544 



NATURE 



[Sept. 19, 1878 



of Hunt's " Handbook of Photography," we read, at p. 

 161, " Niepce de St. Victor has made many experiments 

 to produce the colours upon salts of silver and copper 

 spread upon paper, but without success; the metallic 

 plate appears absolutely necessary, and the purer the 

 silver the more perfect and intense is the impression." 

 The following is recommended as the most effectual 

 mode of manipulating ; — " The plate is highly polished 

 with tripoli powder and ammonia ; being perfectly 

 cleaned, it is connected with the battery and plunged 

 into the bath prepared in any of the ways stated. [The 

 baths were made from ferric chloride, cupric chloride, 

 hydrochloric acid, &c.] It is allowed to remain in the 

 bath for some minutes, taken from it, washed in a large 

 quantijiy of water, and dried over a spirit-lamp. The 

 surface thus produced is of a dull neutral tint, often 

 almost black ; the sensibility of the plate appears to be 

 increased by the action 0/ heat, and, when brought by the 

 spirit lamp to the cerise red, it is in its most sensitive 

 state." 



" The sensibility, however, of the plates is low — two or 

 .three hottrs being required to produce a decided effect in 

 Ihe camera obscura. . . . These, when I first saw them, 

 were perfectly coloured in correspondence with the 

 rdrawings of which they were copies, but the colours soon 

 jfaded, and it does not appear as yet that any successful 

 mode of fixing the colours has been discovered." The 

 ■coloured spectra which Becquerel photographed were 

 produced in a somewhat similar way, the variation from 

 which need scarcely be repeated. 



In Hunt's work we also find that natural colouration of 

 photographs was found to be possible by one or two other 

 processes, but that the above gave the most satisfactory 

 results. Mr. Simpson also noticed when using an emul- 

 sion of silver chloride and after exposing the film to white 

 Jight so as to tint the surface with a lavender colour, that 

 he was able to reproduce on the film the tint of diffe- 

 rent coloured glasses to which such a surface might be 

 exposed. - 



It will be noticed that the coloured spectra were produced 

 on a dark compound of silver which gradually responded 

 the colour faUing on it. We have first a case of total 

 or nearly total absorption of all the rays, and a subsequent 

 production of compounds of varying tints. In order to 

 produce any variations of colour it is only necessary that 

 we should have at the most three molecular groupings, 

 one of which should absorb the blue and green, another 

 the green and red, and the last the red and blue. 

 Whether the number of groupings may be reduced to 

 two is a question for future consideration. In Lockyer's 

 note read before the Royal Society on June 11, 1874, 

 "On the Evidence of Variation in Molecular Structure," 

 we find statements which might have been conceived 

 to be almost too bold at the time when they were 

 made, but which subsequent investigations seem to prove 

 to be exact. In this note he refers to definite mole- 

 cular groupmgs of compounds and the absorption caused 

 by them, and indicates that we may have a group 

 which will absorb at the blue end and another which will 

 absorb the red end of the visible spectrum. It has already 

 been shown that the silver bromide can be reduced to two 

 groupings, one absorbing the blue and the other the red, 

 and it is somewhat remarkable that, by applying pressure 

 to the latter molecular grouping, it is gradually resolved 

 into the former grouping, and passes through all tints of 

 spectrum between the blue and the i-ed. It must bei 

 remembered that these colours are not the colours of thin 

 plates, but are totally independent of the thickness of 

 the film so long as light can penetrate through it. It 

 is not too much to assume that if silver bromide can be 

 made to group itself into these two states, that the sub- 

 bromide when oxidized should also assume a similar 

 molecular condition. With this compound in a state 

 which practically absorbs all rays, it is easy to imagine 



that particular sets of vibrations may cause it to resoh^e 

 itself into groupings which answer to them. We have, in 

 fact, the inverse of the reduction of the silver bromide by 

 different portions of the spectrum. It is found that one 

 molecular grouping can be reduced by a whole series of 

 vibrations ; thus the blue absorbing molecular group is 

 altered by all the radiations from the ultra violet to the 

 yellow, and the red absorbing molecular group by the radia- 

 tions from the ultra-red to the green. If there were a green 

 absorbing molecular group, of which there is a strong 

 suspicion of the existence, it would probably be altered 

 by radiations from the blue to the orange. If, then, 

 one silver compound can exist in two or three states of 

 molecular grouping, it is quite within the range of 

 reason that the oxidised compound should exist in the 

 same three groupings. The black compound to which 

 we have already referred, in fact, does arrange itself thus, 

 probably by a re-arrangement of molecules, as formed 

 when it absorbs oxygen. If a plate be prepared in a 

 similar manner to that described above, and if it be 

 exposed in an oxidising medium, these groupings are 

 attained rapidly, a few minutes sufficing where previously 

 hours were required. The images thus formed, however, 

 appear not to be unchangeable, as exposure to white 

 light, or to any colour except that in which the re-arrange- 

 ment takes place causes the colours to fade. The feat of 

 producing permattent photographs in natural colours is 

 as yet unsolved, but it may not be so far distant as may 

 be imagined. In order to obtain them it is necessary 

 that a method should be found by which the molecular 

 groupings of metallic silver can be formed in either of 

 the two (or three) states already described. As is well 

 known, the absorption by metallic silver in a thin film 

 takes place entirely in the red end of the spectrum, but it is 

 a fact well known to photographers at large, that in certain 

 processes it is perfectly feasible to obtain silver in which 

 the transmitted light is of a pink red colour, whilst 

 tints varying from indigo, passing through olive green to 

 rich brown are familiar. In order to obtain permanent 

 photographs in natural colours, the object to be sought is 

 a method by which the sensitive silver compound may be 

 reduced by the red rays to a molecular grouping, which on 

 development (probably by the alkaline method) shall be 

 grouped into the red transmitting molecular grouping, and 

 so on. When this is discovered, the leap between mono- 

 chromatic pictures, and chromatic, will have been taken, 

 and the once apparent improbability have become more 

 than a possibility. 



We have finally to return to the subject of photo- 

 graphy with the light of those rays which are usually 

 inactive upon sensitive salts, and at which we have 

 already glanced. 



To Dr. H. Vogel, of Berlin, is undoubtedly du^ 

 the new interest which lias been taken in this branch 

 of photography. Towards the end of the year 1873 

 he announced that he had discovered a method of 

 making the non-actinic rays in certain circumstances 

 actinic. We quote his own words ^ : — " I have found that 

 bodies which absorb the yellow ray of the spectrum make 

 bromide of silver sensitive to the yellow ray. In like 

 manner I find bodies which absorb the red ray of the 

 spectrum make bromide of silver sensitive to the red 

 rays. For example, by the' addition of corallm—viYiich. 

 absorbs the yellow ray— to a bromide of silver film, it 

 becomes as sensitive to the yellow ray as to the blue ray." 

 In articles which he published at various times he en- 

 larged on this idea, some of his most striking experirnents 

 being conducted with aniline dyes of various kinds. 

 He and Waterhouse have shown that a silver bromide 

 film becomes sensitive to the part of the spectrum which 

 certain of those dyes absorb, whether the absorption be 

 due to a compound formed between the dye and silver, 

 or to aqueous or alcoholic solutions. This at once opened 



' Photographic Kezvs, December 5, 1873. 



