALATE DEVELOPER. 69 DISADVANTAGES. In comparison with the alkahne developers the oxalate possesses a few disadvantages, ist. It cannot be made so strong, and therefore necessitates a slightly-increased exposure, which facft, however, need not be considered with the positive process. 2nd. It is very sensitive to the influence of perspiration, so that finger marks must be carefully avoided. GENERAL OBSERVATIONS. Composition. — The oxalate developer is composed of solutions of neutral potassium oxalate and of iron sulphate. According to the experiments of Eder and Valenta, a double salt is produced, i.e., protoxide of iron or ferrous oxalate of potash, upon which the developing action depends. Proportions of Combinations.— The mixture must be made in certain proportions ; if too much iron sulphate be used in propor- tion to the potassium oxalate, a yellow powder is formed, i.e., ferric oxalate, which is precipitated to the bottom of the bottle. The proportions given by Eder are the following : — Solution I. Neutral potassium oxalate, 100 grammes (25^ drs.) Distilled water, 400 grammes (14 fl. oz.) Solution II. Iron sulphate, 100 grammes (25^ drs.) Distilled water, 300 grammes (10^ fl. oz.) Pure sulphuric acid, 5 drops. According to later formulae, citric or tartaric acid is to be preferred to sulphuric acid ; from 15 to 30 minims are used. Both solutions should be clear, and must therefore stand for a little time or be filtered. Stock solutions may be made. The oxalate solution keeps for an indefinite time. The iron solution, however, gradually oxidizes and becomes cloudy, owing to the separation of basic sulphuretted iron oxide. If to be kept for some weeks it should be in a narrow-necked bottle, filled as full as possible, and well corked. Method of Mixing. — The mixture must always be made in the following manner : One part of iron sulphate solution (II.) is poured into four parts of oxalate solution (I.) If mixed in the opposite manner the protoxalate of iron is separated as an insoluble yellow powder. The ferrous oxalate developer thus formed should be a clear, reddish-yellow solution, and may be employed at once for developing. Green Crystals in used Developer. — By use the solution gets more darkly coloured by the formation of oxide salts, which, unless the solution be acid, separate a basic ferridoxalate of a rust- brown colour. If allowed to stand still it finally separates potassio 70 BROMIDE PAPER. ferric oxalate in the form of emerald green crystals. The proportion of this oxalate regulates the clearness and absence of fog in developing, and as a restrainer is to be decidedly preferred to the addition of potassium bromide on account of the tone. THE FERROUS CITRO-OXALATE DEVELOPER was suggested by Abney. It is considerably weaker than pure ferrous oxalate, but as with the positive process short exposure need not be considered, and the ferrous citro-oxalate developer gives really fine results with a black tone ; it deserves to be noticed, especially as it is distinguished for its greater durabihty compared with the ordinary oxalate. Formula. — Abney's formulae are no doubt intended for chloro- bromide of silver emulsions, as they give much too weak developers for pure silver bromide emulsion. The author produced a citro- oxalate developer for bromide paper as follows : — loo grammes drs.) potassium oxalate and 100 grammes drs.) potassium citrate are dissolved in 800 grammes (20 fi. oz.) water, and at the same time 62 grammes (16 drs.) iron sulphate (solid) and 2 grammes (30 grs.) citric acid are dissolved by continual shaking. Mixing Ready-prepared Developer. — By dire(51: mixing of equal parts of ready-prepared oxalate with a ready-prepared citrate developer, the latter, in a concentrated form, producing and dissolving stable iron sulphate in neutral ammonium citrate, the author obtained a splendid depth of tone in the shadows. The mixture is always attended with cloudiness from the separation of ferrous oxalate, which must be allowed to precipitate. Neverthe- less, this developer works exceedingly well when the yellow salt has settled. DEVELOPERS FOR CHLORIDE PAPER. As stated on page 48, the developing capacity of silver chloride is much greater than that of silver bromide, because the silver chloride is more easily decomposed ; therefore, the developers used for it must be much weaker. Further, although with chloride of silver the first (red) stage of redudlion appears so strongly that it suffices for a pi(5fure, yet a considerably weaker developer is necessary than for the second (black) stage of redudlion. Therefore, two different developers, varying in intensity, must be discussed for silver chloride development. Of the alkaline developers only three are perhaps suitable — these are the hydrokinone, hydroxylamine and eikonogen. A number of different iron developers may be used, however, as besides the oxalate we have also the citrate, tartrate and acetate, all very useful developers, and several modifications, as for instance, the iron acetate developer with gallic acid, etc. With chloride it is more essential than with bromide paper that the THE PYROGALLIC DEVELOPER. developers should be slightly oxidized, in order to produce a clear image entirely free from fog. The addition of restrainers is always permissible in developing red images, and is to be recommended. It is not, however, advisable for black tones, for the reason that these restrainers cause the black tone to incline to green. The same applies to the acidity of the developer, which may also be considered a restraining influence. Red images, therefore, require the use of a somewhat acid developer ; while black must have only a feebly acid developer if required to be free from veiling and of a pure black tone. This, of course, does not apply to the alkaline developers, the alkalinity of which is always necessary. The acid bath applied immediately after development is indispensable for all iron developers. THE PYROGALLIC DEVELOPER. The pyrogallic developer has only been used in chloride of silver development in exceptional cases, probably owing to its great power. Nevertheless, it may be used. Dr. Just's Formula. — The following formula gives warm brown to sepia tones with an exposure of about 4 to 5 seconds to diffused daylight in a room. A longer exposure gives redder tones. Solution A. no c. c. m. (3 fl. oz. 7 dr.) water. 4 grammes ii dr.) sodium sulphite. 0 2 grammes (3 grs.) potassium metabisulphite. 0-5 grammes (8 grs.) pyrogallic acid. 0-4 grammes (about 7 drops) glacial acetic acid. Solution B. no c c. m. (3 fl. oz. 7 dr.) water. 2-25 grammes (34 grs.) carbonate of potash, Add together equal parts. This developer is almost colourless at commencement, but turns brown by use ; it works clearer after having been used a little, or if to every 100 c.c.m. (3^ fl. oz.) of it 5 drops of potassium bromide solution (i in 50) are added. After development a bath well acidified with acetic acid must be used to remove any stain produced by old or brown developer. Lainer's acid fixing bath may also be employed. Avoid leaving the prints too long in the washing water previous to fixing; the various operations should, in fadf, be done in as rapid succession as possible, otherwise there will be staining. PicSfures developed with pyro are not quite so vigorous or so rich as those obtained with other developers, especially hydrokinone. The addition of hydrokinone may be made with advantage. THE HYDROKINONE DEVELOPER. For silver chloride development hydrokinone possesses many good qualities, at least so far as concerns the first stage of reducflion. The red to sepia tones appear rich and vigorous, and the transition 72 BROMIDE PAPER. is slow, a great advantage in case of faulty exposure. When freshly made it produces at times a cloudiness, but if already used and slightly oxidized, it works very clearly and vigorously. The table on opposite page shows a few of the formulae for the hydrokinone developer that have been recently given, and of which the potassium metabisulphite, and the same with acetic acid, are to be specially recommended. All the formulae are calculated for I part of hydrokinone. THE HYDROXYLAMINE DEVELOPER. For chloride of silver, hydroxylamine is a faultless developer, by means of which fine and vigorous red-brown and sepia tones may be obtained. It keeps very clear, and requires no acid bath, but necessitates the use of an alum bath, owing to the gelatine film becoming slightly loosened in the developer, which contains caustic soda. Blisters need not occur if the following method is stricflly carried out. The addition of potassium bromide is indispensable, in order to keep reduLT:ion within desired limits ; a larger quantity can be used for developing chloride paper than for bromide. Author's Formula. — The author recommends a hydroxy- lamine developer made up in the following manner : — Solution I. 10 grammes (2J dr.) caustic soda dissolved in 1000 c. c. m. (if pints) v/ater. Solution II. 5 grammes (77 grs.) hydroxylamine hydrochloride. 100 c. c. m. (3^ fl. oz.) alcohol (90%). 200 Q. c. m. (7 fl. oz.) water. Solution III. I gramme grs.) potassium bromide in 50 c. c. m. (14 drs.) water. For brownish-black tones (exposure about ^ second to diffused daylight) mix together: — 100 c. c. m. (3J fl. oz.) solution I. 17 c. c. m. (5 drs.) solution II. 25 drops solution III. For sepia tones (exposure ^ to i second) : — 100 c. c. m. (3J fl. oz.) solution I. i2| c. c. m. (3^ drs.) solution II. 25 drops solution III. For red to reddish-brown tones (exposure 2 to 4 seconds) : — 100 c. c. m. (3I fl oz.) solution I. 8^ c. c. m. (2J drs.) solution II. 25 drops solution III. THE HYDROKINONE DEVELOPER. 73 Detaille Bros, j modified. | Ditto j Ditto ( d D Author Pizzighelli ... I Warnerke Name 4^ CO Oj OOOOOOOOO 1 a^l Water Alcohol 1 1 1 1 1 1 1 1 1 8 1 to Hydrokinone. 1 — I 1 ^ to ^ 1 1 1 1 1 ' Cn ' 1 1 1 1 1 Sodium sulphite 1 1 1 1 1 1 1 1 1 1 Potassium metabisulphite 1 1 1 1 ^ 1 1 1 1 M 1 Caustic Potash 1 1 1 1 1 1 1 1 i 1 ° 1 Ammonium carbonate Ln \ U\ 1 4^ M ^ 1 -4^ 1 1 1 Potash 1 1 1 1 1 1 ^ 1 1 1 1 1 Soda - 1 1 1 1 1 1 1 1 1 1 Acetic acid 1 1 1 1 1 1 1 1 1 1 1 Potassium bromide I I I " I Sodium chloride UO MM O Oo O O CD o o < 2 a, ►-I O r^- O O rD O o o >-< o »-! I-! O) p' o o o ^ o o 3 o o a> ft) (/) 0) O) en in <^ a> ^ in u) 74 BROMIDE PAPER. EIKONOGEN. Eikonogen, properly diluted, adls as a very fine developer for chloride paper. The author is of opinion that the potash developer is not sufficiently regular, and in spite of restrainers, is inclined to fog. With Potassium Metabisulphite. — The soda developer with potassium metabisulphite, made up after the following formula, produces excellent results : — Solution I. 300 c. c. m. (loj fl. oz.) water. 4 grammes (i dr.) sodium sulphite (crystallized). I gramme (15J grs.) potassium metabisulphite. Solution II. 300 c. c. m. (loj fl. oz.) water. 4 grammes (i dr.) carbonate of soda (crystals). I c. c. m. (15^ grs.) glacial acetic acid. Mixed in equal parts. With an exposure of 3 to 6 seconds in diffused daylight (in a room) the author obtained with this developer very clear, vigorous, red to reddish-brown pitftures. Place them diredfly after development in an acid bath (acetic acid I in 500 is the best). With Sodium Bicarbonate — According to private asser- tions of Dr. Kriigner, sodium bicarbonate is a valuable substance for preparing eikonogen solutions for development. of chloride paper. The author obtained from his suggestions some excellent results with the following mixtures: — Instead of solution I. the ordinary (Kriigner's) soda developer, consisting of 80 c. c. m. (2f fl. oz.) of water, 4 grammes (i dr.) of sulphite, 3 grammes (46 gr.) of carbonate of soda, I gramme (15I- gr.) of eikonogen, and 10 drops of potassium bromide ; and for solution II. a five per cent, aqueous solution of sodium bicarbonate. A mixture of 100 parts of solution II. with 20 parts of solution I. gives intense red pidfures with an exposure of 3 seconds in a room. A solution made up of 100 parts of No. II. and 40 parts of No. I. gives with an exposure of 2 seconds, reddish-brown, and with an exposure of i second, sepia-coloured images. THE FERROUS OXALATE DEVELOPER. The oxalate is a very useful developer for chloride of silver if properly made and with corre(5l exposure. The same rules as given on pages 68 and 69 must be observed in making up the solutions, as well as in the method of mixing. Here, too, oxidized developer works decidedly better than fresh. On the other hand, the addition of potassium bromide is decidedly to be recommended, provided that blue-black tones are not sought for. The proportions are as follows : — I. For red brown tones (the same as gold-toned albumen paper). Exposure 6 to 8 seconds to diffused daylight in a room. THE FERROUS OXALATE DEVELOPER. 75 A. Dissolve in looo c. c. m. (35 fl. oz.) water go grammes (3 oz.) potassium oxalate. B. 500 c. c. m. (18J fi. oz.) water. 25 grammes (6^ drs.) ferro'is rsulphate. 2 grammes (31 grs.) citric acid. 0 2 gramme (3 grs.) potassium bromide. B is poured into A, well shaking the while, but on no account pour A into B. This developer may be used at once, or may be kept stored in well-corked bottles. Its strength may be decreased by the addition of 2 to 6 drops of potassium bromide solution (i in 50). With this the tone becomes redder, and the development retarded. The strength may be increased by the addition of developer (see following formula). 2. For blue-black tones (expose 2 to 4 seconds to diffused daylight in a room) : — A. B. Dissolve in 100 c c. m. (3^ fl. oz.) water. 600 c. c. m. (21 fl. oz.) water. 30 grammes (yf drs.) ferrous sulphate. 100 grammes (3|oz.) potassium oxalate. 2 grammes (31 grs.) citric acid. Mix as above suggested. By adding from 2 to 10 drops of potas- sium bromide solution (i in 50) the tone changes to a pure black, and then to a brownish-black ; that is to say, without the bromide the tone is blue-black, with a small addition of bromide, pure black, and with a large quantity, brownish-black. Double Development for Sepia-brown Tones (exposure 3 to 6 seconds to diffused daylight). Development is first of all accomplished in developer No. i (above) until the half-tones have appeared and the detail in the high-lights is still invisible. The prints are then placed in developer No. 2, where they are completely developed, and become more vigorous, and the tone alters to a dark-brown. If the development progresses too rapidly an intermediate washing between the two developments should be given. The acid bath must be used direcflly after development with these developers. The method of regenerating the oxalate developer is described on page 68. THE FERROUS CITRATE DEVELOPER. In the manufacSfure of this developer ammonium citrate must be used, as both potassium and sodium salts produce inferior developers. Eder and Pizzighelli employed the acid ammonium citrate, manufadtured after the following manner : 600 grammes (19^ oz.) of citric acid are dissolved in 2000 c. c. m. (3^ pints) water. This solution is then neutralized as accurately as possible with ammonia (any excess may be removed by heating), and then are added 400 grammes (i2f oz.) of citric acid, and as much water 76 ROMIDE PAPER. as will bring the volume of the solution up to 9 pints. Except for the formation of a white skin on the surface this solution keeps well for an unlimited period. The developer is made up after the following formula : — go parts of acid ammonium citrate solution. 30 parts ferrous sulphate solution (i in 3). 6 parts sodium chloride solution (i in 30).. This developer appears perfecftly clear, does not precipitate, and is of a light-green colour ; if exposed to the air the colour changes to a dark-green, the transformation proceeding from the surface. With an exposure of 8 to 10 seconds to diffused daylight (in a room) this solution produces fine reddish brown images, full of vigour, which may be altered to a more agreeable albumen paper tone by means of a gold bath. Preparation of Neutral Ammonium Citrate. — The author has used the neutral ammonium citrate with considerable success. Neutral ammonium citrate is made by dissolving 600 grammes (ig^ oz.) of citric acid in 2 to 3 litres (3I- to 5|- pints) of distilled water, and neutralizing with ammonia with the utmost accuracy possible. Any excess of ammonia may be removed by the appli- cation of heat, or a better plan is to re-obtain a very slight acid readfion by the addition of a small quantity of citric acid, and again neutralize. Finally the total volume is increased to 4 litres (7 pints). This solution may also be preserved, and is less inclined to form mould than the acid salts. Preparation of the Developer. — The mixing of the developer is done as follows : — 150 c. c. m. {^^ fi. oz.) ammonium citrate solution. 50 grammes (if fi. oz.) ferrous sulphate solution (i in 3). This solution obtained should be of a clear, green colour, which does not become cloudy, as with ferrous oxalate, if there be an excess of iron sulphate solution. Excess of Ferrous Sulphate. — A normal development is obtained if there be an excess of ferrous sulphate and no alteration worth mentioning is produced in the tone. The excess of ferrous sulphate solution a(5fs as a weak restrainer. If less iron solution be taken than stated the same depth of colour can no longer be obtained. The developer produces fine, greyish-black and delicate tones, and a sepia colour with a long exposure. Dilution. ^ — If the strength of the developer is reduced one-half to one-third, development does not proceed further than the first stage of development, producing only red tones. Concentrated Developer. — The neutral citrate developer may also be made with increased strength, as follows : — Dissolve 168 grammes (5 oz. 3 dr.) of citric acid in 500 c. c. m. (17I fl. oz.) of water, and neutrahze with ammonia as described THE FERROrS TARTRATE DEVELOPER. 77 above. Increase the volume of the ammonium citrate solution thus cleared to 800 c. c. m. (28^ fl. oz.) by adding water. Now add to every 100 c. c. m. (3I- fl. oz.) of this solution 15 grammes (4 drachms) of ferrous sulphate in crystals, and shake it well up until they have dissolved. Allow the white crystalline precipitate to settle and the solution to become slightly oxidized by exposure to the air, when it is ready for developing. Regeneration. — The citrate developer may be regenerated by placing some pieces of iron wire in the stock bottle, slightly acidifying with citric acid and placing the bottle in diffused day- light. By this means the developer which has become dark green and black by exposure and use will again become clear and bright, and may be used again. THE FERROUS TARTRATE DEVELOPER. In the manufadfure of this developer, also, the ammonium salt must be used, as the commercial potassium and sodium tartrates fail to produce vigorous developers. Preparation of Ammonium Tartrate. — Neutral ammonium tartrate may be prepared by dissolving 300 grammes (8| oz.) of tartaric acid in about litres (2^ pints) of distilled water and neutralizing with ammonia. In neutralizing a thick crystalline precipitate of acid salt is first formed, which, however, re-dissolves on the full addition of ammonia. Finally, the solution is increased to a volume of 2 litres (3^ pints) by the addition of water. Mixture with Ferrous Sulphate Solution. — The mixing of the tartrate with the ferrous sulphate solution, in the proportion of 3 to I, is accompanied by a gradual separation of the acid salt, which dissolves with difficulty. This change, however, has no influence upon the developing process, and is, therefore, harmless. The precipitate is allowed to remain in the stock bottle. The tartrate developer possesses at first a fine yellow, wine- like colour, but becomes much darker after a time without detriment to its qualities. It is a weak developer, and only effetffs reduction in the first or red stage. The pidfures have a colour ranging from a warm reddish-brown (mahogany) to (at farthest) a sepia-brown, and require a rather long exposure, namely, from 8 to 12 seconds in diffused daylight. THE FERROUS ACETATE DEVELOPER, first suggested by the author, belongs to the most distindl: and useful class in the group. It is easily and conveniently produced in varying percentages of concentration, is equal in stren Stereoscopic pbotoorapb^. Translated from the French of F. Drouin by Matthew Surface. Principal Contents: Binocular Vision — The Perception of Relief — Various Forms of Stereoscopes — Applications of Stereoscope— Stereoscopic Photography- Stereoscopic Negatives— Stereoscopic Prints, etc.- 180 pages. More than 100 illustrations. Ipbotoarapbi? for Hrttsts* By Hector Maclean. Contents: The Extent to which Photography is used by Artists — (J:)ncerning Various Kinds of Artists' Studies — The Right to Copy Artists' Studies— Photographic Reproductions of Work of Art— Some Photographic Falsities— The Photographic Misrepresentation of Tones — Falsifications in Photographic Printing — Some Reasons why Artists should Use a Camera— The Choice and Use of Apparatiis, etc., Suitable for Artists— Indoor Photography: Models, Sitters, Copying Pictures— On the Reproduction of Pictures— Illustrations for Photographic Reproduction— Condensed List of Photographs for Artists— List of Reference Books. 152 pages, with an appendix consisting of 16 pages illustrations, besides 19 diagrams and illustrations in the text. IPboto^rapbic Xenses: ibow to cboose m\b ibow to nise* By John A. Hodges. An Elementary and Practical Guide to the selection and use of Photographic Objectives. Contents: Optical Principles— Definition of Terms- Various Defects in Lenses— The Diaphragm or Stop, and its Functions — Single Lenses — Upon the Properties and Use of Single Lenses — The Rapid Rectilinear, or Non- Distorting Doublet — Other Forms of the Doublet, including Wide- Angle Lenses — Portrait and Universal Lenses— New Types of Lenses, Constructed of Jena Glass — On certain Obsolete Lenses— Upon the Choice of a Lens— The Care of Lenses — Upon Focussing — Upon Angle of View — Distortion: and its Avoidance by the Use of the Swing Back — Combination Lenses, Casket Lenses, and the Use of Back Combinations — How to Test a Lens— Lenses of Foreign Construction— On Purchasing Second hand Lenses— Dallmeyer's Tele-photographic Lenses. 148 pages and 36 original illustrations, including eight half-tone engravings. Ube Ibalf^ITone process* By JuLivs Verfasser. A Practical Manual of Photo-Engraving in Half-Tone on Zinc and Copper. Second edition; revised and in great part re-written. Contents: What is Half- rone?— The Studio and its Fittings— The Camera, etc.— The Screen— The Dark Room — The Printing Room — The Etching Room — The Mounting Room — Negative Making — Failures and Remedies in Negative Making — Printing from the Negative — The Etching— Mounting and Proving. 172 pages and 75 illustrations, with four full-page blocks in half-tone by the Author. t>alUZonc on tbe Bmerican Basis. From the Personal Experience of Wilhelm Cronenberg, Translated by William Gamble. Chapters on Photo-Engraving in America— Apparatus for Negative Making — The Negative— Stripping and Reversing the Negative— The Printing Process — Etching- Finishing Work— Engraving— Vignettes. 56 illustrations in the text, and twelve supplementai-y on art paper at end of book. 164 pages. BMates anb papers : *ft)ow /iRaDe mt> insct). By Henry C. Stiefel, Ph.D. Giving Instructions How to Make Albumen, Gelatine, Collodion, Platinum, Carbon, and other papers, and How to Print, Tone, Develop and Fix the Picture upon them, based upon the author's practical experience in factory and studio. 35 chapters, nearly 200 pages, with several illustrations. PERCY LUND, HUMPHRIES & CO., LTD., Bradford and London. Advts.] DR. just's bromide PAPER. THE POPULAR PHOTOGRAPHIC SERIES. IN SIXPENNY VOLUMES, NET. These books deal with the elementary and popular side of photography, and it is intended that the series shall cover the whole field of this fascinating subject. They are elegantly got up, unique in shape and appearance, fully illustrated where necessary, and supply a need long felt for cheap standard literature on photography and its innumerable applications, suited to the purses and requirements of amateurs. No. 1. Drop=Sbuttet pbotoorapbi^. By Fred. W. Pilditch. With 22 Half-tone illustrations. Contents : Advantages- Evolution of Hand Cameras— Instruments— Choice of Camera— Dark Slides— The Lens —Movement of Objects— The Shutter— Finders— Focussing— Light— Suitable Subjects —Street Scenes— Use of Photography to Teachers — Train Views — Development — Formulje — Test your Light — Fixing Baths — Clouds — Seascapes Possibilities — Animals — High Speed— Printing Process— Appendix, etc., etc. No. 2. XTbe H)arF?=1Room anb its Equipment. By H. J. L. J. Masse. Principal Contents : Fitting up a Dark-Room— Lighting Day, Oil, Gas, Electric— Ventilation — Chemicals and Bottles required — Weights and Measures— Home-made Apparatus and Appliances — Notes on some Developing Formulae — Photographic Poisons and their Antidotes — Storing of Plates and Films. (Second Edition. Sixth Thousand.) No. 3. ^Lantern SUbes : ^Tbcir production anO THec. By J. Pike, with a Preface by J. Pattison Gibson. Contents: Introductory — A Consideration of Negatives — Printing by Contact — Printing by Reduction — Printing by the Carbon Process — Clouds —Exposure and Development: Formulas — Toning and Intensification— Mounting and Finishing— Conclusion. (Second Edition.) No. 4. H)epelopers : ^beir xase and Bbuse* By Richard Penlake. Contents: Theory of Development — Treatment of the Plate — Pyro- Ammonia — Pyro-Soda — Pyro-Potash—Hydroquinone— Ferrous Oxalate — Amidol —Eikonogen—Metol—Glycin—Rodinal— Failures and How to Avoid Them— Various Formulae— Weights and Measures— Prices of Chemicals. (Second Edition. Sixth Thousand.) No. 5. XTbe Camera anb its Bppurtenances, By H. J. L. J. Masse. Principal Contents: Cameras— Changing Bags, etc. — Cycling and Photography— Dark Slides— Diaphragms— Exposure Tables— Focussing, Aids to— Hand-Cameras — Lenses, etc., etc. No. G. TTbe H B C of IRetoucbin^. By Andrew Young. With examples of both portrait and landscape retouching, and a guide to the anatomy of expression. Principal Contents: Character of the Sun-drawn Image— Methods of Correction— Preliminary Examination of the Film— Removal of Accidental Flaws— How to Apply the Work— Retouching the Head and Bust— Facial Anatomy— Landscape- Retouching the Print. A Permanent Library.— Binding Cases, in neat cloth, gilt lettered, to hold the first six volumes of this series, price 1/0 ; post-free, I/l. No. 7. ipbotoQcapbi? anJ> Brcbitecture : ^^Jfte^^'esVmbe other. By E. Macdowel Cosgrave, M.D. With 43 illustrations from the author's photographs. Deals with Cathedrals— Epochs in Architecture— Early Stone Monu- ments — Camera to Use — Lenses, etc. No. 8. jnboor iPbotograpbg, ff'asbimbt stijwe8 By Bertha M. LoTHROP. With over 20 Half-tone illustrations. Contents: Work not dependent on Weather— Children Fascinating Subjects- Simple Accessories- Daylight V Flashlight. No. 9. Ubc ^Elements of Stereoscopic pbotoGtapby. By C. P. Seymour Rothwell, F.C.S. Principal Contents : Advantages— Principles Simply Explained— The Twin-Lens Camera— Size of Plate— Stereoscopic Hand Cameras —Selection of Lenses— Separation of Lenses— Exposure Shutters— Selection of Subjects, etc. No. 10. xibe 'X' iRa^s» By Arthur Thornton, M.A. With 25 Half-tone and other illustrations. Contents Sound— Longitudinal Vibrations— Ether Vibrations— Hertz Electrical Vibrations— Light —Electrical Discharges through Gases— Discovery of X Rays— Fluorescent Effects- Photographic Methods - Radiographs— Uses of Radiography— The Nature of X Rays. In the Press. No. 11. Ube Camera anb tbe BJcn. By T. C. Hepworth, f.C.S. „ No. 12. Ipbotograpbig as a Ibobbs. By Matthew Surface. PERCY LUND, HUMPHRIES & CO., LTD., Bradford and London. DR. just's bromide PAPER. [Advts. GENERAL PHOTOGRAPHIC BOOKS. Burton's /IDanual of ipbotoGtapbi^. By W. K. Burton, C E. A practical handbook for all who are taking up photography. An explicit guide to all ordinary photographic manipulations. The latest information. 184 pages, illustrated. Paper covers, 1/0 net; post-free, 1/3. UaWiB on pen anb JnF?. By Elizabeth M. Hallowell. Many photographers would like to be able to sketch, so as to be able to dot down ideas or little bits too small to spare a plate for, or too inconveniently situated to enable the camera to perform its work properly. 52 pages. Crown 4to. 1/0 net; post-free, 1/1^. Zbc practical pbotoarapber's fiVBt 1bant)booF?» By Matthew Surface, Editor of The Practical Photographer and The Junior Photographer, with a Preface by H. P. Robinson. 6d. net ; post-free, 7d. Hntboni^'9 international HnnuaL The recognised leading Photographic Year Book of America. Issued about the first of December every year. Price 2/0 net ; post-free, 2/4J. 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With a Chronological Record of Discoveries, Inventions, etc., Contributions to Photographic Literature and Personal Reminiscences extending over forty years. Crown 8vo. 812 pages. Illustrated with 10 collotype portraits. Neatly bound cloth. Price 3/6 net ; post-free, 3/9. H Ibistor^ of pbotograpbp. Written as a Practical Guide and an Introduction to its Latest Developments by W. Jerome Harrison, F.G.S., with an Appendix on the Discovery of the Gelatino- Bromide Process. Demy 8vo. 150 pages, twelve illustrations of the Fathers of Photo- graphy. Paper, 1/6 net ; Cloth, 3/6 net. Post-free, l/8i and 3/9. PERCY LUND, HUMPHRIES 8c CO., LTD., Bradford and London.