THE PHOTOGRAPHIC IMAGE. 385 



tinctly described. Carey Lea has since contributed largely to our 

 knowledge of these colored haloids, and has made it appear at least 

 highly probable that they are related to the products formed by the 

 action of light. [Red photo-chloride and purple photo-bromide and 

 iodide shown.] 



The photographic image is impressed on a modern film in an inap- 

 preciable fraction of a second, whereas the photo-salt requires an ap- 

 preciable time for its production. The image is invisible simply be- 

 cause of the extremely minute quantity of haloid decomposed. In the 

 present state of knowledge it can not be asserted that the material com- 

 posing this image is identical in composition with the photo-salt, for we 

 know the composition of neither the one uor the other. But they are 

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

 composition, and there is great probability that they are closely related, 

 if notidentical, chemically. Itmay turnout that thereare various kinds 

 of invisible images, according to the vehicle or halogen absorbent — in 

 other words, according to the sensitizer with which the silver haloid is 

 associated. The invisible image is revealed by the action of the de- 

 veloper, 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 mag- 

 nify the deposit of photo salt 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 favor of the chemical theory appear to me to be tol- 

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



The action of reagents upon the photographic film is quite similar 

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

 posed to thei)oint of visible coloration. Reducing agents and halogen 

 absorbents increase the sensitiveness of the film: oxidizing and halo- 

 genizing agents destroy its sensitiveness. It is difficult to see on the 

 physical theory why it should not be possible to impress an image on 

 a film, say of pure silver bromide, as readily as on a film of the same 

 haloid imbedded in gelatine. Everyone knows that this can not be 

 done. I have myself been surprised at the extreme insensitiveness of 

 films of pure bromide prepared by exposing films of silver deposited 

 on glass to the action of bromine vapor. On the chemical theory we 



very pale red body on being transformed by chlorhydic and nitric acids." In another 

 experiment silver arsenite was formed, this being treated with caustic soda, and the 

 black precipitate then treatefl snccessivelj' with chlorhydic and nitric acids: "Silver 

 is dissolved, and there is left a substance - - - [of] a rich chocolate or maroon, 

 etc." This on analysis was found to contain 24 per cent, of chlorine, the normal 

 chloride requiring 24.74 and the subchloride 14.08 percent. The committee which 

 conducted these experiments consisted of Messrs. Maskelyne, Hadow, Hardwick, 

 and Llewelyn. B. A. Rep., 1859, p. 103. 



H. Mis. 129 25 



