490 



NATURE 



{Sept 5, 1878 



who gave their attention to h, many important problems 

 in photography were solved. Succeeding these years, 

 however, were others in which little was done in the 

 absolute science of the subject, though great progress 

 was made in perfecting the processes Avhich had been 

 brought forward. Within the last few years a fresh start 

 in research in all directions seems to have been made, 

 and much that is valuable in elucidating the correct 

 theories on which photography is based has been demon- 

 strated, and it is to this to which attention will be drawn. 

 With the risk of being tedious, ground which has 

 been well trodden must once again be briefly gone over, 

 in order to estimate the progress which more recently has 

 been made. Scheele, the Swedish chemist, as is well 

 known, found that the blackening of silver chloride (which 

 was the basis of Talbot's pictures) gave up chlorine on 

 exposure to light, thus proving, as it were, that the 

 blackening was due to the formation of a new che- 

 mical compound. As far as can be traced not much 

 more M'as known regarding this compound ; but it was a 

 generally-received notion that it was a subchloride of 

 silver ; and up to the present time we find that such is 

 the accepted opinion. In the second edition of 

 Hunt's "Researches on Light," published in 1854, at p. 

 79, a remarkable experiment is noted. He says : — 

 " The exposure (of silver chloride) in the water was, in 

 another case, continued for several days, but no greater 

 degree of darkening occurred; but a curious fact was 

 noticed. It was found that during the night nearly all 

 the chlorine which had been liberated during the day 

 \yas recombined, and that the darkened powder became 

 lighter" .... He then, after recounting other expe- 

 riments, says (p. 123) :— " From other experiments I am 

 inclined to believe that the first action of the solar ray is 

 to liberate one half of the combined chlorine, which is very 

 readily, moisture being present, replaced by oxygen. By 

 the continued action of the exciting cause the oxide is 

 decomposed, and metalhc silver in a fine state of divi- 

 sion, is formed over the surface" (of the paper). 



P. 125 :— "The absorptionof oxygen, or rather its combi- 

 nation, with the decomposing chloride is proved by another 

 very easy experiment. Some pure chloride of silver was 

 arranged in a bent tube closed at one end, and the 

 other end immersed in a bottle of distilled water. In 

 this state the chloride was exposed for many days to the 

 action of sunshine, during which time it was frequently 

 shaken for the purpose of exposing the whole of the 

 powder and its influence. As the chloride darkened, the 

 water rose in the tube, and it gave a precipitate of chlo- 

 ride of silver on the addition of the nitrate, thus appear- 

 ing to prove the substitution of oxygen for chlorine under 

 the agency of solar radiation. It was quite evident that 

 some absorption of atmospheric air had taken place. 

 This explanation will also serve for the iodide, bromide, 

 and some other salts of this metal (silver)." 



This last experiment has lain fallow for years, and 

 it is only recently that it has had any meaning beyond that 

 indicated in the quoted paragraph. It must be borne in 

 mind, however, that the visible change in the chloride is 

 here under consideration, and that the invisible effect of 

 light was not mentioned. 



With regard to the developable and invisible image, 

 till within the last few years, it was a debatable point as 

 to whether the action of light on a sensitive compound 

 was really a chemical change, or simply a physical action ; 

 one school held that the sensitive compound was not 

 altered in composition at all, but that in some mysterious 

 manner the atoms of the molecules composing it were 

 shifted, and possessed a new property which was denied 

 to it in its original form. Diagrams were introduced to 

 render this subtle change clear to the student, ene of 

 which is reproduced (Fig. i). 



In A we see two ovals slightly differing in size, each of 

 which was intended to indicate one of the atoms com- 



posing a molecule of the sensitive salt. When the ovals 

 coincided, the molecule was supposed to be in the ordinary 

 state, but after light acted upon it for a certain time the 

 ovals occupied the positions shown in B, and after a 

 further action of light they occupied the positions shown 

 at c, in which it again became incapable of proper deve- 

 lopment, and gave rise to what was known as solarisa- 

 iion, the part of the " latent image " formed by these 

 solarised molecules refusing to develop. By solarisa- 

 tion was meant the phenomenon which occurred (more 

 especially if silver films containing iodide were used), when 

 any portion of the plate received a lengthened exposure to 

 any very bright part of the lenticular image, such as to 

 that of the sky. In solarisation we have have a term 

 which is as unmeaning as is "polarisation" in some of 

 its applications, but since it has passed into the tech- 

 nical language of photography, we are bound to em- 

 ploy it. By the term latent image was meant the in- 

 visible (and usually) developable image impressed upon a 

 sensitive film, and it will be used, where convenient, with 

 the reservation once for all, that its applicability is not 

 admitted any more than is the term "developer," as 

 applied to a solution which may cause the deposition of 

 metallic silver from a solution of silver nitrate ; since such 

 a solution is effective whether applied to an exposed sensi- 

 tive film or not. The advance of photography has literally 

 been impeded from the neglect of using accurate language. 

 As regards this peculiar condition which the molecule was 

 supposed to have attained after its impact with light, there 

 seems to be no ground for its adoption. The idea seem- 

 ingly arose from a supposed necessity which existed for 

 a difference in condition between the visible and the 



merely developable image. By a strictly logical inference 

 there need be no difference between the two beyond this 

 that there should be a difference in the number of molecules 

 absolutely altered, and in no other respect. Perhaps the 

 most telling experiment giving direct evidence of the simi- 

 larity of the two images was that made by Poitevin, in 

 which he proved the dissociation of iodine from silver 

 iodide, by placing metalhc silver in contact with the film. 

 After exposure to light, on separating the two, he found 

 that the latter had absorbed iodine, as proved by treating 

 it with mercury vapour. The circinnstantial evidence of 

 the truth of the chemical theory of the invisible image, 

 however, is so strong, that on that alone we are bound 

 to accept it, at the same time we are not prepared to say 

 that there are not other physical forces which must play a 

 part in its development ; in fact, it must be so. We may 

 say, then, that at \t^ first formation the developable photo- 

 graphic image is formed by the reduction of the sensi- 

 tive compound to one of a less complex nature. Thus, 

 silver chloride (argentic chloride) is reduced to silver 

 sub-chloride (argentous chloride) with the liberation of 

 chlorine ; and silver bromide to silver sub-bromide with 

 the liberation of bromine ; and so on. 



We must now allude to the development of the photo- 

 graphic image. We may divide the methods of develop- 

 ment of the image on silver compounds into three : (i) 

 j The condensation of mercury; (2) the deposit of metallic 

 silver from a soluble salt of silver by means of a reducing 

 agent such as ferrous sulphate ; and (3) the reduction of 

 the sensitive salt of silver itself to form the image. 



The first method is the earliest, dating from the dis- 

 covery of the Daguerrotype process, and till within very 

 recent times the reason of its efficacy has been a subject 

 of controversy. Quincke has lately thrown a light upon 



