Sept. 19, 1878] 



NATURE 



545 



out a large field for inquiry, and made research in this 

 direction doubly interesting owing to the fact that appa- 

 rently certain physical laws would have to be modified if 

 VogeFs theory were correct. He divided the action of 

 the substances so added into two, the dye he called an 

 optical sensitiser for that particular part of the spectrum 

 which it absorbed, whilst bodies which absorbed the 

 halogen (thrown off by the reduction of the molecule) he 

 •called a chemical sensitiser, and a combination of both 

 properties in a dye made the film sensitive to the absorbed 

 j-ays. The theory of the optical sensitiser seemed to 

 clash with the received notion of molecular motion, but 

 before analysing the results the accompanying figure 

 should be studied, which is taken from Vogel' s work on 

 Photography (Fig, 5). 



Let us take one or two examples from the above figures 

 and see whether they agree with Vogel's assumption. 

 We will take VIII. as a standard of comparison, being 

 the effect on unstained bromide, and this will be fair 

 (though it does not take the form given, shown in Fig. 2, 

 p. 529), as it is presumed that this sample of bromide 

 <was worked with throughout. Comparing say IV. with 

 VIII. we see that in the blue the sensitivenesses, as 



shown by the ordinate of the curved line, are very 

 similar, but that the action is got in the yellow. In 

 examining cyanine blue, the dye used, we find that the 

 absorption takes place just at that part of the spectrum. 

 Similarly examining V. and VI. we an'ive at the same 

 results, and in fact the absorption of the rays invariably 

 corresponds with the photographic action. 



It will be seen then that without doubt the principle 

 Vogel contends for might explain the phenomena. He, 

 however, found that if the silver bromide film had been 

 prepared with an excess of bromide, that the actions 

 indicated did not take place. This seemed to indicate 

 a weak spot in the theory, and it pointed at first sight to 

 the idea that it was necessary to form a coloured compound 

 of the dye with silver, in order to render it sensitive. In 

 the majority of cases this still seems to be, if not a neces- 

 sity, yet a cause of increase of sensitiveness to the region 

 of the spectrum absorbed. Vogel, however, shows that, 

 if the silver bromide film, prepared with an excess of 

 bromide, be washed, and be then treated with a dye, and a 

 chemical sensitiser, such as tannin, that the same action 

 takes place. The theory of a silver compound in this 

 case must evidently be abandoned, and would point to 



cBa, A 



JJffect of the salar spectrum on silver 

 bromide stained with : — 



Naphthaline red. 



VIII. EiTect on unstained silver bromide. 



Fig. 



The curves marked a ihiv/ longer exposure than those mark;d b. 



the correctness of Vogel's theory, did not other experi- 

 ments in^ a certain degree offer an explanation more 

 accordant with our preconceived ideas. Let the dye be 

 mixed with plain collodion and a film of it be exposed 

 to the spectrum, it will soon be visibly evident that 

 there is a marked effect produced by the rays ab- 

 sorbed. Thus cyanine blue will be found bleached in 

 the region near D and below it. If over the dyed film 

 of collodion thus exposed, a film of silver bromide in 

 collodion be poured in the dark, and the alkaline 

 developer be applied gradually, a silver image of the 

 altered portion of the dyed collodion film will make its 

 appearance, although the film of silver bromide has re- 

 ceived no impression by light. The explanation of this 

 remarkable phenomenon is to be found in the theory 

 of alkaline development already given in these articles. 

 The reduced dye acts as a nucleus on which the metallic 

 silver will first adhere, and this first reduction of metallic 

 silver determines the position which the further reduced 

 silver shall occupy, and thus the image is built up. 

 "When there is an excess of soluble bromide in the film 

 the developing action will be retarded. This series 



of experiments seems, then, to indicate that there is 

 no need for optical sensitisers to alter the oscillation 

 of the molecules or molecular groups, but that, in some 

 instances, the same theory may be applied to the action 

 of dyes as may be applied to the deposition of metallic 

 silver on a glass plate which has not been freed from 

 "dirt" a disagreeable phase which is well known to 

 photographers al large. The explanation of the diagram 

 is, therefore, not hard to understand when viewed m this 

 light The theory of rendering the silver salt sensitive 

 to the red has already been explained, and the sanie 

 explanation is naturally applicable to those dyes which, 

 when brought in contact with silver, form a definite 



compound with it. , . , ■. i_- . 



There are many interesting physical researches which 

 spring out of these various experiments, amongst which 

 may be mentioned a method of determining the size of 

 atoms and their arrangement in the molecule, and last 

 and not least, the production of permanent photographs 

 painted in natural colours by light itself. The attempts 

 made of late to form photographs in proper colours by 

 takin<» distinct negatives through blue, through green, and 



