the Developed Photographic Image. 207 



red and in the blue, a portion of the spectrum in the green 

 and yellow remaining unreversed (fig. 6). Now the action of 

 these acids is not a strictly oxidizing action, but is probably a 

 removal of the loose atoms of the silver which goes to form the 

 subiodide, and leaving silver iodide behind as the result of the 

 action. The results of the action of acids do not, therefore, 

 vitiate the above deduction. A plate exposed in benzene or in 

 nitrite of potash showed no reversal even with a very prolonged 

 exposure. It should be remarked that the action of perman- 

 ganate and bichromate of potash when very feeble is sometimes 

 to give feeble negative images in the red and blue in lieu of 

 positive images ; also positive images in the blue, and feeble 

 negative images in the red (see dotted curves in figs. 4 and 5). 

 But this is to be accounted for by the fact that the dilution 

 of these oxidizing agents is so extreme that the reducing action 

 on the unaltered sensitive salt is far greater than the rapidity 

 of the oxidation*. 



Ordinary bromide of silver in collodion or gelatine may be 

 taken as giving almost identical results under the influence of 

 a soluble bromide f. Now ordinary bromide is sensitive as far 

 as B (see clotted curve, Pl.VI. fig. 7); and it might be presumed 

 that this sensitiveness to the rays of lower refrangibility would 

 cause a modification in the action of the soluble bromide. A 

 reference to figures 7 and 8 will show that this is the case, 

 but that at the same time the features which are so marked with 

 the action of bromide on silver iodide are present. Fig. 7 is the 

 curves clue to silver bromide in collodion which had received a 

 preliminary exposure and was then exposed in a 5-per-cent. 

 solution of acid potassium bromide. It will be seen that the 

 curves in figures 7 and 2 are similar, showing that the principal 

 action is due to light acting on the soluble bromide in the 

 presence of an acid. Fig. 8 is a similar plate exposed in an 

 alkaline solution of KBr, in which there is a modification of the 

 curve. The last loop is probably clue to the silver subbromide 

 itself, since the oxidation of this salt by oxidizing agents oc- 

 cupies approximately the same position (see fig. 10). 



Fig. i) shows the effect of permanganate of potash ; and 

 when it is compared with fig. 10, which is the curve due to 

 oxidation by bichromate of potash, it will be manifest that the 



* There is one singular fact to be noted in tins, and which I propose to 

 treat of in another contribution, viz. that the iodide of silver, when given 

 a preliminary exposure, is sensitive in a region of the spectrum lying- be- 

 tween a point near D and one near A. This phenomenon has been described 

 before and not explained, though experiments show that the explanation 

 is easy. 



t As before explained, it is useless to expose such plates in a solution 

 of soluble iodide, since silver iodide is immediately formed. 



