oy 
“98 British Association for the Advancement of Science. 
of Optics and Chemistry united, were insufficient in their present 
state, to give any plausible explanation of this delicate and com- 
plicated process. If M. Arago, who had the advantage of being 
for six months acquainted with the secret, and therefore of con- 
‘sidering its nature in all points of view, was of this opinion, it 
seemed as if a call were made on all the cultivators of science to 
use their united endeavors, by the accumulation of new facts and 
arguments, to penetrate into the real nature of these mysterious 
phenomena. . For this reason he would offer a small contribution 
of new observations, which might perhaps be of service in the 
elucidation of this new branch of science. The first part of 
Daguerre’s process consists in exposing a silver plate to the vapor 
of iodine, by which it becomes covered with a stratum of iodide 
of silver, which is sensitive to light. Mr. 'T’.. stated that this fact 
n known to him for some time, and that it formed the 
basis of one of the most curious of optical phenomena, which as 
it did not appear to have been observed by Daguerre, he would 
here describe. Place a particle of iodine, of the size of a pin’s 
head, on a plate of silver, or on a piece of silver leaf spread on 
glass. Warm it gently, and you will shortly see the particle sur- 
rounded with colored rings, whose tints resemble those of New- 
ton’s rings. Now, if these colored rings are brought into the light, 
amost singular phenomenon occurs; for the rings prove to be sen- 
sitive to the light, and their colors change, and in a short time 
their original appearance is quite gone, and a new set of colors 
occupy their places. These new-colors are altogether unusual 
ones; they do not resemble any thing in Newton’s scale, but 
seem. to have a system of their own. © For instance, the two first 
colors are deep olive green, and deep blue inclining to black, 
which is quite unlike the commencement in Newton’s scale. It 
will be understood that the outermost ring is here accounted the 
first, being due to the thinnest stratum of iodide of silver, farthest 
from the central particle. The number of rings visible is some- 
times considerable. In the centre of all, the silver leaf becomes 
white and semi-transparent like ivory. This white spot, when 
heated, turns yellow, again recovering its whiteness when cold: 
from which it is inferred to consist of iodide of silver in a perfect 3 
state. The colored rings seem to consist of iodide of silver 10 
various stages of development. They have a further 
property; which, however, has not been sufliciently exe nied 
A Ni # ™ 
ats 
