142 Professor Raphael Meldola [May 16, 



the products formed by the action of light. [Red photo-chloride and 

 purple photobromide and iodide shown.] 



The photographic image is impressed on a modern film in an 

 inappreciable fraction of a second, whereas the photosalt requires an 

 appreciable time for its production. The image is invisible simply 

 because of the extremely minute quantity of haloid decomposed. In 

 the present state of knowledge it cannot be asserted that the material 

 composing this image is identical in composition with the photosalt, 

 for we know the composition of neither the one nor the other. But 

 they are analogous in so far as they are both the result of photo- 

 chemical decomposition, and there is great probability that they are 

 closely related, if not identical, chemically. It may turn out that 

 there are various kinds of invisible images, according to the vehicle 

 or halogen absorbent — in other words, according to the sensitiser 

 with which the silver haloid is associated. The invisible image is 

 revealed by the action of the developer, into the function of which I 

 do not propose to enter. It will suffice to say that the final result 

 of the developing solution is to magnify the deposit of j^hotosalt by 

 accumulating metallic silver thereon by accretion or reduction. 

 Owing to the circumstance that the image is impressed with such 

 remarkable rapidity, and that it is invisible when formed, it has been 

 maintained, and is still held by many, that the first action of light 

 on the film is molecular or physical, and not chemical. The 

 arguments in favour of the chemical theory appear to me to be 

 tolerably conclusive, and I will venture to submit a few of them. 



The action of reagents upon the j^hotographic fihii is quite similar 

 to the action of the same reagents upon the silver haloids when 

 exposed to the point of visible coloration. Reducing agents and 

 halogen absorbents increase the sensitiveness of the film : oxidising 

 and halogenising agents destroy its sensitiveness. It is difficult to 

 see on the physical theory why it should not be j)ossible to impress 

 an image on a film, say of pure silver bromide, as readily as on a film 

 of the same haloid embedded in gelatine. Every one knows that 

 this cannot be done. I liave myself been sur^Drised at the extreme 

 insensitiveness of films of pure bromide prepared by exposing films 

 of silver deposited on glass to the action of bromine vapour. On the 

 chemical theory we know that gelatine is a splendid sensitiser — i. e. 

 bromine absorbent. There is another proof which has been in our 

 hands for nearly thirty years, but I do not think it has been viewed 

 in this light before. It has been shown by Carey Lea, Eder, 

 and especially by Abney — who has investigated the matter most 



clilorliydric and nitric acids : " Silver is dissolved, and there is left a substance 

 .... [of] a rich chocolate or maroon, &c. " This on analysis was found to con- 

 tain 24 per cent, of chlorine, the normal chloride requirin^^ 24*74 and the sub- 

 chloride 14 'OS per cent. The committee which conducted these experiments 

 consisted of Messrs. IMaskelyne, Hadow, Hardwich, and Llewelyn. ' B. A. Rep.,' 

 185!), p. 108. 



