= ge = x 
British Association for the Advancement of Science: 99° 
into. It is as follows: it is well known. that gold leaf transmits ie 
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 
insuflicient 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 platé was spread over with 
hitrate 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- 
* ‘im. 
