SOCIETY FOE THE ENCOURAGEMENT 
 
 OF 
 
 ARTS, MANUFACTURES, AND COMMERCE. 
 
 CANTOR LECTURES 
 
 RECENT ADVANCES IN 
 PHOTOGRAPHY, 
 
 CAPTAIN ABNEY, R.E., F.R.S. 
 
 ELIVERED BEFORE THE SOCIETY OF ARTS, JANUARY AND FEBRUARY, 1882. 
 
 LONDON: 
 
 PRINTED BY WILLIAM TROUNCE, 10, GOUGH SQUARE, FLEET STREET, E.C. 
 
 j 
 
 1882. 
 
 Price One Shilling, 
 
Digitized by the Internet Archive 
 in 2018 with funding from 
 Getty Research Institute 
 
 https://archive.org/details/cantorlectureson02abne 
 
RECENT ADVANCES IN PHOTOGRAPHY, 
 
 By CAPTAIN ABNEY, R.E., F.R.S. 
 
 LECTURE I. —Delivered January 30 th, 1882 . 
 
 In beginning this course of Cantor Lectures, 
 
 I make this presumption—that amongst my 
 audience I have those who have commenced 
 photography hut recently; also, those who 
 commenced photography about the time I 
 did, say twenty years ago ; and that there are 
 very few now who practice those old pro¬ 
 cesses which our forefathers, if I may use 
 the expression, were accustomed to manipu¬ 
 late. I have also reason to believe that amongst 
 my audience there are some who know very little 
 of photography at all. I must try and do the 
 best I can to address myself to all. It is a 
 difficult thing ; but I hope those to whom I 
 am not exactly fitting my remarks at any time, 
 will remember that I am addressing others, 
 who I also have to consider. 
 
 I thought, perhaps, it might be useful to 
 go over the very oldest processes with which 
 photography started ; and when you recollect 
 that photography is not very old—in fact, is a 
 chicken compared with some of the sciences, 
 although there are many amongst us who 
 can remember its discovery, I think it may be 
 of interest if you see worked out practically 
 before you these very early processes to which 
 I refer. 
 
 The first germ of photography was when 
 Scheelc investigated the action of light on 
 silver chloride, and found that it discoloured it. 
 It was subsequently found that this discoloura¬ 
 tion of the chloride was due to a liberation 
 of chlorine from the chloride of silver. In 1802, 
 eightyyears ago, Wedgwood reada paperbefore 
 the Royal Institution, in which he described a 
 'method of taking profiles, and also copying 
 painted pictures, by means of what we should 
 how call photography. These profiles were 
 faken on paper washed over with silver nitrate. 
 In other words, the profile of the person to 
 0e portrayed was thrown against sensitised 
 
 paper, in a strong light, and the outline was 
 photographed or made visible by the strong 
 light acting outside the profile. Of course it 
 produced a white profile on a black back¬ 
 ground. About the same time that Wedg¬ 
 wood read this paper, Sir Humphry Davy 
 was practising this process, and he found 
 that silver chloride was preferable to silver 
 nitrate. He also found that if he used white 
 kid as a basis of what I will call the photo¬ 
 graph, he got a stronger impression than if he 
 used simply paper. In 1814, twelve years 
 later, Niepce devoted himself to studying the 
 action of light upon asphaltum, and he worked 
 out a process which up to this day is still used, 
 and which I shall not refer to more particularly 
 now, as I shall hope to do so in a subsequent 
 lecture. He developed the process which was 
 then called heliography, which consisted in the 
 production of a picture in bitumen on a metal 
 plate, the light causing the oxidation of the 
 bitumen or asphaltum, and rendering insoluble 
 or less soluble those parts which had been 
 acted upon. 
 
 In 1827, Niepce came to England, and 
 endeavoured to introduce the results which he 
 had obtained to the notice of the Royal Society; 
 but, owing to the process being a secret one, 
 the communication was not received. 
 
 In 1824, Daguerre, a name to be remembered, 
 a French painter, began similar experiments 
 in the same direction, and, in 1829, he and 
 Niepce entered into partnership to work out 
 that which eventually received the name of 
 photography. Now, Niepce has left it on 
 record, that in his bitumen pictures he had one 
 considerable trouble : he found that, instead of 
 the lines of an engraving being represented 
 by black stains on the white metal, the lines 
 were represented by white metal on black 
 bitumen. In order to overcome this difficulty, 
 
after coating the silver plate with his bitumen, 
 and developing it by dissolving away the un¬ 
 altered parts, he treated the metallic silver 
 plate beneath with any ingredient which fie 
 might choose, and amongst others, he has left 
 on record that he treated it with iodine, 
 and then rubbed off the bitumen. This, in 
 the case of an engraving, would of course 
 give dark lines on a white background. Here 
 we have the first mention of iodine and 
 silver. 
 
 Now, we cannot suppose that two such 
 shrewd observers as Niepce and Daguerre 
 could have used this combination of iodine 
 and silver without noticing that it was altered 
 by light; and I may say that there are a 
 number of grave grounds for considering 
 that this was really the origin of the daguerre- 
 otype process, which consists, as you know, 
 in the treatment of a silver plate primarily 
 with iodine, and subsequently with other sub¬ 
 stances, to cause sensitiveness. Daguerre and 
 Niepce must have noticed the alteration by 
 light in the silver iodide which they had formed 
 on the silver plate, but up to that time of which 
 I am talking, they had not discovered the 
 secret of what we now call development. And 
 when we come to consider the early history of 
 photography, it is a most remarkable fact, that 
 the two first modes of development were dis¬ 
 covered, if I may say so, by flukes. The dis¬ 
 covery of the development of the daguerrean 
 image was as follows : — Daguerre had 
 been trying to reduce the exposure of iodide 
 of silver to light, in order to get an image in 
 a reasonable time. Instead of exposing 
 his plate for a quarter of an hour or an 
 hour, he wished to obtain an image in one 
 or two minutes, which, to him, would have 
 been very quick. Many months were passed 
 in these apparently fruitless researches, until 
 one day he placed one of his iodised plates, 
 which he had exposed to light for some short 
 time, in his chemical cupboard. Now, in that 
 chemical cupboard were a variety of bottles of 
 all descriptions. He had worked hard, and as 
 chemists know perfectly well, when a man is 
 working at research, there is usually a collec¬ 
 tion of all sorts of chemicals. He placed his 
 daguerreotype plate in the cupboard over night, 
 and to his astonishment, next morning, when 
 he opened the cupboard, he found that he had 
 a fully developed daguerrean image. This was 
 a discovery which he immediately turned to 
 account. He exposed other plates, shifting 
 first one bottle then another, and by this pro¬ 
 cess of elimination, at last he was able to find 
 
 that it was an open bottle of mercury which 
 had caused the development of the image. 
 This was really, I believe, the true history of; 
 the daguerreotype process, so far as the mer¬ 
 curial development was concerned. It was 
 considered a very short exposure in those 
 days, if you sat five or ten minutes in the sun¬ 
 light to have your portrait taken. We have 
 changed all that now, and I dare say before 
 this course of lectures is over, not only five or 
 ten minutes will be considered a long exposure, 
 but perhaps T^ths of a second will not be con¬ 
 sidered short. 
 
 I have the pleasure to-night of intro¬ 
 ducing to your notice, an old worker of the 
 daguerreotype process, Mr. England. He has 
 kindly consented to show the whole manipula¬ 
 tion of the process from beginning to end, 
 thinking it might interest what I may call 
 a juvenile audience, for juvenile I suppose 
 most of you are, as regards photography. Mr. 
 England has left me to do the talking, and 
 therefore I must do the best I can to explain 
 the operations which will be gone through. 
 We have a little box, divided, into two parts, 
 
 in one of which is placed iodine, and in the 
 other is placed what is called bromide of lime, 
 that is to say, caustic lime, impregnated with 
 bromine. You will find that, first of all, having 
 polished it, he will place the plate over 
 one of these apertures—over the iodine; the 
 iodine vapour will be allowed to play upon it 
 a certain time, and then he will shift the plate 
 over to the bromine, and thus he bromises the 
 silver. He will finally give it another touch of 
 iodine vapour, and the plate will be ready for 
 exposure. Mr. England has taken the trouble 
 to polish these plates beforehand, since their 
 polishing is one of the chief difficulties of 
 
5 
 
 the daguerreotype process. The fact is, the 
 plates require the most elaborate polish, such 
 as even a silversmith can scarcely give them. 
 One of these plates he will now place over 
 the iodine, and for two minutes allow the 
 iodine vapour to rise and attack the silver. 
 [Mr. England here explained that the first 
 process is to bring the plate to a deep yellow 
 colour, by means of the iodine, and then to 
 place it over the bromine.] In the first place, 
 it is necessary to be able to see the colour, so 
 that a reflector is placed on the one side of 
 the sensitising box, the other being open. By 
 this artifice, the plates may be viewed, and the 
 colour of the daguerreotype plate can be 
 watched, till it has attained a deep yellow 
 colour. It is passed on, to be acted on by the 
 bromine, and that is continued until it attains 
 arose colour. It is a little difficult, by artificial 
 light, to see the exact colour; this process 
 was seldom worked by artificial light, and, 
 therefore, one may be misled by false colour. 
 
 We propose, between us, to take a daguerreo- 
 type picture. Now, the operation of sensitising 
 is very nearly finished. It has been treated 
 with iodine, and then with bromine, and now it 
 is having a final submission to iodine. The plate 
 Mr. England prepared has now been exposed 
 one minute; Mr. England is rather doubtful 
 about the sensitiveness of the plate, but I am 
 afraid, myself, it is rather over-exposed. Mr. 
 England is now going to develop the picture, 
 and he will do so by the use of mercury. He 
 has here the original mercury box with which 
 he was accustomed to manipulate. He will 
 hang it against the wall, and light a small 
 spirit lamp under a bowl of mercury beneath it. 
 The mercury fumes will rise through an orifice 
 into the box, and so on to a plate; he will 
 watch it from time to time, until it is de¬ 
 veloped, when we will show it to you. [A 
 copy of an engraving was here taken whilst 
 illuminated by the electric light.] 
 
 I should like you to understand exactly 
 what takes place in the daguerreotype plate 
 when it is exposed and developed. On the 
 surface of the plate we have a mixture 
 of silver iodide and bromide, but, for sim¬ 
 plicity’s sake, I will suppose that there is 
 simply silver iodide on it. When light acts 
 on such a compound, the action is to split up 
 the silver into a salt, which we call silver 
 sub-iodide, and iodine is liberated, Ag 2 I - 2 — 
 A ga I + I. The iodine is taken up by the 
 silver plate at the back of the sensitive film. 
 To develop the picture, mercury vapour is 
 caused to coijdeijse on the sub-iodide, and leave 
 
 the iodide intact. Amongst some pictures 
 which Mr. England has kindly brought, we 
 have an instantaneous view of New York 
 Harbour, taken about 25 years ago ; and I 
 doubt very much whether there are any wet 
 plate instantaneous pictures equal to that; from 
 age it has become a little bit tarnished, but 
 otherwise it is a most perfect picture—the size 
 is about s in. by 4 in. I have a transparency 
 taken from this, and one from a negative, also 
 in the possession of Mr. England. This last 
 is a portrait of Daguerre, taken by himself. 
 This is interesting as showing one of the 
 very earliest daguerreotypes known. 
 
 The next process is the Talbotype process. 
 In Talbot’s original process, chloride of silver 
 formed the basis, and from that sprung our 
 silver printing process of the present day. But 
 the great improvement he made was by the 
 introduction of iodide of silver. I may mention 
 that he introduced this in January of the same 
 year in which Daguerre introduced the daguer¬ 
 reotype process in France, namely, in 1839. 
 The outline of the talbotype process is as 
 follows:—First of all, he gave a wash of 
 nitrate of silver to plain paper—-some papers 
 answered better than others—this he dried, and 
 then washed over it the iodide of potassium, 
 so forming iodide of silver, then dried the paper, 
 and again washed over it the nitrate of silver. 
 Such a paper was found sensitive enough to 
 take camera pictures. But how about the 
 development ? Curiously enough, the develop¬ 
 ment of the talbotype picture was discovered 
 in an independent way by the Rev. j. B. Reade. 
 He was practising with the solar microscope, 
 and copying certain objects. One afternoon, 
 having given his paper, which he had washed 
 over with gallic acid, a short exposure, he 
 was obliged to leave it for the night; next 
 morning he found that his image was fully 
 developed. That gave him the idea of the 
 development of what was called the latent 
 image. Talbot introduced this mode of de¬ 
 velopment into the calotype process. This 
 calotype process was the subject of litigation, 
 and the claims of the Rev. J. B. Reade to be 
 the discoverer of the gallic acid development 
 was disputed. I do not wish to enter into the 
 legal question as to who was the discoverer, 
 but my own opinion is Mr. Reade should have 
 the credit which belongs to the discovery. 
 From the negative pictures so produced, an 
 unlimited number of portraits would be made 
 by what we call the ordinary silver-printing 
 process. A daguerreotype, on the other hand, 
 could be only reproduced by electrotype, and 
 
6 
 
 Mr. England has kindly lent me such a repro¬ 
 duction, which can be examined afterwards. 
 I propose, however, to show you how Talbot was 
 able to make a picture on paper. I intended to 
 have taken a camera picture by the process, 
 with the aid of the electric light, but I find it will 
 take up too much time. I have here a negative 
 with some talbotype paper upon it, prepared 
 as described. I will expose this to the light 
 from a bit of magnesium wire, and then will 
 develop it in the way in which that development 
 was practised in the old time. I have a solu¬ 
 tion of gallic acid, which I place in a little 
 dish, and with it will mix a solution of nitrate 
 of silver and acetic acid. The acetic acid is 
 meant to retard the action of the gallic acid, 
 to prevent the reduction of the silver before 
 the development takes place. I should say 
 the usual method of using the paper was as 
 follows :—A final wash of gallic acid and silver 
 nitrate was given it, and it was then exposed 
 while damp in the camera. This is what I 
 intended to do, but the minutes steal on us so 
 very rapidly, that I have been obliged to leave 
 out the wash of gallic acid and silver, and 
 to give it a rather longer exposure behind a 
 negative, and thus I shall produce a positive 
 on development. The undeveloped picture is 
 now placed on this piece of glass, and now I 
 proceed to develop it. The glass is placed on 
 
 Stand for Developing Paper Pictures. 
 
 a stand, and the light from the lantern, after 
 passing through orange glass, is allowed to 
 illuminate it. The lines begin gradually to 
 appear, and they must now be visible to you 
 all. I continue brushing over the picture the 
 solution of gallic acid and nitrate of silver, with 
 acetic acid, and it becomes still more visible. 
 A paper process is a most fascinating process, 
 because you can dab about with your brush and 
 do exactly what you like ; it is not like the 
 gelatine plates of the present day, which you 
 have to leave to come out mechanically. With 
 paper, if you want to bring it out a little better 
 
 detail in one place there, you can dab it out; 
 and if you want to keep it back, you can put a 
 little water over the place. There is no process 
 like the paper process to please an artist; not 
 that I profess to be an artist, but, belonging 
 to the Science and Art Department, I have a 
 sort of art instinct, I suppose. I think you can 
 see that the image is very fairly out now. In 
 the old days, I dare say it would be considered 
 passable for a beginner, not that I am a 
 beginner, because I practised it in India; but 
 it is a longtime ago since I did so. So much, 
 then, for the talbotype process. Now, what is 
 the meaning, I would ask you, of that develop¬ 
 ment. It is a curious thing that a small ex¬ 
 posure to light should cause an image to< 
 appear. I.will ask Mr. Woods to throw on; 
 the screen something which perhaps will illus¬ 
 trate how development arises. This morning 
 I was in my laboratory, and I saw lying on the | 
 bench a feeble negative which I had badly 
 developed, and which I had fixed with hypo¬ 
 sulphite of soda. On taking it up, I found the 
 salt had crystallised over the surface in a most 
 beautiful manner, and I do not think I could 
 point out to you anything which would give you 
 a better idea of what development is than thos ; 
 crystals whose images you see on the screen, 
 When you have silver precipitated from a 
 solution by any means whatever, you have i! 
 always in a crystalline form, and as all crystals! 
 possess polarity, so crystals of silver possess 
 polarity; and where one silver particle is; 
 deposited, there another silver particle will 
 deposit, exactly in the same way as those 
 crystals of hypo-sulphite of soda follow one 
 another in regular course in those beautifu 
 feathers. Thus, then, I look upon it as e 
 physical development; we have a crystalline! 
 action going on during development, anc 
 nothing else. The iodide of silver is alterec 
 into sub-iodide, and this, like the pole of e 
 magnet, attracts the precipitating silver, anc] 
 from that time, where the first crystals of silve: 
 are deposited, other crystals of silver are de 
 posited. That is what I call physical develop 
 ment. 
 
 Now, there is another kind of development 
 which some call chemical development; anc 
 here I differ from some of my brethren whe 
 live on the other side of the water, in Germany 
 who have chosen to name as chemical develop' 
 ment something quite different to what i; 
 my idea of chemical development. I say che 
 mical development is only a development whicl 
 is shown by a change in the colour or materia 
 of the substance acted upon, and not b) 
 
7 
 
 a building-up process, such as we have 
 had before us. Now, I have something 
 which I hope will show you what I mean by 
 this chemical development. I have here a 
 picture, which has been printed on nitrate of 
 uranium ; I am going to use simply silver 
 nitrate for a developer. I do not want a 
 reducing agent in the developer in this case, 
 for since the uranic salt is reduced to the 
 uranous state by the action of the light, we 
 shall have a picture formed of silver oxide, 
 because the uranous salt will reduce the 
 nitrate of silver to the state of oxide, and that 
 of itself can be reduced afterwards to the state 
 of metallic silver. This paper was exposed, 
 just before you came to the lecture, to the 
 electric light behind a negative. There is an 
 outline of it now, but when I apply the silver, 
 you will see that it springs out into vividness. 
 It has been rather over-exposed, but this I do 
 not mind, as I merely wish to illustrate 
 a principle. The whole of that picture 
 is formed by silver oxide reduced by the 
 particles of uranium nitrate which have 
 been acted upon by light, and by nothing 
 else. The silver oxide reduced is an exact 
 equivalent of the uranium salt which has been 
 altered by light. That is what I call chemical 
 development, and I know no other meaning for 
 the term. It is impossible, except where you 
 have a change in a large amount of the material 
 that is acted upon by light. I hope I have 
 made you understand the difference between 
 this development and the development you saw 
 just now. The gallic acid, in the one case, 
 reduces the silver solution to the state of 
 metallic silver; and, in the other case, the 
 uranous image itself reduces it to the state of 
 silver oxide ; but still that silver oxide was only 
 equivalent to the amount that had been acted 
 upon by light, and nothing more. I have here 
 a wet plate prepared. I dare say most of you 
 know what a wet plate is, though some of you 
 may not if you only began photography when 
 the gelatine process came in. To such, I 
 suppose, a wet plate is as much a matter of 
 novelty as a daguerreotype is to many who 
 know what the collodion process is. I will 
 ask you to bear with me, however, while I 
 produce the picture, because I want to show 
 you a use of the electric light. Of all the 
 recent aids to photography, the electric light 
 is one of the most powerful, because with it you 
 can photograph by night or by day. In fact, in 
 some of the advertisements I have seen it 
 stated that they prefer to take portraits by 
 night; I do not know why, but in a country 
 
 town I saw an advertisement in which they 
 said distinctly they preferred to take photo¬ 
 graphs by night, as it was more convenient. 
 [An engraving was focussed on the camera- 
 screen, and the wet plate exposed for twenty 
 seconds.] A short exposure will do for this. 
 Now I will try to show you the development on 
 the screen. I hope you will see the crystalline 
 action taking place before you, and the image 
 built up by the crystals as they deposit, gradually 
 coming out on the screen. A cell containing 
 ferrous sulphate in solution, and a small 
 quantity of glacial acetic acid, is focussed on 
 the screen. The plate of sensitised (and now 
 exposed) collodion is fairly transparent. After 
 placing a piece of yellow glass in front of the 
 lantern, the plate is immersed in the cell, and 
 the image is seen gradually being built up. 
 
 There is another mode of development which 
 our friends across the water have chosen 
 to call chemical development, though I cannot 
 agree with them, and that is the method which 
 is called alkaline development. I dare say I 
 could very rapidly demonstrate that to you. 
 The rationale of the alkaline development is 
 that when you have a strongly oxidising agent 
 in the presence of an alkali and a silver com¬ 
 pound, solid or in solution, then you have the 
 last reduced to the metallic state. Such an 
 oxidising agent we have in the pyrogallic acid, 
 and the alkali generally used is ammonia. 
 Now, this kind of reduction is evidently useless 
 unless it can discriminate between a compound 
 which has been acted upon by light and one 
 which has not. When pyrogallic acid is used 
 in order to make this discrimination, or, in 
 other words, in order that those parts acted 
 less on by light may alone be reduced, some¬ 
 thing more has to be added, which is usually a 
 solution of a bromide of an alkali. This 
 restrains the reduction inducing the change to 
 take place in the part acted upon by light. I 
 may say that without a restrainer the tendency is 
 for the exposed parts to be first reduced, but the 
 action extends to those which have not been 
 acted upon by light. It has been usually said 
 that alkaline development is only available for 
 bromide of silver, and until recently I dare say 
 that was the case, or rather it was not the case, 
 but it was thought to be so. I hope on a future 
 occasion to show that iodide of silver is as 
 amenable to alkaline development as bromide 
 of silver, although not so rapidly, and that 
 chloride of silver is very amenable to alkaline 
 development, giving in fact most beautiful 
 pictures. The plate I am going to expose 
 now is a bromo-iodide plate. I have shown 
 
8 
 
 you this form of it, simply because it is trans¬ 
 parent. I have here a negative, and the ready 
 prepared sensitive plate behind it. Now I 
 expose it to the light from the magnesium 
 wire for a few seconds. I will take a cell, in 
 which I will place some of the alkaline develop¬ 
 ing solution. The pyrogallic acid and bromide 
 are mixed, and the ammonia is ready for 
 adding. I mix a small portion of this latter 
 with the pyrogallic acid and bromide—the 
 bromide, remember, being for restraining pur¬ 
 poses. I place the plate in the cell, which is 
 
 seen in focus on the screen. The picture is one 
 of Mr. England’s beautiful statuary studies, 
 and you can see the outline of the bust 
 gradually appearing. That, I say, is another 
 case of physical development. 
 
 Now, I want to show you why I say it is 
 physical development. I throw on the screen 
 a slide, which is a picture of a young lady, 
 seemingly cut in half, part of her figure being 
 intense, and part not intense. This difference 
 in intensity was produced by exposing a plate 
 behind a negative, and then before develop¬ 
 ment, coating a portion of the plate with bro¬ 
 mide of silver. The top layer thus had no 
 exposure whatever, and yet you see where the 
 unexposed film is, there we have the picture 
 very much denser than in the case where there 
 was only the one film. This shows that here 
 we have a case of physical development, for 
 the image is built up from the film itself, being 
 partly fed, as it were, from the film which had 
 received no exposure. There is thus crystalline 
 action taking place in the film, just as much 
 
 as there was in the silver reduced from the; 
 nitrate of silver. I want you to understand 
 why I object to the words chemical develop¬ 
 ment, when we are talking about alkaline 
 development. 
 
 There is another mode of development, 
 which is now very much in vogue, and that is 
 with ferrous oxalate. In this case we have an 
 organic salt of iron in the ferrous state, which 
 is capable of reducing silver bromide, iodide 
 and chloride, to the metallic state, whilst 
 itself is reduced to the ferric state. That was 
 a development which also required a restrainer. 
 If I had had time there were several more 
 experiments I should have gone through, but 
 1 must omit them. There is, however, another 
 kindred developer, which I think is one of the 
 most recent advances in photography, about 
 which I am supposed to be lecturing. I speak 
 of it with great tenderness, because it is my 
 own child. I have now got an iron developer, 
 which is capable of being used without 
 any restrainer whatever. I call it ferrous 
 citro-oxalate; it is rather a long name, 
 and some of my audience may not under¬ 
 stand it, but I will tell you how it is made. 
 You take a solution of citrate of potash, 
 and then you add ferrous oxalate to it, till no 
 more will dissolve, the resulting compound is. 
 probably citrate of iron, but in a stronger form 
 than is usually found, and much more energetic 
 than that form which was recently recom¬ 
 mended by Dr. Eder, to whom I shall have to 
 refer more at length presently. I propose 
 to use some of that same paper which I 
 used just now. Its use for the purpose I 
 have in view is a novelty. This paper can 
 be developed by the alkaline method, or 
 by the ferrous oxalate method, or by the 
 ferrous citro-oxalate method ; it is an excellent 
 paper, and was prepared as I described about 
 two years ago at the Photographic Society, 
 before the gelatine papers came into vogue ; it 
 gives a very clear picture, and I dare say that 
 you will also see that it gives very satisfactory 
 negatives. I expose a piece of the paper 
 behind a negative to the light of the magnesium 
 wire again. The negative is used in this case 
 was kindly lent me by Mr. William Bedford. 
 It is rather a dense one, and requires longer 
 exposure than I should have given to a thinner 
 negative. The exposure is now given, and on 
 withdrawal from the frame, it apparently 
 is a perfectly blank piece of paper. [The 
 paper was wetted and placed on the develop¬ 
 ing stand already described, and brushed 
 over with ferrous citro-oxalate without any 
 
9 
 
 bromide being added. The lantern was covered 
 with yellow glass, and the light of the 
 disc thrown on the paper.] You see it is 
 beginning to develop; see how beautifully it 
 comes out. This, I say, is a novelty in photo¬ 
 graphy, for here we have an iron developer 
 which needs no restrainer, and a paper which 
 can be developed by an energetic developer. 
 I think I may claim that as an advance over 
 the calotype process. 
 
 I must defer what I have to say on other 
 developers to the next lecture; but before 
 I close, I am gratified to be able to bring 
 before the meeting some very early calo- 
 types, taken by my distinguished friend, 
 Mr. James Glashier, who is President of 
 the Photographic Society, and who has 
 worked at photography from its very earliest 
 beginning. These are some of his beautiful 
 
 fern impressions he got with the talbotype, 
 and printed afterwards. I will ask the 
 members to look at them, as being early 
 examples, both of talbotype and calotype; the 
 one being developed pictures on iodide of 
 silver, and the other being the printing process 
 of the negative on chloride of silver paper, 
 such as Talbot first introduced. I have also to 
 show you Mr. England’s developed daguerreo¬ 
 type. I will undertake to say that under the 
 circumstances it is a very good daguerreotype 
 indeed. Of course a lecture-room is not the 
 place to show all the niceties of the art, and, 
 in fact, it is very difficult to demonstrate before 
 such a large audience, but I hope those experi¬ 
 ments which, luckily, have all been successful, 
 will at any rate have afforded some instruc¬ 
 tion to the junior members of the photographic 
 world. 
 
 LECTURE II. —Delivered February 6tii, 1882. 
 
 At the last lecture I introduced to your 
 notice the ferrous citro-oxalate developer, for 
 which I claimed the advantage that it was 
 able to work without any restrainer whatever. 
 To-night, I propose to bring before you 
 another developer, which is also able to 
 work without a restrainer ; but before I do 
 that, I should like you to take note of an 
 improvement, which has been lately intro¬ 
 duced by Mr. Berkeley, in the ordinary alkaline 
 developer. With the pyrogallic acid is mixed 
 four times the weight of sulphite of soda. The 
 action of this sulphite, apparently, is this—the 
 sulphite of soda absorbs the oxygen with 
 greater avidity than does the pyrogallic acid, 
 thus leaving the pyrogallic acid free to do its 
 work, and, consequently, we have a developer 
 which remains uncoloured for a very long 
 period indeed. I hold in my hand a bottle 
 which was presented to me by Mr. Berkeley, 
 
 some time ago, which I read was made up on 
 October 13th, 1881. This contains four times 
 the quantity of sulphite of soda that it does of 
 pyrogallic acid. Here is another bottle of 
 pyrogallic acid, made considerably later— 
 some time in January—and you will see the 
 difference in colour between the two. The one 
 is slightly yellow in tint; the other, a deep 
 muddy brown. I consider that this introduc¬ 
 tion of sulphite of soda into the developer is one 
 of the most remarkable improvements in alka¬ 
 line development, and I hope that those prac¬ 
 tical photographers present will not hesitate to 
 try the formula which Mr. Berkeley has lately 
 published. 
 
 I now propose to show you the practical 
 use of the developer to which I referred 
 when I commenced. It is a new one, but 
 has not been used to any very great extent, 
 I am afraid, on account of the high price 
 
10 
 
 it used to fetch. I refer to hydro-kinone; the 
 first quantity I bought cost me 12s. a drachm, 
 but, I am happy to say, it is now only 8s. an 
 ounce, and one grain of it is as active as two 
 grains of pyrogallic acid, for it is a much more 
 powerful absorber of oxygen than pyrogallic 
 acid; its main advantage, however, is, that 
 you are able to use it without any restrainer. 
 You can even develop chloride of silver without 
 adding any soluble alkaline haloid to it, which 
 photographers know is essential when using 
 even a very weak pyrogallic and alkaline 
 development. To illustrate this, I have an 
 opal glass plate which is covered with a 
 film of collodion containing chloride of silver. 
 Until quite recently, this was a film which it 
 was almost impossible to develop by the 
 alkaline or iron methods; but, thanks to Dr. 
 Eder, we are able now to develop chloride of 
 silver exactly in the same way as we can bromide 
 of silver. To save time, I gave the plate a 
 short exposure just before the lecture. In this 
 cell I have now simply hydro-kinone and 
 ammonia, and I think you will see that it 
 developes the plate without the slighte trace 
 of fog. I have taken an opal glass as the back¬ 
 ground, so that you will be better able to see the 
 result; it comes out very gently and gradually. 
 Thus you have a practical demonstration that 
 this new developer works without any restrainer, 
 even when such a troublesome salt as silver 
 chloride is used. Not having any restrainer, it 
 is able to give a better detail, and allow a shorter 
 exposure in the camera than if the ordinary 
 alkaline developer be used. This development 
 is applicable not only for collodio-chloride, 
 but for collodio-bromide plates, or for gelatino- 
 bromide plates—in fact, any plate with which you 
 work, even iodide. [The perfectly developed 
 image was then shown.] 
 
 I promised last lecture that I would show 
 you how very easy it was to develop iodide of 
 silver with either the alkaline or the ferrous 
 oxalate developer. You saw in my last lecture 
 how we could develop the iodide with silver 
 and gallic acid, but it becomes a very different 
 thing—at least it was always so considered— 
 when you have to develop a plate containing 
 simply iodide with nothing but an oxalate 
 developer, such as I have here. In this frame I 
 have a paper which is coated with pure iodide 
 of silver, not a particle of bromide or chloride 
 in it; I will give it a slight exposure, and I 
 propose to develop it on the screen before you. 
 I will simply immerse it in water, and use some 
 of the same ferrous citro-oxalate which I intro¬ 
 duced to your notice last week. It will be 
 
 apparent with what facility the iodide of silver 
 can be developed. The paper, where viewed 
 in the yellow light, is apparently colour¬ 
 less, although really it is yellow. If everything' 
 be correct, you will see that we get an image 
 developing out rapidly, which is formed of pure 
 metallic silver, reduced from the sub-iodide at 
 first, and then fed, as it were, from the adjacent 
 iodide. [The picture was fully developed.] 
 We now have a practical demonstration that 
 you can develop iodide just as easily as you 
 can bromide, which is a desideratum when you 
 come to consider the composition of gelatine 
 plates, with which we may have to deal more fully 
 in subsequent lectures. Having fulfilled my 
 promise with regard to this, the next point I wish! 
 to call the attention of the meeting to is the 
 action of sensitisers. I should like to explain to 
 some of you who are not initiated in the mysteries 
 of photography, what a sensitiser is. I will 
 throw a diagram on the screen, and then you 
 will be able to understand better what I mean. 
 When you have chloride of silver, for instance-, 
 exposed to light, you have a new compound 
 formed, which is called sub-chloride, or 
 argentous chloride (Ag., Cl 3 = Ag 2 Cl -j~ Cl),I 
 and chlorine liberated. This chlorine is a 
 very obstinate thing to eliminate, if you do not 
 give it something that can take it up; fc.: 
 instance, if you place perfectly dry chloride of 
 silver in vacuo, without any trace of organic 
 matter present, you will find, if it is exposed 
 to brilliant sun-light for months, you get no 
 darkening action. If such a white powder was 
 submitted to you, to test by its darkening, you 
 would say at once that it was not chloride of 
 silver, because it was not darkened, one ol 
 the tests of chloride of silver, amongst chemists, 
 being, that it shall darken in the light. Here. 
 I have- a little bulb of it which was prepared,! 
 dried carefully, and sealed up, the vacuum 
 being made by means of a Sprengel pump. 
 This tube has been exposed for months to 
 light, and it is as pure a white as the Very 
 first day it was put into the bulb. Another 
 experiment was made at the same time; 
 a bulb was prepared, but in using the 
 Sprengel pump, unfortunately, as I then 
 thought, a small globule of mercury got in the i 
 vacuum, and was sealed up with the chloride ; i 
 the consequence was, that the chloride of silvet i 
 immediately darkened; although the mercury i 
 was not in contact with the chloride of silver, ; 
 the chlorine flew to the mercury, and formed i 
 chloride of mercury. This is an instructive i 
 experiment, showing that merely in the pre-; 
 sence of something that will mop up thr 1 
 
chlorine, chloride of silver will darken. Here 
 is another darkened bulb of chloride of silver, 
 which was sealed up, not in vacuo, but 
 in hydrogen. Now, hydrogen, as you are 
 perfectly aware, can combine with chlorine to 
 form a gas, which we call hydro-chloric acid, 
 so, when the chlorine was in the presence 
 of hydrogen, it combined with it, instead of 
 returning to re-combine with the sub-chloride 
 of silver. On the screen is a diagram showing 
 these reactions of the two other haloid salts of 
 silver. I have already shown you the diagram 
 of silver chloride, and its splitting up by light. 
 
 Silver iodide splits up into silver sub-iodide 
 and iodine. 
 
 Ag 2 I 2 = Ag a I + I. 
 
 Ag 2 Br 2 = Ag 2 Br + Br. 
 
 Silver bromide, when exposed to light, splits 
 up into silver sub-bromide and bromine. 
 
 Now, in order that there shall be a ready 
 darkening of either of these, you must have 
 something which will absorb the iodine or bro¬ 
 mine (or in the case of the latter, allow it to 
 escape), whichever you expose to the light. 
 Now, the image which you just now saw de¬ 
 veloped on the iodide, was, before development* 
 precisely in the same condition as the darkened 
 image on the chloride ; it is a matter simply of 
 degree. Of course, if you took a few shot, 
 and hid them in a bushel of flour, you would 
 not know that there was anything black in 
 the flour; in the same way, if you alter two 
 or three particles of iodide into sub-iodide, or 
 chloride into sub-chloride, you do not notice 
 it, because they are so mixed up with the par¬ 
 ticles that are not altered. It is these little 
 altered particles that are the nuclei for the 
 developed image. In order to demonstrate 
 practically the use of sensitisers, I have 
 prepared a sheet of paper, a quarter of 
 which has been washed over with that 
 vulgar beverage, beer; another quarter 
 with potassium nitrite, and another quarter 
 with gum; the fourth quarter has nothing 
 on it. Now, all these three are—but in 
 greater or less degree—absorbents of iodine 
 and bromine, and you will see, when I expose 
 the paper and develop, that there is a de¬ 
 cided increase of sensitiveness where those 
 substances have been applied. You may 
 say, “according to what you have said 
 already, the particles not coated with any¬ 
 thing ought not to develop,” but you 
 must recollect that we are in contact with 
 air, and air contains moisture, and if you 
 have chlorine, for instance, in the presence of 
 
 moisture, light is able to split it up into hydro¬ 
 chloric acid and oxygen, so that, in the case of 
 a paper like this, you have a sensitiser present, 
 but only a feeble one. This paper is the 
 bromo-iodide paper that I introduced to 
 your notice some little time ago. I have 
 marked on the different quarters of it what is 
 the sensitiser on each. I will brush it over with 
 this ferrous citro-oxalate developer, and we 
 shall see if there is any difference in the 
 rapidity of coming out. The part coated with 
 nitrite in coming out very rapidly, it has a very 
 inordinate affection for iodine and bromide, the 
 vulgar beverage beer does not seem to be doing 
 its duty as it ought to be ; now the gum is flash¬ 
 ing out; now the beer is coming. You see the 
 nitrite came out first, then the gum, and then 
 the beer, finally that in which the sensitiser was 
 merely the air and the paper, and you will see, 
 when the picture is finished, there is a vast 
 difference in the amount of development. If 
 there had been no moisture in the atmosphere, 
 if it had been exposed in dry air, there would 
 have been no action whatever where it was 
 not coated with some sensitiser. Here, then, 
 we have a proof that a sensitiser is necessary 
 in order to give the greatest possible sensitive¬ 
 ness, and I hope that this will explain to you 
 what I shall have to allude to further on. 
 
 I next have to call into use the spec¬ 
 trum. Here we have it on the screen, and I 
 have produced it by the same arrangement of 
 apparatus as if I were going to use it for 
 photography. You will see that we have a 
 tolerably clear spectrum. We have the light, 
 
 Fig. i. 
 
 A, and a lens, L, to throw the image on the 
 slit B, and then the lens, c, to make the rays 
 parallel, then a single prism, D, then a photo¬ 
 graphic lens, E, which focusses the spectrum on 
 the screens. Now I want to point out to you 
 not only that we have red, green, yellow, blue 
 and violet rays, but we have rays beyond the 
 violet also which we cannot see, but which by 
 an artifice I shall be able to show you. For 
 instance, if I place this card on this screen, 
 you see we have a vast extension of the spec- 
 
 P tea) 
 
trum beyond the violet, but if I take it away, 
 it is absent. The reason of that is, that I 
 have brushed over the card a solution of sul¬ 
 phate of quinine, which lowers the wave lengths 
 of the ultra-violet rays, making them visible. 
 I was anxious to bring this before you, for 
 this reason, that it is the principal part 
 of the spectrum which is useful to photo¬ 
 graphers, though I do not believe they are 
 aware of it. When we expose one of these 
 haloid salts of silver to the spectrum, it behoves 
 us to know what part of the components 
 of light it is that acts upon this haloid 
 salt. Now, a very simple way is, of course, 
 to place a film containing the haloid salt in 
 the spectrum. But what I want first to 
 point out to you is, that we must go upon 
 scientific principles in dealing with the facts 
 that come before us, and the main principle 
 involved is the principle of work. When you 
 have what you call light radiating from a 
 source, be it the sun or be it a gas flame 
 it is only when its radiant energy is stopped or 
 absorbed that you are able to get any work 
 out of it all ; that is to say, supposing you 
 have a transparent glass plate, you know that 
 the light, as you call it, passes straight through 
 the plate, without leaving any very visible effect 
 on the glass. But if you have a red glass, why 
 does it appear red ? Simply because a part 
 of the rays have been stopped; but what 
 becomes of those rays that are stopped ? 
 they must take one of two forms, or rather, 
 the work they do must take one of two 
 forms. The work they can perform may 
 be shown by the heating the glass, or else the 
 energy must be exhausted by doing chemical 
 work upon the substance on which it acts. 
 The effect then, is this ; if you have an haloid 
 salt of silver, the principal part of the energy 
 which is radiated, and which is absorbed, does 
 chemical work, and it is only those regions 
 of the spectrum which are absorbed which are 
 capable of doing chemical work. I cannot 
 show you a better example than I have upon 
 
 Fig. 2. 
 
 the screen. The second part of the slide 
 shows the whole spectrum, taken with a sub¬ 
 stance which I shall be able to show you by- 
 
 and-bye. You see the darkening caused by 
 the light; at the far end we have the ultra¬ 
 violet rays, and at the other end we have the 
 red rays. In front of the slit of the spectro¬ 
 scope was placed a cell of bichromate of potash, 
 and you see it cuts off, not only the blue 
 and the violet, but a part of the green, and 
 leaves the red and yellow intact (B, Fig. 2). 
 
 Now, photographers know that those beauti¬ 
 ful prints that go by the name of carbon prints 
 depend for their manufacture on the fact that 
 bichromate of potash is altered by light in the 
 presence of organic matter. Here we have the 
 spectrum taken on carbon tissue, in which you 
 will see the absorption exercised by bichromate 
 of potash exactly coincides with the work done 
 on the bichromate (a, Fig. 2) ; notice that the 
 part of the spectrum which is impressed ends 
 exactly w'here the bichromate absorption begins. 
 I think that is a very excellent illustration of 
 the work done by radiation. I propose now to 
 pass two or three films through the spectrum 
 (I., Fig. 3), to see what kind of work we may 
 
 Fig. 3. 
 
 expect to be done with them. First, we have a: 
 film of chloride of silver (II.), and you will see 
 that the violet part of the spectrum towards 
 the yellow is subdued, whilst the other parts 
 pass through. Next, we have a bromide film 
 (III.) in which you see some parts of the 
 spectrum are cut off which before were visible. 
 Next, I take iodide (IV.) where you see the 
 violet is cut off entirely, leaving a trace of 
 blue, which was not the case with the bro- 1 
 mide. Here, again, we have the bromo-iodide 
 (V.) cutting out the blue, going right into the!; 
 green. That is interesting, because I propose j 
 to show you next the fact that the absorption jl 
 coincides with the work done on the films J 
 themselves. 
 
 I have here a board with four strips of paper 
 upon it. The first coated with chloride of | 
 silver, the second with bromide of silver, the | 
 third with iodide of silver, and the fourth with j 
 bromo-iodide of silver. I will expose each olj 
 them for twenty seconds, to the spectrum, and j 
 
*3 
 
 develop them before you. In order to recog¬ 
 nise the parts acted upon, there are strings 
 put across the different parts of the spectrum. 
 [The exposures were made, and the strips 
 damped and placed on the developing board.] 
 I brush each strip over with the ferrous- 
 citro-oxalate developer. [The prints developed 
 rapidly, the chloride taking rather longer than 
 the others to appear.] The chloride sometimes 
 takes a little longer to develop, and I ought to 
 have given it a little longer exposure, because it 
 is not quite so sensitive as the other salts. Here 
 are the four spectra (Fig. 4). The maximum 
 
 Fig. 4. 
 
 BA a T DCA 
 
 
 
 
 
 
 5#t§fi§f§li 
 
 ip|I| 
 
 
 
 
 
 
 if n 
 
 
 
 
 
 
 . 1 
 
 
 
 
 VI B Q OR 
 
 sensitiveness of the chloride, A, is about the line 
 H on the spectrum. The iodide, C, goes as far 
 as G; the bromide, B, half-way between G and 
 F, and faintly extends down as far as B ; the 
 bromo-iodide, D, extends as tar as F in the 
 spectrum. I think I can show you this more 
 directly by photographs taken in the spec¬ 
 trum, in a more accurate manner than 
 is possible in a lecture experiment. When 
 I passed the different silvers through the 
 spectrum, 1 only passed the haloid salts in 
 the state in which they are usually prepared 
 photographically, but now I wish to show you 
 how different coloured salts of silver can be 
 obtained. Let me take bromide, for instance. 
 Here you have the ordinary form of bromide, 
 such as you saw developed on the screen—the 
 red or orange bromide. By boiling it, as you 
 do, with gelatine emulsion, you can change 
 the molecular condition into a gray blue form, 
 such as you see here. From what I have told 
 you of the principle of work and absorption, 
 you will be quite prepared to learn that these 
 particular forms are sensitive to different parts 
 of the spectrum ; so they are, but their maxi¬ 
 mum place of sensitiveness is but very slightly 
 changed; that is a point worthy of attention. 
 Again, we have two forms of chloride, the 
 ordinary form, which, as a rule, is decidedly 
 yellow by transmitted light; and the other is 
 the blue molecular form, which is got by boiling. 
 Once more, we have the iodide in the yellow 
 form, and also the blue form. Now, I will 
 
 show you photographs which were taken to 
 show the maximum of the different haliod salts 
 (Fig. 5, p. 14). The first one is the red form of 
 chloride (No. 1), and you see that the maximum 
 sensitiveness is half-way between H and h in 
 the violet part of the spectrum; if we go to 
 the boiled chloride (No. 2) you see it has 
 the same place of maximum intensity as the 
 orange, and that the intensity is very great as 
 far down as the yellow line, D. We next 
 come to the red form of bromide (No. 3), and 
 we see that its maximum place of sensitive¬ 
 ness is half-way between G and F. When we 
 come to the blue form (No. 4) we have an 
 immense extension towards the red, but still 
 it retains nearly the same place of maximum 
 sensitiveness between G and F. Next, we 
 have the yellow form of iodide (No. 5), and 
 it shows that it ends sharply at G. There is 
 no spectrum impressed beyond G. In one case 
 where I gave it a very prolonged exposure, I 
 got what is called a reversal of the image, 
 but still the iodide spectrum is cut off at that 
 particular point (No. 6). 
 
 The next spectra are exceedingly interest¬ 
 ing—to me at all events—and I should like 
 them to interest you. They represent a new 
 form of molecule. 1 believe very few people 
 know anything about them. The peculiarities 
 of the mixtures were discovered by me, and 
 a paper about them sent to the Royal Society 
 a very short time ago. The type of the 
 new molecule is this : it is not a combina¬ 
 tion between iodide and bromide in the 
 ordinary acceptation of the word, that is 
 to say, it is not Ag 2 Br 2 + Ag 2 I 2 , or not 
 simply one equivalent of each of bromide and 
 iodide of silver added together, but an absolute 
 combination, Ag 3 Br I, or a double molecule, 
 which has been formed, and on which the 
 spectrum has a totally different effect. You 
 will see (No. 7) that, instead of having the 
 maximum intensity at G, its maximum is far 
 into the green, and it has a minimum about G. 
 In the same way another molecule is formed by 
 a combination of molecules of chloride of silver 
 and iodide of silver, and that, too (No. 8), has 
 no maximum about G, but has a minimum there, 
 which is the place where the maximum sensitive¬ 
 ness is found in the ordinary iodide of silver. 
 
 I will try and explain how these last 
 curves are produced; if you take iodide 
 and bromide of silver together, and expose 
 them to the spectrum, what happens is 
 this; first of all iodine and bromine are 
 liberated. What becomes of the iodine and 
 bromine ? The bromine immediately attacks 
 
the sub-iodide of silver, and forms a new 
 molecule, Ag s Br I; the bromine undoes the 
 work of the iodide as fast as it is done, 
 so that we have, in the place of maximum 
 intensity of the iodide, the least work done. 
 In the same way with the chloride. The chlorine 
 liberated from the chloride undoes the work 
 which is done by the iodine ; the consequence 
 is that, in the place of maximum intensity of 
 the iodide, we have a diminution of intensity in 
 the case of the double salt; in other words, the 
 dry bromo-iodide of silver is less sensitive for 
 the spectrum about the part G than any other 
 part of the spectrum to which it is sensitive. In 
 order to prove that, I will narrate a little experi¬ 
 ment which was carried on by myself. Chloride 
 
 of silver was absolutely blackened in the light, 
 and sub-chloride was formed; then it was 
 treated with iodine again, and it become a buff 
 colour. When this new molecular form was 
 placed in the spectrum, we got exactly the same 
 result as we do here, showing that we have a 
 new molecule of chloro-iodide of silver. I think 
 this will very likely throw a great deal of light 
 upon some of the phenomena which are met 
 with in ordinary dry-plate work. 
 
 Now, I crave your attention to one thing. 
 You may ask if bromine always undoes the 
 work done on the iodide. The fact is, that it 
 does not, if you have free nitrate of silver 
 present; since this salt is able to take up the 
 iodine and bromine, therefore the bromine can- 
 
 Fig. 5. 
 
 H h G F E DCBA 
 
 AgCl in collodion. 
 
 Grey form of AgCl in gelatine. 
 
 Orange AgBr in collodion or gelatine. 
 
 Grey AgBr in gelatine. 
 
 Agl exposed in presence of sensitiser. 
 
 Agl+AgNO^ prolonged exposure. 
 
 AgBr-j-Agl in collodion acid. 
 
 Agl+AgCl in gelatine. 
 
 not act on the sub-iodide of silver; it leaves it 
 there, and you have the impressed spectrum 
 totally different; in fact, the bromide and 
 iodide spectrum are superposed one on the 
 other. There are a great many facts I could 
 show you to prove this. We come to the con¬ 
 clusion then, for sensitiveness, you must have a 
 sensitiser present. In gelatine plates, you may 
 say that gelatine|is a sensitiser, but it certainly 
 is a very feeble one; anyhow, if you want to get 
 sensitiveness in a plate, you must have a sensi¬ 
 tiser present which will mop up, not only iodine, 
 but bromine. Whenthisisthecase,brominecan- 
 not go and destroy the work done on the iodide. 
 
 Before concluding this part of my lecture, I 
 may also say I that have found that the printed 
 
 image and the developed image have precisely 
 the same characteristics. Thus, if you get 
 pure iodide of silver, you get the termination 
 of both the printed and developed spectrum 
 close to G. A curious thing with regard to 
 iodide is this, if you have the smallest trace of 
 impurity in it, it is immediately shown in the 
 spectrum by a prolongation below G. With 
 such a tail present in a spectrum on silver 
 iodide, you might tax the chemist with having 
 given you iodide which is not pure. 
 
 Now there is one point that has exercised 
 the minds of a great many photographers, 
 and that is the illumination of their dark 
 rooms; and I hope I may be excused if I 
 go over what may be apparently well trodden 
 
 I 
 
ground, but I believe that there are many here 
 present who have no absolute certainty as 
 to why they are using certain glasses 
 to glaze their developing rooms. I want 
 to show photographers the absolutely safest 
 light they can use in developing their pic¬ 
 tures. Here we have a spectrum, and 
 beyond it you see the fluorescent rays. I will 
 ask Mr. Woods to pass through the spec¬ 
 trum one or two glasses. First of all, we will 
 take ruby glass (No. 2), and you will see 
 that when the spectrum is very bright a 
 certain amount of blue light comes through 
 the glass ; I can see a trace of yellow, and a 
 trace of green. Now I will take the yellow 
 glass; here is one which photographers are 
 very fond of, but I dare say you will see that 
 
 there is a great deal of blue light comes 
 through that. That was the ordinary glass 
 used for photographic dark rooms, within the 
 last half dozen years, and people were perfectly 
 content if they glazed their rooms with it. But 
 there are yellows and yellows, and I will show 
 you the difference between two. Now we have 
 deep orange (No. 5), which you see cuts off 
 a great deal more light. The next (No. 6) is a 
 stained red glass, which cuts off the green too. 
 Now there is one combination which I wish to 
 show you, because people often complain 
 that their photographic studios are not safe 
 as to light, and that they use ruby glass 
 and get foggy plates. I have great doubt 
 about fogging bromide plates with proper red 
 glass, where ordinary precautions are taken, 
 
 Fig. C. 
 
 1. Cobalt glass. 
 
 2. Ruby glass. 
 
 3. Chrysoidine. 
 
 4. Magenta. 
 
 5. Flashed orange. 
 
 6. Stained red glass. 
 
 7. Bottle green. 
 
 8 . Aurine. 
 
 9. Quinine. 
 
 though not with the ruby glass which lets 
 blue light through. If people would only use 
 the spectroscope with a strong light, they 
 would very soon see where their glazing is at 
 fault. Now, I will throw on the screen the 
 light through a piece of blue glass (No. 1), 
 and I think you will see that there are bands 
 in the red and yellow ; there is plenty of blue 
 coming through and some green. Now, I will 
 ask Mr. Woods to give us a combination of 
 stained red and blue ; you see a red line and 
 nothing else. If photographers want to be 
 absolutely safe, let them glaze their studios with 
 cobalt glass and stained red, and they will get 
 nothing but the light of that particular refrangi- 
 bility, which I will warrant is totally unable to 
 affect any gelatine plate, of the ordinary type, at 
 all events, You may glaze and glaze with ruby, 
 
 but you will never get rid of blue light entirely. 
 Of course it diminishes with every thickness 
 you take. If you want to use ordinary plates, 
 which are not so sensitive that you cannot look 
 at them, my advice is to use a combination of 
 stained red (No. 6), and ruby glass (No. 2), 
 which will give you a comfortable light to work 
 in, for it cuts off the blue, and leaves the red 
 in a brilliant patch. Those that are using that 
 combination ought to have nothing to fear ; 
 but if they still fear, let them use a com¬ 
 bination of cobalt glass (No. 1) and stained 
 red glass (No. 6), and then they will only get 
 the colour of that narrow strip of light which 
 you saw just now. I should like to show you 
 the green glass in the spectrum (No. 7), that 
 you may judge what kind of light gets into 
 it. It was at one time seriously recommended 
 
i6 
 
 as an admirable thing- for not trying the eyes, 
 and for being safe to use in the developing 
 room. A combination of red and green is a 
 fairly safe light for iodide plates or ordinary 
 plates, but not for gelatine plates, which are 
 extremely sensitive. Next we come to a series 
 of pretty colours, which may be very useful to 
 us. Here is the spectrum of magenta (No. 4); 
 the yellow and the green are cut out entirely, 
 leaving the blue, violet, and orange. Now I 
 will try aurine (No. 8) and chysoidine (No. 3). 
 Here you see the blue is gone entirely. Some 
 have said that the ultra violet rays go through 
 aurine, but I will leave it to you to say whether 
 they do or not. There is no trace here of the 
 ultra violet rays when I apply this card with 
 the sulphate of quinine on it. They should 
 appear if there were any. We have next a 
 combination of magenta and aurine, which 
 is a perfect red light, and is very good 
 indeed for the photographic studio. I think I 
 may say, without a breach of confidence, that 
 I believe a certain “ruby medium” in the 
 market is made according to the directions 
 that I gave some time ago. You may ring the 
 changes of combinations of these glasses; 
 you may have scarlet and aurine, or magenta 
 and aurine, and either one or the other will 
 give you this red band only. If all other 
 means fail, the photographer may use the 
 ferrous oxalate developer. You will see that 
 it cuts off the blue light at all events. I 
 should not at all mind developing a plate in 
 such a light as that. In fact, you may bring 
 the most sensitive plate out into a white light, 
 when developing in a dish, with a covering of 
 ferrous oxalate over it. I hope that, having 
 seen these spectra, you will lay to your heart 
 what I have been saying to you about the 
 glazing of your dark rooms. I thought this 
 was a subject that I ought not to pass over 
 without some reference. 
 
 In 1874, we had a most astonishing dis¬ 
 covery made, and that was, that by dyeing 
 a film of sensitive collodion you were able to 
 get an increased action by the spectrum. 
 In common with others I took up that subject 
 and investigated it, and I wish to draw your 
 attention very briefly to it to-night, because 
 I think it is one of the advances made in pho¬ 
 tography which ought not to be passed over. 
 If you take one of these aniline dyes, such as 
 a blue dye, and expose it to light behind a piece 
 of black paper, with an aperture in it, such as 
 I have here, you get an image on the dye, 
 such as you see. What is the meaning of 
 that ? The meaning is that the dye is oxidised, 
 
 for if you apply an oxidising agent you get the 
 same result. Dr. Vogel found that if you dyed 
 plates with some of these fugitive dyes—they 
 were all fugitive to a great extent which he 
 used—he was able to obtain an extension of 
 the impressed spectrum exactly in those parts 
 of which the dyes absorbed, and he intro¬ 
 duced the term optical sensitiser to describe 
 the fact. Now, I am going to quarrel with that 
 expression, because he explains it in this way ; 
 he says, in effect, that the sensitive aniline dye 
 is able to take up a vibration, and to direct it 
 to something else ; in other words, that when 
 light has done all it can in endeavouring to 
 bleach a dye, that its period is transformed,! 
 and it does something else on the silver salt! 
 which is in contact with it. Well, that is not 
 a philosophical way of looking at things, be¬ 
 cause if you could only arrive at that principle, 
 you might just as easily arrive at the principle 
 of perpetual motion, which is a thing which I 
 am not prepared to admit. Now, I honestly 
 say, that for a long time these experiments 
 staggered me, and the arguments Dr. Vogel 
 brought forward seemed fully to justify his term 
 of optical sensitiser, as applied to the dye 
 stuff, but eventually I was able to come tc 
 a different conclusion, and that conclusion 
 I wish to put before you. If you havt' 
 a substance which is oxidised in the pre¬ 
 sence of an haloid salt of silver, what dc 
 you expect ? You expect that the haloic 
 salt of silver would be reduced. If you place 
 pyrogallic acid in contact with the haloic - 
 salt of silver, and help it to be oxidised by 
 ammonia, you expect that the haloid salt of silvei 
 will be reduced, and I lay the action of the 
 dye to precisely the same principle as the 
 pyrogallic acid, which helps to develop the 
 image ; in other words, the oxidation of the 
 dye causes a reduction of the bromide of silver 
 or iodide of silver, as the case may be, anc 
 simply provides a nucleus on which develop 
 ment can take place. I wish, however, to poin 
 out that Dr. Vogel has an objection to my ex 
 planation. He says, “ Oh, but you requir 
 time to bleach things, you require time tf 
 alter them.” So you do; you require timet 
 blacken the chloride of silver, nevertheless 
 the slightest exposure to light begins th 
 change in it, and so the slightest exposure t 
 light begins the change in the dye, and on thi 
 principle, of course, the action of dyes upo 
 sensitive films can readily be explained. 
 
 I have got behind the screen three piece 
 of sensitive bromide paper, one dyed with 
 blue dye, and another with eosine, the thirc 
 
pure bromide of silver without anything. I 
 propose to develop those on the screen. I 
 must give this rather a longer exposure than 
 last time, because blue dye takes away the 
 sensitiveness to a material extent. You can 
 see the colour, it is somewhat of a lavender 
 grey, which no doubt will show that it 
 absorbs exactly in the same position where 
 the extra portion of the spectrum is developed. 
 In the eosine, I dare say you notice a peculiar 
 dimming of light about the green ; this is a 
 very fluorescent dye, and it is in that position 
 where we shall expect the action to take place. 
 I will, however, show you where the absorption 
 of these two dyes we have used takes place. 
 This is a spectrum of the dye, it lets the ultra¬ 
 violet rays through ; in the green we have a 
 great cutting off. Here we have the cyanine, 
 which is a different colour altogether. It 
 cuts off the yellow, and also the green; 
 therefore, on the principle of work, we should 
 expect that where absorption takes place in 
 the dye, there the dye would be oxidised, and 
 the silver reduced in consequence. I now 
 
 expose the three slips of paper above alluded 
 to, and I will develop them before you. [The 
 papers were developed in yellow light, as 
 before.] The impression of the spectrum on 
 
 Fig. 7. 
 
 ThI . 
 
 wi 11 H 
 
 r | r 
 
 
 |§ ii! If EEHHni II I 
 
 
 
 the eosine dyed paper flashes out, the part in 
 the green comes out very strongly indeed. 
 The blue dyed paper shows, besides the ordi¬ 
 nary spectrum of the bromide, a band in the 
 yellow. Finally, we have the undyed paper. 
 The above diagram shows the effect obtained 
 on cyanine blue. 
 
 LECTURE III.—Delivered February 13th, 1882. 
 
 I finished my last lecture by showing you 
 the effect that dyes had upon sensitive salts. 
 In my previous lectures I have not touched 
 on the method of producing sensitive surfaces, I 
 except by the calotype or paper process, or 
 by the wet collodion process ; but I should be 
 remiss if I did not mention the subject of 
 collodion emulsions, which have been in 
 vogue now for seven or eight years, but which 
 have been supplanted, to a large extent, by 
 gelatine emulsions. Whether that be an im¬ 
 provement upon the collodion process, except 
 for certain things, I hesitate to say; but cer¬ 
 tainly, in a collodion emulsion process, we 
 have a process which is admirably adapted, 
 at all events, for landscape work. The collo¬ 
 dion emulsion was originated, I believe, in 
 September, 1864 , by Messrs. Bolton and Sayce, 
 
 and a new era was opened out when they 
 made their first emulsion. 
 
 Now, an emulsion is very simply formed, 
 particularly a silver bromide or chloride emul¬ 
 sion ; but, as far as I know, it has been 
 rather a crux amongst photographers to make 
 an emulsion with silver iodide. However, 
 it is a very easy thing to do, and I propose 
 roughly to show those who are initiated in 
 emulsion-making how an emulsion can be 
 made, and further, how iodide of silver can be 
 made amenable to treatment. The point in 
 emulsion making seems to be, to get the pre¬ 
 cipitate in as fine particles as possible, and it 
 is said that this can only be obtained, except at 
 very great cost of time and trouble, by first 
 adding the soluble bromide, or iodide, to the 
 collodion. If you take the trouble to add the 
 
i8 
 
 silver to the collodion, first of all, the aspect 
 of emulsion making is entirely changed, and 
 you can get any amount of fineness by adding 
 the iodide or bromide to the silver contained 
 in the collodion so long as you keep the silver 
 nitrate in excess. In this bottle is some ether, 
 and some gun cotton which is not dissolved 
 up. In this test tube I have some nitrate 
 of silver dissolved in alcohol containing a very 
 little water. I simply pour this solution 
 into the mixture of ether and gun cotton, 
 shake it, and I find an emulsion of nitrate 
 of silver in collodion , since the alcohol and 
 ether dissolve up the cotton. In the old days, 
 emulsion makers would have said there 
 is something wrong in having an emulsion of 
 silver nitrate, but I think I shall soon demon¬ 
 strate to you that in reality it is all right. Now 
 I will add an iodide to this silvered collodion. I 
 pour a solution of potassium iodide into it, 
 not drop by drop, but somewhat carelessly, 
 and shake it up in the bottle at intervals, and 
 as a result, we get a very fine emulsion. 
 You would find if you reversed the opera¬ 
 tion, and put the iodide into the collodion 
 first, and then added silver nitrate, you would 
 have precipitated the iodide of silver at 
 the bottom of the bottle, and in a form which 
 would not emulsify at all. My advice to those 
 who wish to make collodion emulsion (and 
 gelatine emulsion) is to add the silver to the 
 collodion (or gelatine), and then add the haloid 
 salts afterwards, and you will get as perfect 
 an emulsion as you choose, This formation, 
 in so simple a manner, of an iodide emulsion, 
 is a practical lesson which I hope will not 
 be thrown away. I would just remark 
 that if you examine this newly-made emul¬ 
 sion, you will find the iodide is in most 
 minute particles, and that by transmitted 
 light it is perfectly orange. 
 
 I have already shown you the colours of 
 different haloid salts of silver under different 
 conditions. I showed you two bromides, two 
 iodides, and two chlorides, one of each of 
 which had been boiled in gelatine, and the 
 other unmodified, and you saw the different 
 colours they transmitted, To-night I wish to 
 show you a new form of emulsion, which is 
 adapted to certain purposes, and which is 
 sensitive to the red and the ultra red rays of 
 the spectrum. If I had time I could show 
 you that green is the most likely colour to 
 absorb all below the red, and it was for this 
 reason that I worked till I obtained a green- 
 blue colour for the purpose I had in view. [The 
 colour of the new form of bromide was shown 
 
 on the screen.] I propose to try and take a 
 photograph with the dark rays of the spec¬ 
 trum which lie below the red. These rays, 
 which were discovered by Sir W. Herschel, and 
 whose properties were examined by Tyndall, by 
 means of the thermopile, we can now analyse 
 by means of photography. I will first, however, 
 show you how very sensitive this form of bro¬ 
 mine is to the ultra-red rays. There is what 
 looks a very uninteresting picture on the 
 screen, but still, it is sufficiently interesting for 
 me to explain it to you. There are now, ap¬ 
 parently, four black smudges, and these 
 smudges were produced as follows :—A film of 
 green bromide was prepared, and over it was put 
 a card, punctured with holes of the same shape 
 as the smudges, one-eighth of an inch away) 
 from it. Above the card, and about half an inch 
 away, was placed a kettle of hot water, having 
 a blackened surface. After considerable ex¬ 
 posure, on development, I got the image of 
 those holes in the card developed on the plate, 
 and the result is the photograph on the screen. 
 So you see that the dark rays of the kettle, or 
 rather the energy of radiation from the kettle, 
 was able to produce an image. This particular 
 kind of bromide is very difficult to manipulate, 
 in a lecture, but I am going to be so bold as tot 
 make an experiment. I dare say some of you. 
 here who had the satisfaction of listening to Pro¬ 
 fessor Graham Bell in his late visit, heard himi 
 discourse about the photophone ; that is, as 
 you know, an instrument which produces 
 sounds in a telephone connected to a selenium 
 cell acted on by light. In one form of this 
 instrument, a beam of light impinging on the 
 selenium, is intercepted by a revolving disc 
 with alternate opaque and transparent sectors.. 
 He found, when he introduced between the 
 source of light and the rotating disc a piecet 
 of ebonite, he still got the same effect, which 
 was a matter of some interest. There were 
 all kinds of explanations given about this 
 remarkable fact. Some people supposed the 
 ebonite took up the radiation, and gave it out. 
 on the other side, but I h®pe to show you that 
 ebonite is transparent to the dark rays of the 
 spectrum. I have some of this green bromide 
 as a film on a plate. In front of that I 
 have a piece of moderately thick ebonite. 
 
 I propose to let the image of the carbon 
 points forming the electric arc fall on the 
 plate through the ebonite, by means of a 
 lens. If the ebonite be permeable by the 
 dark rays, we ought to get an image of the 
 points on the bromide film when it is I 
 developed. As the rays have to penetrats 
 
19 
 
 through a mass of ebonite, I propose to give 
 an exposure of about thirty seconds, and I 
 dare say we shall then find we get the points 
 fairly delineated. It would be impossible to 
 develop this plate in this light, so I will give 
 it to Sergeant Jackson, and ask him to place 
 it in the developer, and by-and-bye I will 
 throw it on the screen. [This was subse¬ 
 quently done.] 
 
 For the sake of those who are uninitiated, 
 I will show how very easy it is to develop 
 an ordinary collodion emulsion plate. I 
 am going to develop it with ferrous oxalate 
 by means of a brush. Behind a negative 
 is a plate coated with a collodio-bromide 
 emulsion, and I will illuminate it by the 
 magnesium wire. On taking it away from 
 the negative, I place it in a little water, 
 in order to make the developer flow, and 
 then I brush the developer on the surface. 
 It is rather a bold proceeding to manipulate 
 a collodion film with a brush, but still I dare 
 say it will answer. [The image was developed 
 and shown.] The usual plan of developing 
 these plates is to place them in a dish, and 
 pour the developer over them. They can also 
 be developed in the hand in the same way as 
 a wet plate. 
 
 Now there is a great comfort in the collodio- 
 bromide process, in that you are able to give 
 local intensity to the image ; and I hold that, for 
 real artistic work, the great object is to be able 
 to give local intensity to any part which the 
 manipulator may judge expedient, so as to 
 give it that tone and that vigour which an 
 artist may think necessary. I have often 
 asserted this before, and I am glad of the 
 opportunity of repeating it. I do not believe 
 any process is perfect until that power is placed 
 in the hands of the manipulator ; and the fact 
 is that the next process which I shall describe 
 is not perfect, as in it you are at the mercy of 
 your plate, and you must let come out what will. 
 
 The process which I refer to is the gelatine 
 process, which may be described as silver 
 bromide, held in suspension in gelatine, in 
 the same way that it is in collodion. The 
 originator of the process, I believe I am 
 correct in stating, was Dr. Maddox, who took 
 the first picture on gelatine. I know there 
 have been other claimants for the credit; but 
 I believe, after consideration of the subject, 
 that Dr. Maddox was the first person who used 
 the gelatine emulsion with success. But of all 
 people to whom we are indebted, there is no 
 doubt, first of all, we are indebted to Mr. 
 Kennett for the very perfect way in which he 
 
 brought the emulsion before the public. Mr. 
 Kennett laboured many years in improving 
 gelatine emulsions. He, no doubt, bore the 
 burden and heat of introducing this process 
 commercially; but for the extreme rapidity with 
 which gelatine plates can at present be pre¬ 
 pared, Mr. Bennett must be credited with the 
 full honour. Mr. Bennett showed how a 
 gelatine emulsion could be rendered very 
 sensitive by keeping it at a comparatively 
 speaking low temperature in a liquid condition 
 for many days. Seven days was not too long 
 a time to get full rapidity of a gelatine emul¬ 
 sion. Colonel Wortley afterwards claimed 
 that he could get the same sensitiveness with 
 heating up to 150 13 Fahr. for a short time and 
 then Mr. Mansfield capped it by showing that 
 in a very few minutes you could get the same 
 sensitiveness by boiling. Another method was 
 then introduced of producing very sensitive 
 gelatine emulsions, by Dr. Monkhoven, and 
 that was by the introduction of ammonia with 
 the bromide of silver. The ammonia process 
 found many admirers, amongst whom was 
 Dr. Eder, and no doubt the process which 
 he has described lately, in which a large 
 quantity of ammonia is added, gives very 
 sensitive ]dates, and at the same time very 
 vigorous pictures when the sensitiveness is 
 not too great. But I think I may say that in 
 England, a gentleman who is well-known 
 amongst photographers, Mr. Cowan, has intro¬ 
 duced an ammonia process, which certainly 
 beats that of Dr. Eder. He emulsifies his bro¬ 
 mide in a very small quantity of gelatine with 
 ammonia, and then adds sufficient gelatine when 
 the emulsion is ripened. Dr. Eder’s method was 
 to add the full amount of gelatine with the 
 ammonia, by which means, of course, he got a 
 certain amount of rapidity. By Mr. Cowan’s 
 method, you certainly get greater rapidity, and 
 greater certainty. I thought it necessary to enter 
 into this, because it has been imagined that 
 your lecturer has been always an opponent of the 
 ammonia process. I am not; but I honestly 
 say I prefer what is called the boiling process 
 in gelatine emulsion making; but if anything 
 would convert me to the ammonia process, it 
 would be Mr. Cowan’s method. 
 
 Now what is the reason of this sensitiveness 
 in the gelatine emulsion; for pictures can 
 be taken in a tenth of the time necessary 
 for a wet plate, and perhaps a thousandth 
 that necessary for one of the old dry- 
 plates ? First of all, we have seen on the 
 screen, in the last lecture, that the bromide 
 has a blue form. But you must not run away 
 
20 
 
 with the idea that because anything is blue, 
 therefore the molecules composing that emul¬ 
 sion are blue. It may be an agglomeration of 
 red molecules, agglomerated together in such 
 a way, that when light is scattered by it, it 
 gives you the effect of blue light; and I believe 
 I am right in stating that the most sensitive 
 emulsion is that in which the agglomeration 
 is in a red state, that is to say, the molecules 
 themselves are red, and the agglomeration 
 appears blue—the blue is there by accident, if 
 I may say so. One reason why the gelatine 
 emulsion appears so much more sensitive than 
 the collodion emulsion is, that you can use a 
 more powerful developer. If you separate bro¬ 
 mide of silver which has been emulsified in 
 gelatine, and which gives very rapid plates 
 from the gelatine—and there is an easy way 
 of getting bromide of silver out of gelatine— 
 and place it in collodion, the extreme rapidity 
 will be found to be gone, simply because you 
 cannot use as strong a developer as you can 
 with the gelatine emulsion ; in fact, the pro¬ 
 perty that gelatine possesses of acting as a 
 physical restrainer comes into play; each little 
 particle, or aggregation of particles, is sur¬ 
 rounded by gelatine, which prevents the de¬ 
 veloper acting rapidly on them. 
 
 Again, the fact that by boiling, or by the 
 ammonia process, you get a coarser deposit of 
 bromide of silver, also points to increased 
 sensitiveness, for the reason that I tried to give 
 point to in my first lecture. For when sub-bro¬ 
 mide of silver is acted on by the developer, you 
 have metallic silver left, the last atom of bro¬ 
 mine being taken away from the silver; and the 
 moment you have freshly reduced silver in the 
 presence of bromide of silver, and in contact with 
 it, the bromide of silver can no longer exist, and, 
 therefore, you have fresh sub-bromide formed, 
 thus :—Ag 2 + Ag 2 Br 2 = 2 Ag 2 Br, and this, 
 in its turn, is reduced, and so on, until the 
 whole of the particle is reduced. The 
 faintest action of light on one of these par¬ 
 ticles will therefore show itself, which it would 
 not do were this aggregation not so marked as 
 it is seen when looked at under the microscope. 
 Another reason of the increased sensitiveness 
 is this (and I have no doubt it aids it very 
 materially), that if you boil or heat bromide of 
 silver, or any haloid salt of silver, with an 
 organic substance, it has a tendency to 
 separate into the metallic state. So by 
 boiling, for instance, a gelatine emulsion of 
 silver bromide, the silver has a tendency to 
 separate into a metallic state ; in fact, the 
 bromide of silver then is in the state of very 
 
 tottering equilibrium ; the bromine is ready t ■ 
 be given off at the very slightest disturbanc 
 of the molecule, much more so than before i 
 is boiled. I think the fact that you so ofte 
 get fogged emulsion when you over-boil, i 
 proof of the statement that I am making 
 Now, if you were to ask me to illustrate th 
 sensitiveness of a gelatine plate, I should tak 
 you down to the exhibition below, and show yo 
 first, not some of those marvellously instar 
 taneous photographs which are there, but tw 
 less interesting-looking pictures, one by Mi 
 Henderson, which is a photograph by moor 
 light [afterwards exhibited on the screen at th 
 concluding lecture], and another by Mr. Williai 
 Brooks, which is a photograph of some undei 
 ground cellars at Reigate, taken by lamg 
 light. I think, if anything can show wh; 
 gelatine plates can do, it is the fact th; 
 candlelight and moonlight can be utilised fc; 
 impressing the gelatine surface with a 
 image. I have here another proof, perhap 
 which will interest you as showing yc 
 how rapidly a gelatine plate can act. 
 propose to take an image of a rotating disi 
 having alternate transparent and opaque sei 
 tors, the illumination being the light of a sing 
 small spark. I have here an ordinary pla 
 electric machine, and six small Leyden ja- 
 together with a discharging machine, ti 
 knobs of which are placed five niillimetri 
 apart. If the Leyden jars are charged lr> 
 certain point, the spark will leap across bi 
 tween these two knobs. The light from th, 
 
 feeble spark will illuminate the rapidly rotatii 
 disc which I hold in my hand, behind whi<. 
 is placed a gelatine plate. [The apparat. 
 was charged, and the spark passed betwe . 
 the knobs, illuminating the disc. The pla: 
 was then developed and shown.] The imag 
 you see, of the rotating disc is perfectly shar 
 [The disc was apparently motionless.] I da 
 say it will interest you to know with what spe 1 
 that photograph was taken. The durati 
 of the electric spark, for a distance such a; 
 have here, has been measured, and found to 
 42-millionths of a second. You have now soi 
 idea of the immense velocity with whi 
 
the spark travels, and the extreme sensi¬ 
 tiveness of the plate which I employ. This 
 recalls to my mind a very early experi¬ 
 ment, made in the daguerreotype days, where 
 a piece of newspaper was placed on a 
 rotating wheel, and by the light of a spark ob¬ 
 tained from a battery of fifty large Leyden jars, 
 a photograph was taken of the rotating 
 newspaper. I believe the words were tolerably 
 well defined. The time of passage of the 
 spark was greatly longer than that which I 
 showed you first, and it was immensely thicker 
 and more luminous. I should suppose the spark 
 from the large battery at the Royal Institution 
 would take not less than a thousandth part of a 
 second to travel, so that you see they were 
 labouring under better circumstances than we 
 are here. I have taken a photograph of this 
 rotating disc in the camera, with only a spark 
 from one jar, but it was rather a ticklish thing, 
 so I preferred to carry out the experiment in the 
 way I have done, rather than to risk a failure. 
 But you may take my word for it, that if you 
 had fifty Leyden jars, you could take a portrait 
 of our Chairman with the greatest ease, by a 
 flash from the spark, which would have a devia¬ 
 tion of about a thousandth part of a second. 
 
 Dr. Vogel has recently introduced a new 
 kind of emulsion, which is a sort of mongrel 
 between collodion and gelatine emulsion. It 
 is made with acetic acid, gelatine, pyroxylin, 
 and bromide of silver; but I will not coat a plate 
 to-night with it, as the smell of acetic acid is 
 very strong. It is a very good emulsion, very 
 clean, and very fairly rapid. I pass on from 
 this, merely saying that plates are more readily 
 coated than by gelatine emulsion, but less so 
 than with collodion emulsion. 
 
 But we come now to a decided advance in 
 photography, and that is the doing away with 
 glass as a support for the emulsion. Anybody 
 who has travelled, and had to make mountain 
 ascents, will know perfectly well that glass is 
 a serious drawback to the mountaineer; he 
 would do anything to save himself the infliction 
 af taking twelve whole plates up to the top of 
 Mount Blanc. My friend, Mr. Warnerke, who 
 is generally to the fore in all matters of photo¬ 
 graphy, was to the fore in doing away with 
 glass for carrying collodion films, and is now to 
 :he fore in doing away with glass for gelatine 
 films. He has perfected a process by which 
 :he photograph is taken on paper instead of 
 m glass, and he has kindly brought here 
 some illustrations to show the method pursued. 
 First of all, he has a sensitive tissue, which can 
 ie made of any length. I do not know whether [ 
 
 he has made it a mile long, but certainly a 
 great many yards, and it can be rolled on a 
 roller, and exposed in the dark slide. By 
 simply turning another roller, a fresh surface 
 is brought into the plane of the focussing- 
 screen. The sensitive tissue is developed 
 in the ordinary way with alkaline develop¬ 
 ment. Here we have it in the developed state. 
 The film can be either stripped off, or else trans¬ 
 ferred to glass. In the latter case, we come to 
 another point, which is a distinct advance in 
 photography. Mr. Warnerke has found that 
 when you develop a gelatine plate with alka¬ 
 line development, that is to say, with pyrogallic 
 acid and ammonia, the parts which have been 
 acted upon by light, and which have been de¬ 
 veloped, become insoluble in hot water. He 
 thus is able, after development, instead of 
 using the hyposulphite bath to fix the print, 
 to transfer it to glass, and wash away with 
 hot water the parts of the film which have not 
 been acted upon by light, and he gets a 
 transparency such as I hold in my hand, 
 and which no doubt you will examine with 
 great interest by-and-bye. It should be 
 noticed that, to do this, it is necessary that the 
 back surface of the gelatine film should be 
 exposed to the water, as in carbon printing, 
 and this is secured by transfer to glass. Mr. 
 Warnerke is not satisfied with doing away with 
 glass for the camera, but he does away rvith 
 glass for printing; and in order to accomplish 
 this, he re-transfers the negative from the glass 
 to a sheet of gelatine. I may say that the glass 
 is freshly collodionised, which enables the film 
 to strip off readily. In the picture before you 
 we have an illustration of the gelatine film 
 ready for printing, and here we have the print 
 taken from the negative. An advantage of these 
 film negatives is, that you can print from either 
 side, each one yielding sharp prints, which is a 
 desideratum when using the single transfer 
 carbon process, and other such processes where 
 reversed negatives are required. I think that 
 Mr. Warnerke has really made a very great 
 advance by enabling us to do away with 
 glass. I only hope the time will come, before 
 very long, when not only he himself, but his 
 friends and the public at large, will be able 
 to avail themselves of this excellent inven¬ 
 tion which he has now been cogitating 
 for some time. I, myself, know he has been 
 working at it for nearly two years. There is 
 another part of his process which I shall have 
 to speak about presently. But before leaving 
 this part of the subject, I wish to Call your 
 attention to the “ reason why” of the invention. 
 
22 
 
 I do not like taking anything on trust, and I 
 do not like seeing a thing and not knowing 
 the reason of it. I hope that Mr. Warnerke 
 will agree with me, when I say I think I have 
 found the explanation of the insolubility of the 
 film which has been acted upon by alkaline 
 development after exposure to light. If you 
 take a gelatine plate and expose it from the 
 back very strongly to the sun-light, behind a 
 negative, and then put it in tepid water, you 
 will find there is a tendency for the part which 
 has not been blackened by light gradually to 
 dissolve away, and to leave the black image 
 formed by light undissolved. What is the 
 meaning of that ? It means this, that bromine 
 is liberated from the silver bromide, and the 
 bromine renders the gelatine insoluble. If you 
 take bromine and brush it across a plate, and 
 put it in hot water, you will find where the 
 bromine has acted, there the film will refuse to 
 wash away. Again, if you take bromine water, 
 and act in the same way upon gelatine emul¬ 
 sion with which plates have been coated, you 
 will find the gelatine emulsion becomes insolu¬ 
 ble. I believe the rationale of Mr. Warnerke’s 
 process is that bromine is liberated in the gela¬ 
 tine, during the act of development. Now, you 
 may say that the bromine is absorbed ; so it is ; 
 but what becomes of it ? If you add sulphite 
 of soda, as in Mr. Berkeley’s developer, I 
 think Mr. Warnerke will bear me out in 
 saying that there is no insolubility created by 
 development. If you use a ferrous oxalate 
 developer, there is no insolubility of the film 
 created by such development. Why is that ? 
 Simply because the sulphite of soda is able to 
 mop up the bromine, as is also the ferrous 
 oxalate developer, as quickly as it is liberated, 
 whereas the pyrogallic acid and ammonia is 
 not able to do so. I have thrown this out as a 
 hint for experimenters who wish to work further 
 in this direction. 
 
 In the matter of gelatine films, we have Prof. 
 Stebbing’s, which are really workable. The 
 gelatine emulsion is apparently flowed on 
 an insoluble gelatine film on glass, which is 
 then stripped. They are developed by placing 
 them on glass. I have here examples of nega¬ 
 tives taken on such films. 
 
 But I must now hurry on. The next point I 
 am going to touch upon is the enlargement of 
 negatives. Now, the question arises, how are 
 you going to get an enlarged negative ? Of 
 course you will at once say “get a transparency 
 by contact, and then enlarge it in the best way 
 you can.” “The best way you can is,” of course, 
 to do it very perfectly indeed ; but, unfortu¬ 
 
 nately, by the wet process, an enlargement is 
 very often sadly defective. The best way which 
 I know of getting an enlargement of a nega¬ 
 tive, is one which was brought forward a few 
 years ago by Mr. Valentine Blanchard, and it 
 is this. He takes the original negative which 
 he wishes to enlarge, and places it in an enlarg- 
 ing camera. He then takes a transparency oi 
 the exact size which he wants his negative 
 to be. He next takes a piece of common 
 albumenised paper, and prints that trans¬ 
 parency upon it, and by this means gets a 
 very soft and beautiful negative. If you have 
 a hard negative, it is almost impossible to get 
 a soft transparency by the wet plate process, 
 but by this artifice of “printing out” your 
 transparency, and using that as a negative, you 
 get a decidedly soft paper negative. If you trace 
 the reason of this, it is as follows :—When the 
 albumenised paper is acted upon by light, we 
 know it blackens immediately, and, of course, 
 light acts less violently on the underneath 
 portions where it has been blackened. Thus, 
 in printing from a transparency, the most 
 transparent portions first blacken, then the 
 next most transparent part blackens, before 
 the first blackened part becomes too intense, 
 and so on. So you see, by the shielding oi 
 the film by the blackening action, you are abl: 
 to get a softness which you otherwise woulc 
 not get. I have here such a negative, whict 
 I prepared several years ago, by which yor 
 see we get some of this softness. Mr. Blan¬ 
 chard, I am sorry to say, has gone back from 
 his old love, and recommends now, that yoi 
 should use a gelatine plate instead of the albu 
 menised paper. A gelatine plate is a very gooc 
 thing, but I think the albumenised paper nega 
 tive beats it, when you have to enlarge from 
 very small negative to a very large one. I thinli i 
 the grain of the paper, too, is an improvemen 
 in large photographs, and you get no grain in r 
 gelatine negative. I will next show you hov 
 to obtain a paper negative by the ordinan 
 lantern, but I must ask you to bear with mi 
 if I keep you rather long. I will make ai 
 enlarged negative from a transparency takei 
 from one of my Egyptian negatives, deve!opin|, 
 it before you. This is ordinary paper, to whicl 
 a couple of washes of bromide of potassiun 
 were given, and which, when dried, wa; 
 floated on silver nitrate. Half a minute* 
 exposure to the light coming through th- 
 transparency will be sufficient. I shall us 
 the developer, which I recommended to you 
 notice some time ago, the ferrous-citro-oxalate < 
 [The negative was developed and shown t 
 
the audience.] So much, then, for paper 
 enlargements. We now come to another 
 great advantage of the gelatine process, which 
 is that by it you can get soft transparencies. 
 I am aware that it is said there is a certain 
 amount of blurring in gelatine transparencies, 
 but I am not prepared to say that is necessary, 
 in fact, I should say it ought not to be the case, 
 as I think I shall be able to show you in the 
 next lecture. Therefore, if you have a hard 
 negative by the common process, it is always 
 possible, by the gelatine proccess, to get a 
 soft negative from a gelatine transparency, 
 in which case you can substitute for the 
 paper, such as I used just now, a gelatine 
 plate, or a collodion plate, or a wet plate, 
 whichever you prefer. The main point to aim 
 at in getting enlarged negatives is, to get 
 thin, delicate transparencies, with every atom 
 of detail present in them. If you get that, you 
 can always get a decent enlarged negative 
 from them. If the detail is blocked up, you will 
 not do so, unless you use a paper printing pro¬ 
 cess, such as I showed you, with the albumen- 
 ised paper negative. 
 
 After these preliminary remarks, I propose to 
 show you one of the new applications of the gela¬ 
 tine emulsion process, and that is the develop¬ 
 ment of a print on paper coated with gelatino- 
 bromide. The paper is prepared by coating 
 ordinary paper with gelatino-bromide (of the 
 most sensitive kind if you like). Such paper 
 can then be exposed to the image formed by 
 an ordinary magic lantern; by that means 
 you can get can get an enlarged print. The 
 lantern before you is a triple-wick oil lamp, 
 and I think you will be able to see that I shall 
 get a very good print indeed from the negative, 
 which is at present placed in front of the con¬ 
 denser. [The negative in the lantern was a 
 view taken by the lecturer of one of the caves 
 of Ellora, in Western India. An exposure of 
 half a minute was given, and then it was 
 developed by the brush, with a ferrous oxalate 
 developer.] 
 
 For my own part, I prefer gelatino-chloride 
 paper, as it gives a very black image which 
 beats the bromide out of the field altogether. 
 The reason why the tone is black, is that 
 chloride of silver when developed, is de¬ 
 veloped in two states, in a red, or ruddy 
 State, and also in a green state, and when 
 you put red and green together, the result is 
 not far from black. I think you will see, when 
 this is developed, that we shall have the ruddy 
 state first. I use the ferrous-citro-oxalate. It 
 will not bear the ferrous oxalate at first, 
 
 although it may be brought out more quickly 
 with it. Now, if I turned up the light, you 
 would see it in the pinky state ; as it goes on, 
 the pinky state gives place to the green, and 
 the two together make up the black, and you 
 will see afterwards that this black is of a 
 most beautiful hue. I want particularly to call 
 attention to the beautiful black tone it has. 
 On the table there are some plates prepared 
 with gelatino-chloride, to show the same black 
 tone. 
 
 [An enlargement on gelatine chloride paper 
 was here made, and developed before the 
 audience, the developer used being the ferrous- 
 citro-oxalate.] 
 
 We may thus say that an advance has 
 been made, when, by an ordinary magic- 
 lantern, with a good negative, you can get a 
 perfect enlarged paper print by development. 
 Perhaps they will not have that lustre which 
 albumenised prints have, but it is a matter 
 of taste whether you like that gloss or not. 
 
 I have now to allude to Mr. Warnerke’s in¬ 
 vention again. He has found that he is able to 
 mix with his gelatine emulsion colouring matter, 
 and you will at once perceive the value of 
 this. When you wash awayfrom the gelatine the 
 unaltered bromide of silver that is not acted 
 upon by development, you get silver left behind, 
 plus the colouring matter surrounding it. I 
 have a developed print containing colouring 
 matter, and to save time, I have placed it on 
 a transfer paper. The image was developed 
 with alkaline pyrogallic acid, and was placed 
 in contact with gelatinised paper, rendered 
 insoluble by chrome alum. I propose to wash 
 away the soluble gelatine containing the bro¬ 
 mide of silver and the pigment, to show what a 
 power this discovery places in our hands. I first 
 place it in hot water, and as soon as it has 
 soaked a little, the outside paper on which 
 it was developed will peel off, leaving the 
 gelatine film on, what we may call, the 
 transfer paper. The subject is one of Mr. 
 England’s statue subjects, of which he 
 was kind enough to give me the negative. 
 Stripping away the paper on which the print 
 was developed, the picture remains on the 
 transfer paper, and we have the image waiting 
 to be freed from the soluble matter. The 
 print before you was taken in my laboratory 
 with the electric light; and had a very short 
 exposure. The soluble matter is now gradually 
 washing away, and the whites are gradually 
 coming out, and we get a picture in the colour 
 of alizarine. You can get almost any variety of 
 colour; but it is not every colour the operator can 
 
24 - 
 
 use, because some would destroy the sensitive¬ 
 ness of the gelatino-bromide, but most colours 
 he can use. I can hardly thank Mr. Warnerke 
 enough for the trouble he has taken in illus¬ 
 trating this part of my lecture for me. He has 
 furnished me with pictures showing a variety 
 
 of tints, from sepia and photographic purple 
 to almost any colour. This I may claim as a 
 great advance in photographic printing, and I 
 hope, before long, the public will have the 
 advantage of this invention, which at present 
 they have not. 
 
 LECTURE IV. —Delivered February 20th, 1882. 
 
 1 have already shown you the example of a 
 production of a positive from a negative, 
 having previously to that demonstrated the 
 production of a negative from a positive. I 
 wish to commence this evening by showing 
 you the production of a positive picture from 
 a positive picture ; not that there is anything 
 essentially novel in it, but it points a moral 
 to which I shall presently have to draw your 
 attention. What I propose to do, is to take some 
 sensitive iodide paper, expose it to light, and then 
 soak it in a weak solution of potassium bromide, 
 and next expose it to an enlarged image from a 
 positive on glass. On developing, I think, the 
 probabilities are that we shall get a positive pic¬ 
 ture. It is perfectly immaterial, as I am going 
 to expose it to light, whether the paper sees 
 the light now or not, and the small modicum 
 of light we have here will make very little dif¬ 
 ference to this comparatively insensitive paper. 
 [The paper was shown to the audience.] I 
 now burn a small quantity of magnesium 
 wire, in order to fully expose the sensitive sur¬ 
 face. If I were to put a developer on that 
 paper as it is now, it would at once blacken. 
 Instead of applying the developer, I will place 
 it in the solution of potassium bromide, then 
 expose it under a transparency, and subse¬ 
 quently develop it. I will give it rather a pro¬ 
 longed exposure, for this paper is not very 
 sensitive, and of course, the time in which this 
 positive is produced depends on the sensitive¬ 
 ness. [A lantern image was thrown on the 
 paper.] Half a minute with this light would 
 be sufficient, but for safety’s sake I will give 20 
 Seconds longer. I will now develop the picture 
 with ferrous oxalate, and a positive appears 
 instead of a negative, which it would do under 
 ordinary circumstances. In other words, the 
 lights are reversed. 
 
 Before I draw any conclusion as to this, I 
 will show what rays of the spectrum produced 
 this positive picture, because this is an im¬ 
 portant item in the deduction I shall have to 
 draw. I will take another similar piece of 
 paper, and throw the spectrum upon it. You 
 will recollect that we applied a solution of 
 bromide of potassium to an exposed piece of 
 paper, and that it was then exposed to the 
 image. I will, therefore, expose this second: 
 piece of paper to the light from the mag¬ 
 nesium wire, as before, and immerse it ii 
 a dish in which there is a solution of potassiumjj 
 bromide, and place it in the spectrum. I leav : 
 it to be acted upon for about half-a-minute 
 This I propose to develop on the screen, anc 
 by cutting off the light during development anc 
 again throwing it on, you will absolutely 
 see the part of the spectrum which affect: 
 the paper. If everything goes right, then 
 ought to be a bleaching action in the blui 
 rays. The red rays will also exert a bleach 
 ing action if the exposure is continued fo 
 some time. I am not going into this actioi 
 of the red rays, as it would carry me into to< 
 much theory ; I simply want to show you thi 
 practical result of bromide of potassium on ai 
 exposed paper. On developing, you see w 
 have the bleaching action caused by those por 
 tions of the spectrum which I mentioned. 
 
 Fig. 1. 
 
 Now, if you will allow me, I will now poin 
 the moral. These may seem paltry experi 
 ments to bring before an audience, but thos 
 who are interested in gelatine plates, and 
 believe there are a great many here who arc 
 
will now, perhaps, be able to draw an inference 
 equally well with myself. All gelatine plates, 
 without exception, I believe, are prepared with 
 an excess of soluble bromide, and I will now 
 ask you to consider what happens, supposing 
 any bromide is left in the film. While the 
 light is acting on the film, the bromide of 
 silver is decomposed, and at the same time 
 the bromide of potassium (or ammonium), 
 is also being decomposed, and undoing the 
 work that the light has done upon the bromide 
 of silver; so that you see that the sensitiveness 
 really got by gelatine plates with an excess of 
 soluble bromide is only the difference between 
 the effect of the light on the silver bro¬ 
 mide, and on the bromide of potassium (or 
 ammonium). I think those two experiments 
 ought definitely to settle that gelatine plate- 
 makers have not, as yet, obtained the highest 
 sensitiveness possible, simply because they 
 prepare their emulsions with an excess of 
 soluble bromide. I may say, with reference to 
 this, that Dr. Eder has recently shown that on 
 immersing gelatine plates in a weak solution 
 of silver nitrate, (in which is a little citric 
 acid), though strong enough to neutralise 
 any soluble bromide which may be left 
 behind, an increase of sensitiveness, two or 
 three-fold, is produced. The problem to be 
 solved by gelatine plate-makers is this—How 
 to get rid of any possible excess of soluble 
 bromide in their gelatine films ? When they 
 do that, no doubt they will get more rapid 
 plates even than they have done at present. 
 I hope the effect on the minds of the audience 
 of these two experiments will not be thrown 
 away, as now you have seen that the same 
 rays which act upon the silver bromide, act 
 upon the potassium bromide. I will next show 
 on the screen those interested in the chemistry 
 of the subject the formula which represents 
 the action which takes place— 
 
 (I.) 2Ag 2 I -f- 2KI -f H a O + O = 4AgI + 2KHO 
 (II.) 2Ag a Br + 2 KBr+H 2 0 -f 0 = AgBr-f 2KHO 
 
 In the first equation (I.) we have silver iodide, 
 but a similar formula (II.) applies to bromide 
 of potassium, and bromide of silver. In (I.) 
 there is sub-iodide of silver, iodide of potas¬ 
 sium, water and oxygen, and that forms iodide 
 of silver, potash, and iodate of potassium. 
 When you have more iodine or bromine 
 liberated, we get— 
 
 (III.) 61 -f 6KHO = 5KI 4- KIO, -f 3H s O 
 
 or the iodine and potash combine, and form fresh 
 potassium iodide, water, and iodate of potash. 
 
 When this reaction is finished, we have to go 
 back to the equation (I.), and the same decom¬ 
 position is repeated. The oxygen of course is a 
 crux; where does it come from ? It comes from 
 the air. It will find its way through the gela¬ 
 tine. Gelatine is not impervious to the air. Dr 
 Leeds has very recently found that this re¬ 
 action does not take place when there is no 
 oxygen present. So that you see, in order for 
 this reaction to take place, there must be 
 oxygen obtainable from some external source. 
 If you prepare your emulsion with hydrochloric 
 acid, as some do, you have nitric acid left in 
 the film, and that will supply the necessary 
 oxygen for this formula to take effect. I need 
 not enter into details of what takes place with 
 the nitric acid, it is sufficient to say that the 
 nitric acid will supply the oxygen. 
 
 This leads me next to consider what causes 
 the destruction of the photographic image. 
 You may destroy the image on a photographic 
 plate, or photographic paper, by any substance 
 which will readily part with oxygen. You can 
 destroy it, for instance, by bichromate of 
 potash ; you can destroy it by any of the ferric 
 salts; or you can destroy it by oxygen- 
 yielding substances, like permanganate of 
 potash; ozone, per-oxide of hydrogen (hy¬ 
 droxyl) ; in fact, there is hardly any sub¬ 
 stance which will part with oxygen which 
 will not destroy the developable image; the 
 photographic image remains behind as a rule, 
 though not always, but these reagents pre¬ 
 vent it becoming developable. Here is paper 
 prepared with bromide of silver, which has 
 been already exposed behind a negative. After 
 immersion in water, I propose to apply to it 
 some of these destructive agents which I have 
 already mentioned. First I will give it a 
 streak of bichromate, next ferric oxalate, and 
 next permanganate of potash. If everything 
 goes right, on those parts to which these solu¬ 
 tions have been applied, the image will be 
 destroyed, or almost destroyed, and a little 
 longer application of the solution would entirely 
 destroy it. I now develop ; parts of the image 
 appears, but you see those parts which have 
 been brushed over with the above solutions 
 entirely refuse to appear. This demonstrates 
 then that oxidising agents will destroy the 
 developable image. 
 
 Now, I do not wish to detract from any 
 developer, but this shows you the care you 
 should take in seeing that your ferrous oxalate 
 developer is tolerably fresh, for if you have a 
 large proportion of ferric oxalate present with 
 it f it will be very liable t9 “$1 qw” your 
 
z6 
 
 plates, and those who are photographers 
 will know what that means. If you take 
 your pyrogallic acid which has been fully 
 oxidised, you will find that it will play you the 
 same trick as this ferric oxalate does, so that 
 neither the ordinaiy alkaline developer nor the 
 ferrous oxalate developer is certain to give you 
 the best results unless it be fresh. Photo¬ 
 graphers take warning, and when using a 
 ferrous oxalate developer, use it fresh! If 
 you use the ferrous-citro-oxalate which I have 
 introduced so often to your notice, you get the 
 same result; it has the same bad qualities that 
 the ferrous oxalate has. 
 
 I now come to the one other destructive 
 agent of the photographic image, and that is 
 bromine; and while I am on the subject, I 
 should like to show you a neat way of exposing 
 a transparency. It is not mine, and, therefore, 
 I can praise it. Through Mr. Bolas’s kind¬ 
 ness I have here a phosphorescent plate 
 made with calcium sulphide, which I propose 
 to expose to light, then to use it as a source 
 of illumination for obtaining a transparency. 
 I now expose the phosphorescent plate to the 
 light from magnesium wire, and then placing 
 it over a negative, behind which is a sensitive 
 plate, expose for 30 seconds. I next take a 
 little bromine water, and brush it across the 
 exposed sensitive plate; I rinse with water, 
 and develop with ferrous oxalate. As I showed 
 you in a former lecture, the ferrous oxalate is 
 a very fair medium for cutting off the radiation 
 which affects a bromide plate, and so I can 
 develop it before you. The image begins to 
 appear, and by degrees we have the image 
 pretty perfect, except that, where the bro¬ 
 mine has acted, there the image is totally 
 destroyed, or very nearly so. This is a very 
 simple experiment, but I did it for the purpose 
 of leading you to another important point 
 with regard to gelatine plates. Most of you 
 are aware that when you have a pure bro¬ 
 mide plate, it is very easily reversed as they 
 call it, that is to say, instead of a negative 
 there is a tendency for it to become a 
 positive. The reason of that, I think, will 
 be found in a measure in the experiment 
 which I have just showed you. When you 
 expose a bromide plate to the effects of light, 
 anybody who has at all a keen sense of smell 
 will perfectly well know there is some substance 
 liberated which is somewhat pungent, and it 
 will be found that it has a smell of bromine. 
 If, therefore, you expose a plate for a consider¬ 
 able period—longer than it should be, that is to 
 say—to light, what is the first effect ? The first 
 
 effect is for the silver bromide to be decomposed 
 at the surface, and bromine to be liberated in 
 a gaseous state, part of it will ascend into the 
 atmosphere, and part of it will have a tendency 
 to go down into the film. As the light pene¬ 
 trates still further, the energy of the radiatior 
 will split up the bromide below into bromine anc 
 sub-bromide, and so on, and the part beneatl 
 the first part will be passed over by bromine 
 liberated from the molecules immediate!} 
 below; so that, by a continuation of this 
 emission of bromine, you get, first, the toj 
 practically saturated, as it were, with the 
 bromine from the lower particles, and, as w< 
 have just seen, when you have bromine appliec 
 to a plate that has been acted upon by light 
 it destroys its developable powers—in othe 
 words, it forms fresh bromide of silver. I 
 such is the case, you ought to have a reversa 
 of the image at the top of the plate, and non 
 reversal probably at the bottom. I think 
 can show you this in the lantern. I exposer 
 two gelatine films, such as are prepared b]j 
 Mr. Stebbing, behind a plate which is preparec 
 with squares of different thicknesses of colouret 
 gelatine; that is to say, it allows differen 
 gradations of light to pass through. On thes 
 graduated thicknesses of gelatine are painte: 
 opaque numbers. No. 1 shows the thinne : 
 amount of tinted gelatine, and No. 25 shows th 
 thickest amount of gelatine. With a moderat 
 exposure to light of a gelatine plate behim 
 such a screen, the whole of these opaqu 
 numbers ought to appear transparent on 
 more or less opaque background plate,' whe: 
 developed and fixed. 
 
 Now I will show you an experiment demon; 
 strating what over-exposure will do. Two c 
 Stebbing’s films were placed face to face, am 
 exposed for a considerable time to light behin 
 a graduated screen. The results are befor 
 you. On the film nearest the graduated screer 
 the whole of the numbers as far as 20 ar 
 reversed ; that is, appear opaque on a trans 
 parent background. On the bottom filni 
 except No, 1, the whole of the gradations at 
 perfect; that is to say, there is not a reverse; 
 image at all. Thus you see that a reverse 
 takes place in the top part of the film, and nc 
 at the bottom. That is the point I wish t 
 bring before you. No doubt this reversin 
 action is partly due to the bromine whic 
 passes over the molecules which lie imme 
 diately below it. Not only that, however, bu | 
 if you enter into the chemistry of the thing 
 you will find that gelatine will take up tlj 
 bromine, as it is liberated, though slowl} 
 
*7 
 
 and that, when bromine combines with gela¬ 
 tine, one of two things happen—it either 
 replaces and liberates hydrogen, or else a 
 molecule which has the properties of hydroxyl 
 Either of these will combine with bromine 
 to form hydrobromic acid, or yet another 
 compound of bromine, which is equally ready 
 to destroy the photographic image as bromine 
 itself, or as any oxidising agent such as I 
 showed you just now. 
 
 Now, before I quit the subject of oxi¬ 
 dising agents, I should like to introduce to 
 your notice a very remarkable utilisation of 
 this oxidising process of photography which 
 was proposed, and not only proposed, but 
 carried out—there is a great distinction 
 between the two, for we often hear propositions 
 made by men who perhaps never have the pluck 
 to carry them out—by Mr. Bolas. He, first 
 of all, took an ordinary gelatine negative 
 with proper gradations of light and shade. 
 Then he wanted to reproduce it, so he took a 
 gelatine plate, and immersed it in bichromate 
 of potash, allowed the film to dry, and then 
 exposed it to light behind the negative to be 
 reproduced. You will see that in this exposure 
 to light he had an oxidising agent present in his 
 film, and that the oxidised parts were acted 
 upon by light, leaving the other part intact, 
 and by that means he got a reversed image. 
 Now, he was aided by the fact that the gelatine 
 is rendered insoluble to a large extent by 
 bichromate of potash, but still that is not 
 everything. On the screen is the original 
 negative, and also the reproduced negative. 
 These I had the pleasure of seeing manipu¬ 
 lated at the Photographic Society. In regard 
 to this process, we have a curious case 
 of re-invention. We have lately had in the 
 papers devoted to photography letters from 
 France, in which a certain Captain Biny has 
 been supposed to have invented this process. 
 Across the Channel they have not the same 
 facility for reading English, I am afraid, that 
 we have for reading French. However, the 
 discovery of Captain Biny’s is nothing more 
 or less than Mr. Bolas’s discovery, and I 
 hope Mr. Bolas will put in a claim for it. 
 
 Another use of oxidising agents is to enable 
 us to get rid of fog. If you have a gelatine 
 plate which has been exposed to light, and so 
 has been fogged, you can get rid of that fog 
 by immersing it in bichromate of potash. I 
 have here a fogged emulsion plate of which the 
 upper half has been immersed in bichromate 
 of potash, the bottom half is completely veiled, 
 but you will notice how beautifully clear the 
 
 top half is, where the plate has been immersed 
 in the bichromate. 
 
 The next subject I wish td treat of is one 
 which has been much misunderstood by 
 many. The theory involved is not new to 
 myself, but I think the results I shall show 
 will be new as far as the audience is con¬ 
 cerned. On this plate is pasted tin-foil, with 
 various figures cut on it; first, there is a 
 circle, then a line across, and so on. The 
 image of these figures I now throw on a 
 monster gelatine plate, the thickness of the 
 glass being some £ inch. Now, I ask you 
 to observe what you see on it. You see every 
 figure surrounded by a halo. Thus, you see, 
 the circle is surrounded by a ring. 
 
 You have a halo round every part IG ' 2 ' 
 of the images. If you come to analyse 
 it, you will find that the halo sur¬ 
 rounding the bright cross is made up 
 of a series of rings similar to that ring which 
 surrounds the clot of light. Now, I have here 
 some wonderful elixir to get rid of these haloes. 
 
 I touch the back of the 
 plate where the cross 
 falls with asphaltum, 
 the halo vanishes. I 
 move the plate a little, 
 to get a fresh surface, 
 and with red varnish I 
 again touch the back of 
 this plate, behind the 
 cross, and now I have a 
 white cross on a red 
 background; to put the matter in words, 
 according to the medium you place on the back 
 of the plate, so is the reflection toned down. If 
 I use a red varnish, this halation will have but 
 little effect on the photographic plate, because 
 it is red, and these red rays do not much effect 
 the production of a change in the sensitive 
 salt. The most perfect cure for halation is 
 Brunswick black; there is no reflection from 
 the back of the plate, and by that means you 
 get rid of any tendency to fuzziness of the 
 image, which was often a disgrace to photo¬ 
 graphers’ pictures. I will now throw on the 
 screen one or two of the absolute photographs 
 showing different halations. One is from an 
 ordinary plate, the other from a thick glass 
 plate ; when the thick one comes on the screen, 
 instead of the cross remaining where it was, it 
 was spread out considerably. [Shown.] When 
 you come to analyse it mathematically, you will 
 find the diameters of these rings depend on the 
 thickness of the glass, together with the critical 
 angle of reflection for the glass employed : that 
 
 Fig. 3. 
 
28 
 
 the greatest intensity in the ring is fixed by the 
 . critical angle alone. That is a very important 
 point, because there has been a good deal 
 
 of controversy 
 Fig. 4. about it, which 
 
 I will not enter 
 into now. I 
 think what you 
 saw on the 
 gelatine plate, 
 and what you 
 see on the 
 screen now, 
 ought to settle 
 that point for 
 ever. I will 
 show you some 
 plates taken with an asphaltum backing; it 
 is of no use giving improper backing; if you 
 have anything which will reflect blue light, you 
 get this blurring of the image. With a plate 
 prepared with a backing of asphaltum, you can 
 even expose it to a bright image of the sun 
 without getting a halo round it. An astronomer 
 in Germany has recently deluded the French 
 Meteorological Office into believing that he 
 has photographed the corona in full sunlight, 
 but when you come to inquire into the matter, 
 this corona is nothing else than the halation ; 
 not only that, but he has found the most extra¬ 
 ordinary rays shooting out from it at certain 
 times. When, however, you come to investigate 
 the question, you will find that these wonderful 
 rays, which are supposed to proceed from the 
 sun, and thus to influence our earth, are 
 nothing more than a certain reflection which 
 the laws of optics tell us ought to be there. 
 
 Here is a simple method 
 by which photographers can 
 always be sure of seeing 
 what amount of blurring 
 they may get. If they take 
 the trouble to place a gela¬ 
 tine plate, or any plate they 
 wish to try, in contact with 
 a slit cut in tinfoil, and then 
 look through the back of the 
 glass at a strong light, they 
 will find it has more or less 
 blurring round the slit. 
 Having so far digressed, 
 I may say that the reason why you get ap¬ 
 parently greater reversal in a photograph 
 taken on a glass plate than you do in a paper 
 negative, is because there is none of this hala¬ 
 tion to help it. 
 
 There is Mso another reason -vyhy reversal is 
 
 more difficult to secure on paper, in that tl 
 bromine has two ways to escape, one throu^i 
 the paper, and the other through the surface 
 the bromide. I hold in my hand an examp 
 of reversal on paper. 
 
 Now I will show you how you can g 
 rid of this reversal altogether. The fil 
 before you was exposed 60 seconds behir 
 a negative to the full sunlight, yet the 
 is no reversal on it. How is that ? Simp 
 because I gave it something which, instei 
 of allowing the bromine to attack the bromi< 
 of silver, that had been altered by ligh 
 at once took it up. The substance er 
 ployed was potassium nitrite. This shov 
 that, if you want to get rid of the reversal 1 
 the image, you must give the plate somethii 
 which will very rapidly absorb bromine, and 
 should say something, if possible, which is m 
 organic, for the reason I have already state I 
 to you in regard to its action on gelatin 
 Now, is it possible that you can give 
 film something of that kind, I say it is qui, 
 possible to do so, and those photographe 
 who are experimenters no doubt might tui 
 their attention to this point. In the mea 
 while, I may say that the addition of iodide 
 bromide in a plate is sufficient to a very lari 
 extent, because, as I showed you in a fori":' 
 lecture, the sub-iodide of silver acts as a sen 
 tiser to the bromide of silver; the consequem 
 is, that those films which contain a lar 
 quantity, or even a small quantity of iodide, 
 not reverse in the same horrible manner aa 
 pure bromide film will do. Perhaps a certs 
 amount of anxiety may have been created 
 photographers’ minds by a recent article in 0; 
 of the photographic papers, in which a doubt w| 
 raised as to whether they can get iodide in th 1 
 films. Let me once for all reassure them. S 
 you have iodide of potassium, and br®mide 1 
 potassium to combine with silver nitrate, iodf 
 of silver will always be present in your fihr 
 for the iodide is formed always long before 1 
 bromide, and, therefore, the scare as to whetl 
 you have iodide in your plates or not, is c 
 which need not be seriously entertained, 
 far as chloride goes, I leave that for anotl i 
 occasion to remark upon ; but once 
 all, if you have soluble iodide present wl 
 making an emulsion, you must have sil 
 iodide in the film, and this will act as) 
 sensitiser to prevent the reversal of the ima: 
 
 I now come to a description of a m' 
 useful instrument introduced by Mr. W 
 nerke, which is known as a sensitomet r 
 it consists of squares of coloured gelatin* i 
 
 Fig. 5. 
 
different opacities through which light is 
 allowed to fall on a sensitive plate. This 
 sensitometer is meant to enable you to judge 
 of the comparative rapidity of your plates. 
 You have seen on the screen some of the 
 images produced on glass, on paper, or on 
 films, by the exposure of such a tablet as this, 
 and it is a most useful instrument. To my 
 mind, no photographer, be he amateur or 
 professional, should be without one, or one 
 similar to it. Mr. Warnerke has been the 
 first to adapt this sensitometer to practical 
 purposes. There are others in the field, of 
 which we have a notable instance in that of 
 Mr. Spurge ; but for the time being I propose 
 simply to 
 call your at¬ 
 tention to 
 Warnerke’s 
 sensitometer 
 The method 
 of operating 
 is very much 
 the way in 
 which I 
 operatedjust 
 now. First, 
 a phosphor¬ 
 escent tablet 
 is exposed to 
 magnesium 
 light, and 
 then allowed 
 sto rest a mi¬ 
 nute, placed 
 in contact 
 with the sen- 
 sitometer 
 plate, which 
 is in front of 
 the plate to be tried. The exposure lasts for 
 half a minute, and the plate is then de¬ 
 veloped.' ?The last distinct number seen upon 
 the plate by reflected light, before fixing, is 
 read off. By a simple table you are able to 
 see the comparative sensitiveness of two par¬ 
 ticular plates of the same kind. Here, owing 
 to Mr. Warnerke’s kindness, I have an enlarged 
 apparatus, showing the way in which you read 
 off the sensitometer. Suppose, for instance, 
 you find the last number on one plate to be 20 
 in the one case, and 16 on another, if you take 
 two such plates out in the field in the same 
 light, and you want to know how much ex¬ 
 posure to give in the one that registers 16, 
 knowing that necessary for the one reading 20, 
 how are you to do it ? You take Mr. 
 
 Warnerke’s instrument, such as we have 
 here (but in miniature, of course), you place 
 this opening at 20, you then cast your eye 
 on the ring, look at 16, and find that the 
 one that registers 20 is three times more 
 rapid than the one that registers 16, and 
 expose accordingly; so, in the same way, if 
 you had one that registered 25, which is a very 
 high degree of sensitiveness, and another 12, 
 you would find that you would have to give 
 the one 36 times more exposure than the other. 
 I recommend this to the notice of practical 
 photographers; I do not wish to act as an 
 advertiser to Mr. Warnerke’s sensitometer, but 
 I merely advise them to get some sensitometer, 
 
 so as to pre¬ 
 vent groping 
 about in the 
 dark as to 
 what ex¬ 
 posure to 
 give to plates 
 of different 
 degrees of 
 se nsitive- 
 ness. There 
 is nothing 
 like picking 
 a hole in 
 your friends, 
 if you possi¬ 
 bly can, and 
 now I am 
 goingtopick 
 a hole in 
 Mr. War¬ 
 nerke’s sen¬ 
 sitometer; it 
 is not a very 
 grave one, 
 but still, one which ought to be noted ; I want 
 to show you what happens, supposing you 
 measure all kinds of plates by such a sensito¬ 
 meter. First, let us examine what kind of light 
 is emitted by this phosphorescent tablet. You 
 will see, if I expose it to the light, it will phos¬ 
 phoresce, and the stronger the light the more 
 it phosphoresces. Now, here we have a spectrum 
 of the phosphorescent light, and when you come 
 to examine it by the eye, it is found that there is 
 one bright band in the indigo : there is a very 
 faint light coming down as far as the red. 
 When you photograph such light in the photo¬ 
 spectroscope, nothing is indicated except the 
 existence of the indigo band of light, and 
 nothing in the violet or beyond; it is almost 
 a monochromatic light which it emits, as 
 
 Fig. 6 . 
 
 S/iee/mm of 
 
 F Phosplro/'p-ire/)/ 
 
 1 TT i i- 
 
 Iodide 
 
 Ch / or/dp 
 
 1 Bromide 
 
 . 1 I 1 | J3/'o>/io-iodicie 
 
 B G Y 0 R (Wet P/oPe) 
 
.3° 
 
 far as photography is concerned. Supposing 
 we have to try some plate, the com¬ 
 position of which is unknown, with such a 
 sensitometer, it will be seen that we may fall 
 into serious error. On this diagram the 
 mountain represents the intensity of light 
 which is painted by the phosphorescent 
 light on a plate. Supposing the plate only 
 contained iodide, you will really see that 
 the little mountain, which only affects the 
 iodide, is only J 0 th part of the whole mountain 
 which would be there if you are using white 
 light. If you only use chloride, it would be 
 showing one-third only; whereas, if you use 
 bromide, it would be unity, or as giving the 
 maximum amount. If you try a wet plate by 
 the sensitometer, you will find that it indicates 
 only about one-fifth of the true value ; that 
 is to say, you would be wrong supposing 
 you exposed a wet plate, and a gelatine 
 plate according to the sensitometer; you 
 would find that you had given five times 
 too much exposure to the wet plate, simply 
 because the bromo-iodide is most sensitive to 
 those rays of light which are not present in the 
 phosphorescent light. Hence, when using 
 such a sensitometer with phosphorescent light, 
 you must beware what you are about, and 
 hesitate before you draw any exact conclusions. 
 
 This leads me on to the measurement of the 
 intensity of light, for if we want to know any¬ 
 thing about our plates, we must know what 
 brightness of light we have. I again have to 
 allude to Mr. Warnerke; he is a most facile 
 inventor, and the photographic world is 
 largely indebted to him for many ingenious 
 contrivances. He has introduced an actino- 
 meter which is dependent on phosphor¬ 
 escence for its value. I have here a phos¬ 
 phorescent tablet which has been as little 
 illuminated as possible. Now I will throw the 
 spectrum upon it. [Shown.] This bright spec- 
 
 Fig. 
 
 I. Visual spectrum of phosphorescent light. 
 II. Photographic spectrum of ditto. 
 
 TII. Rays exciting phosphoresence. 
 
 IV. Rays which extinguish phosphorescence. 
 
 trum of phosphorescent light extends into the 
 ultra-violet, but stops short at the place in the 
 indigo, where the phosphorescent light is 
 
 emitted, that is to say, one spectrum end 
 where the other begins. Mr. Warnerke ha 
 shown us that by exposing such a phosphor 
 escent tablet to the action of light, by a 
 ingenious contrivance, such as you will se 
 downstairs, he is able to tell the photographi 
 value of the particular light. This is a mos 
 valuable discovery, because phosphoresenc 
 is induced by very nearly the same rays a 
 those which affect bromide of silver. If I alloi 
 the spectrum to play upon a fully excited phos 
 phorescent tablet, we get another actior 
 You will see that the phosphorescent ligl 
 is diminished where the red and green hav 
 acted. These rays extinguish phosphoresenc 
 to a very large extent. With reference t 
 this, there is a curious fact that belo’ 
 the red, as we know, there are rays whic 
 exist, and which, to a certain exten- 
 “ de-sensitise ” the phosphorescent film, an 
 we have a darkening going on in that regior 
 I have shown you this experiment, to demons 
 trate how you can desensitise this phosphe 
 rescent plate, viz.,—by allowing white ligl 
 to pass through red or green glass, since the 
 permit the passage of those particular raj 
 which act as extinguishing rays. There 
 another very simple way, which I recommen 
 to amateurs, of telling the amount of expose i 
 to give to their plates, and that is by M 
 
 Woodbury’ 
 
 Fig. 8. 
 
 photomete 
 which is tl 
 simplest thir 
 in the worli 
 Here we ha\ 
 a piece 
 bromide pap 
 exposed 1 
 light for 
 minute, th< 
 readoff again 
 one of the 
 tinted circle 
 accordingas 
 whichever ti 
 
 it agrees with; knowing the “light value 
 of the tint, you know what is the intensi 
 of the light, and, therefore, what to give to j 
 plate. A simple rule to remember is this, th 
 if you use a bromide plate, only use a bromii 1 
 paper for securing this tint; if you are usir " 
 a chloride plate, use a chloride paper. Rece ' 
 researches of mine have shown that the darke j 
 ing intensity and the developing intensity 
 hand in hand; therefore, it will be found th 
 when you have the number which gives the rig 1 
 
tint, you may always be sure of getting the 
 right exposure. If a tint require two minutes 
 to obtain, which corresponds with an exposure 
 of a plate of two seconds, and you get that tint, 
 and give two seconds, you will not be wrong. 
 If it .’requires half a minute to obtain it, give 
 the plate half a second’s exposure, and you 
 will be right. This is as useful an apparatus 
 as you can have for the field. I do not know 
 about the studio, but in the field some such 
 device as this is almost a necessity. 
 
 I want to show you the theoretical as well 
 as the practical necessity of using a photo- 
 
 Fig. 9. 
 
 UlxrouViolet Violet Blue Green* Yellow Red/Dark; 
 
 H G F E D C A 
 
 Jill 
 
 Mg 
 
 
 
 
 
 m 
 
 1 
 
 
 iimli 
 
 r 
 
 
 
 
 
 Iodjde, 
 
 * ^ iL 
 
 ik 
 
 
 
 
 
 
 Chloride 
 
 
 ggHll 
 
 *=51 
 
 
 
 
 
 BroTTude 
 
 p ^ mSSli 
 
 
 
 
 
 
 
 hlodtde 
 h Chloride 
 
 
 __ 
 
 
 
 
 
 
 hlodidLe, 
 
 '/iBromicU 
 
 The, KorVwniaJb }ux/ehing represents the 
 
 the ulxrw rioLet/ Ught is deficient The* standard/ 
 light is represented by Ova whole of One, shaded/ 
 figures 
 
 meter or actinometer of some description. Here 
 we have these little mountains which signify 
 the intensity of ’different parts of the spec¬ 
 trum. There[are some bright days in summer, 
 the light of which, when spread out in a 
 spectrum, let us represent by a rectangle (I.), 
 that is to say, they are all of equal intensity. 
 I do not say they are, but you can represent 
 them as such. There are other days in 
 autumn or winter in which the ultra - violet 
 will very much decrease. If you judge the 
 xposure by the eye, you may be entirely 
 wrong, and if you are using an iodide plate, 
 pou may only give one-third of the exposure 
 /ou ought to give; for chloride only one- 
 burth ; for bromide about one-half. If you 
 ire using one-half iodide and one-half bro¬ 
 mide, you would give only two-thirds of what 
 i'ou ought to give, so that the eye is no exact 
 udge of the exposure you ought to give; it 
 ^epends on the amount of rays which exist in 
 he ultra-violet, and that can only be told by 
 1 photometer of some description. 
 
 The next thing I propose to show you is with 
 regard to drop shutters. I could talk to you 
 for a whole evening on drop shutters, and 
 even then I should not have finished, for 
 there is a great deal in the philosophy of 
 a drop shutter of which people have not 
 dreamed. I am sure if I were to enter 
 into minutiae, I should carry you beyond 
 any endurable limits of time. I therefore, 
 propose to show you one very easy plan 
 of knowing what your drop shutter can 
 do for you. We often hear said, “Oh, 
 I exposed that plate with a drop shutter,’’ 
 and if the exclaimer is asked what length of 
 exposure that was, he will probably confess 
 that he does not know, or will say anything 
 from 1 -5th to i-5oth of a second. Now that is 
 a very untidy way of photographing. It is 
 decidedly misleading to say i-5th of a second 
 when it may be i-2oth. I will show you a way 
 in which you can tell whether it is i-2oth 
 or 1-5th of a second of exposure you may have 
 given with a drop shutter. I have here 
 a monster lens, with which, had I time, I was 
 going to show you some tricks. It is not very 
 valuable; I think it cost 6s. In front of this 
 gigantic lens I have a gigantic drop shutter, 
 and attached to the sides of that drop shutter 
 there is a tuning-fork which cost is. I claim 
 no monopoly. You take a common iron clamp, 
 and attach it to the side of the shutter. Then 
 you blacken a piece of albumenised paper by 
 the side, and make the tuning fork vibrate. 
 As the shutter drops, it traces its own vibra¬ 
 tions, and tells you how many it has per¬ 
 formed during the fall of the shutter. You 
 know that an E tuning-fork vibrates so many 
 times per second, and by simple measurement 
 
 Fig. 10. 
 
 you can tell at what speed your shutter is going 
 at any part of its path. I dare say we shall 
 be able to see that the tuning-fork leaves its 
 mark on the smoked glass, which I have sub¬ 
 stituted for the albumenised paper above 
 described. Here is another trace (II) made 
 when gravity was aided by an elastic band. 
 [The shutter fell, and the diagram was shown 
 on the screen.] 
 
 I find my time is more than up ; but before I 
 close I s uld like to show you one or two 
 
3* 
 
 curiosities, as regards the scientific application 
 of photography, and these are some composite 
 photographs which were kindly lent to me by 
 Mr. Galton, showing you what aid photo¬ 
 graphy can give to anthropology. Here is a 
 typical family composite portrait, composed of 
 a mother and two daughters, all three faces 
 being blended together. We thus get a like¬ 
 ness of the female branch of the family ; then 
 we have the father and mother, two sisters, 
 and two brothers, giving the typical family 
 group. Here we have another image, which is 
 a typical group of engineer officers, and I am 
 glad to see that they are so good looking. 
 Here is one more, which is a typical group of 
 sappers. I wish I had more time to go into 
 the subject. 
 
 I cannot touch on printing processes at this 
 late period of the evening, and therefore, I 
 must ask your permission to close. I feel that 
 what I have done has been very imperfect, but 
 I have done it to the best of my ability, so far 
 as time would permit. I may have done things 
 I ought not to have done, but I have certainly 
 left undone a great many things which I ought 
 to have done, but I hope for all my short¬ 
 comings you will find some excuse. That 
 this course of lectures has caused a certain 
 amount of interest, is evinced by the continued 
 large audiences I have seen before me, and I 
 hope some abler lecturer, at some future time, ! 
 will be able to complete what I feel I have 
 only commenced to do, viz., give a resume of 
 recent advances in photography. 
 
 $4 
 
 hinted by w. trounce, io, gough-square, fleet-street, London, tv.c. 
 

 
 
 
 
 

 
 
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