British Association for the Advancement of (Science. 99 



into. It is as follows : it is well known that gold leaf transmits 

 a bluish green light ; but no other metal has been described as 

 possessing colored transparency. These rings of iodide of silver, 

 however, possess it, being slightly transparent, and transmitting 

 light of different colors. In order to see this, a small portion of 

 the film should be isolated, which is best done by viewing it 

 through a microscope. Mr. T. said that he had considered the 

 possibility of applying a silver plate thus combined with iodine, 

 to the purpose of photogenic drawing, but he had laid it aside as 

 insufficient for that purpose, because its sensitiveness appeared to 

 be much inferior to that of paper spread with chloride of silver, 

 and therefore in an equal time it takes a much feebler impression. 

 Now, however, M. Daguerre has disclosed the remarkable fact 

 that this feeble impression can be increased, brought out and 

 strengthened, subsequently, by exposing the plate to the vapor of 

 mercury. Another experiment was then related, in which a par- 

 ticle of iodine was caused to diffuse its vapor over a surface of 

 mercury. In order to this, a copper plate was spread over with 

 nitrate of mercury, and then rubbed very bright and placed in a 

 closed box along with a small cup containing iodine. The result 

 was, a formation of colored rings of the greatest splendor and 

 of large size. But they did not appear to be in any degree, sen- 

 sitive to light. The next point of Daguerre's process is, the ex- 

 posure of the picture to the vapor of mercury, and this is by far 

 the most enigmatical part of the whole process. For he states 

 that if you wish to view the picture in the usual manner, i. e. 

 vertically, you must hold the plate inclined to the vapor at an 

 angle of 45°, and vice versa. Now this is altogether extraordinary ; 

 for who ever heard of masses of vapor having determinate sides, 

 so as to be capable of being presented to an object at a given an- 

 gle ? From the hasty consideration which he had been able to 

 give to it, his first impression was, that this fact bore a certain 

 analogy to some others which he would mention. If a piece of 

 silver leaf is exposed to the vapor of iodine, however uniform the 

 tension of the vapor, it does not combine uniformly with the me- 

 tal, but the combination commences at the edge of the leaf and 

 spreads inwards, as is manifested by the formation of successive 

 bands of color parallel to the edge. This is not peculiar to silver 

 and iodine, but occurs when other metals are exposed to other 

 vapors, not always with entire regularity, but it displays a ten- 



