130 ANNUAL OF SCIENTIFIC DISCOVERT. 



the dense portions of the negative, which it has contributed to 

 form. 



"To settle this point, I exposed and developed an iodo-brom- 

 ized plate in the ordinary manner. Tlien, instead of removing 

 the unchanged iodid and bromitl by fixing in tlie ordinary nianii(;r, 

 I tooic measures to remove the developed image witiiont allecting 

 th(! iodid and bromid. This I succeeded in doing with the aid of 

 a very weak sohitiun uf acid per-nitrale uf mercury. Now, if the 

 iodid, or bromid, or both, had been in any way decomposed, to 

 form or aid in forming the developed negative image, when this 

 came to be removed, there should have been left a more or less 

 distinct positive image, depending upon varjing thicknesses of 

 iodid ami bromid in the film, nuich like a fixed negative that has 

 been completely iodized. Nothing of this sort was visible ; the film 

 was perfectly uniform, just as dense where an intense light had 

 been, as in tliose parts whicii had scanvdy received any actinic 

 impression, and looking exa(;t]y as it diil when it first left the 

 camera, and before any developer had In^cn applied. This ex- 

 periment seems sufficiently decisive. But the following is far 

 stronger. 



" Experiment 2. — A plate was treated in all respects as in No. 1, 

 except that the application of the nitrate of Jiiercury for removing 

 tiie developed image was made by yellow light. The plate, now 

 showing nothing but a uniform yellow film, was carefully washed, 

 and an iron developer, to which nitrate of silver and citric acid 

 had been adiled, was applied. In this way the original image 

 was reproduced, and came out quite clearly with all its details. 

 Now, as every trace of a picture and all reduced silver had Vjeen 

 removed by the nitrate of mercury, it is by this experiment 

 absolutely demonstrated that the image is a purely physical one ; 

 and that, after having served to produce one picture, that picture 

 may be dissolved oft', and the same physical impression may be 

 made to produce a second picture by a simple application of a 

 developing agent. 



"I have repeated the experiment with a pyrogallic develop- 

 ment with similar results. Both the first and second develop- 

 ments may be made with an iron developer, or both with a 

 pyrogallic' The experiment succeeds without the least difficulty 

 in either way." 



The same author, in " Silliman's Journal " for September, 1866, 

 concludes a paper on this subject, as follows : " I have endeavoi'cd 

 to show that the action of light upon pure iodide of silver isolated 

 cannot be a chemical reduction: 1. Because that effect, even 

 when carried many hundred thousand times further than in the 

 ordinai-y photographic processes, perfectly disappears in a few 

 hours, spontaneously, under circumstances which render it im- 

 possible to suppose that iodine could have been restored to replace 

 that which (had reduction taken place) must have been disen- 

 gaged. 2. Because, even where the action of light is prolonged 

 many hundred thousand fold the ordinary time, no reduced silver 

 nor sub-iodid can be detected as present. 3. I have shown that 

 another metal, mercury, is capable of developing these images 



