PhotograjiJiy in the Daguerreotype Process, S77 



I have observed that the iodide of silver without bromine is 

 about 100 times more sensitive than the bromo-iodide to the 

 action of light, which produces the decomposition of the com- 

 pound forming the white precipitate of silver, while it is 100 

 times less sensitive for the effect which gives the affinity for 

 mercury. This seems another reason for supposing that the two 

 actions are different. It may be that, in the case of the iodide 

 of silver alone, the decomposition being more rapid, and the 

 affinity for mercury slower than when bromine is added to 

 the compound, the red, orange, and yellow rays having to 

 act upon an incipient decomposition, have the power, by their 

 own photogenic influence, of continuing the decomposition 

 when it has begun. This may explain the development of 

 the image under red, orange, or yellow glasses, according to 

 M. Gaudin's discovery. But in the case of the bromo-iodide 

 of silver, the red, orange, or yellow rays have to exert their 

 action on the affinity for mercury, begun a long time before 

 the decomposition of the compound ; and they have the pro- 

 perty of destroying that affinity. 



So that it would appear that all the rays of light have the 

 property of decomposing the iodide of silver in a longer or 

 shorter time, as they have that of producing the affinity for 

 mercury on the bromo-iodide of silver; with the difference, 

 that on the former compound the separate actions of the 

 several rays continue each other, and that on the second com- 

 pound these separate actions destroy each other. We can 

 understand that, in the first case, all the rays are capable of 

 operating the same decomposition ; and that in the second, 

 the affinity for mercury when imparted by one ray is destroyed 

 by another. This would explain the various phaenomena of 

 the formation of the two different deposits I have described, 

 and also explain the anomaly of the continuation of the action 

 of light by the red, orange, or yellow rays, according to M. 

 Ed. Becquerel's discoveries on the iodide of silver ; and of the 

 destruction of that action by the same rays, according to my 

 own observations on the bromo-iodide of silver. 



The red, orange and yellow rays, when acting on an un- 

 affected surface, are considerably less capable than the most 

 refrangible rays of imparting the affinity for mercurial vapour 

 on both the iodide and bromo-iodide of silver; and they de- 

 stroy diat affinity when it has been produced on the bromo- 

 iodide of silver by the photogenic rays. It follows from this 

 fact, that when the red, orange, or yellow rays are more abun- 

 dant in the light than the most refrangible rays, the photo- 

 genic effect is retarded in proportion to the excess of these 

 antagonistic rays. This happens when there exists in the 



