1890.] on the Photographic Image. 139 



in the cliloride, less in tlie bromide, and least of all in the iodide. 

 The latter must be associated with some halogen absorbent to render 

 the change visible. [Strips of paper coated with the pure haloids, 

 the lower halves brushed over with silver nitrate solution, were 

 exposed.] The different degrees of coloration in the three cases 

 must not be considered as a measure of the relative sensitiveness : it 

 simply means that tlie products of photo-chemical change in the three 

 haloids are inherently possessed of different depths of colour. 



From the fact that halogen in some form is given off, it follows 

 that we are concerned with photo-chemical decomposition, and not 

 with a physical change only. All the evidence is in favour of this 

 view. Halogen absorbents, such as silver nitrate on the lower halves 

 of the papers in the last experiment, organic matter, such as the 

 gelatine in an emulsion, and reducing agents generally, all accelerate 

 the change of colour. Oxidizing and halogenizing agents, such as 

 mercuric chloride, potassium dichromate, &c., all retard the colour 

 change. [Silver chloride paper, painted with stripes of solutions of 

 sodium sulphite, mercuric chloride, and potassium dichromate, was 

 exposed.] It is impossible to account for the action of these chemical 

 agents except on the view of chemical decomposition. The ray of 

 light falling upon a silver haloid must be regarded as doiug chemical 

 work ; the vibratory energy is partly sj)ent in doing the work of 

 chemical separation, and the light passes through a film of such 

 haloid partly robbed of its power of doing similar work upon a 

 second tilm. It is difficult to demonstrate this satisfactorily in the 

 lecture-room, on account of the opacity of the silver haloids, but the 

 work of Sir John Herschel, J. W. Draper, and others, has put it 

 beyond doubt that there is a relationship of this kind between 

 absorption and decomposition. It is well known, also, that the more 

 refrangible rays are the most active in promoting the decomposition 

 in the case of the silver haloids. This was first proved for the 

 chloride by Scheele, and is now known to be true for the other 

 haloids. It would be presumption on my part, in the presence of 

 Captain Abney, to enlarge upon the effects of the different spectral 

 colours on these haloids, as this is a subject uj)on which he can speak 

 with the authority of an investigator. It only remains to add that 

 the old idea of a special " actinic " force at the more refrangible end 

 of the spectrum has long been abandoned. It is only because the 

 silver haloids absorb these particular rays that the blue end of the 

 spectrum is most active in promoting their decomposition. Mauy 

 other instances of photo-chemical decomposition are known in which 

 the less refrangible rays are the most active, and it is possible to 

 modify the silver haloids themselves so as to make them sensitive 

 for the red end of the spectrum. 



The chemical nature of the coloured products of photo-chemical 

 decomposition is still enshrouded in mystery. Beyond the fact that 

 they contain less halogen than the normal salt, we are not much in 

 advance of the knowledge bequeathed to us by Scheele in the last 



