382 THE PHOTOGRAPHIC IMAGE. 



o£f under these circumstances is either the free halogen or an oxide or 

 acid of the halogen, according to the quantity of moisture present and 

 the intensity of the light. I have found that the bromide affects the 

 iodide and starch paper in the same way, but silver iodide does not give 

 oft' any gas which colors the test paper. All the silver haloids become 

 colored on exposure to light, the change being most marked in the 

 chloride, less in the 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 differ- 

 ent degrees of coloration in the three cases must not be considered as a 

 measure of the relative sensitiveness ; it simply means that the prod- 

 ucts of photo-chemical change in the three haloids are inherently pos- 

 sessed of different depths of color. 



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

 we are concerned with photochemical decomposition, and not with a 

 physical change only. All the evidence is in favor of this view. Halo- 

 gen absorbents, such as silver nitrate on the lower halves of the papers 

 in the last experiment, organic matter, such as the gelatine in an emul- 

 sion, and reducing agents generally, all accelerate the chauge of color. 

 Oxidizing and halogenizing agents, such as mercuric chloride, potas- 

 sium dichromate, etc., all retard the color change. [Silver chloride 

 paper, painted with stripes of solutions of sodium sulphite, mercuric 

 chloride, and potassium dichromat^^, 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 doing chemical work; the vibratory energy is 

 partly spent 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 film. It is difficult to demonstrate this sat- 

 isfactorily 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 be- 

 tween absorption and decomposition. It is well known also that the 

 more refrangible rays are the most active in promoting the decomposi- 

 tion 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 colors on these ha- 

 loids, as this is a subject upon 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 par- 

 ticular rays that the blue end of the spectrum is most active in pro- 

 moting their decomposition. Many other instances of photo-chemical 

 decomposition are known in which the less refrangible rays are the most 



