Mr. Talbot on Coloured Rings by Iodine on Silver. 95 
I was in hopes it had become known to the scientific world; 
but as that appears not to be the case, will you allow me to 
occupy a page of your Journal with an extract or two from the 
above-mentioned report contained in the Athenzeum? 
After alluding to M, Daguerre’s process (then just divulged) 
of exposing a silver plate to the vapour of iodine, by which it 
becomes covered with a stratum of iodide of silver which is 
sensitive to light, I stated to the Section that this fact had been 
known to me 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 M. Daguerre, I would de- 
scribe to the Meeting. Place a small particle of iodine, the 
size of a pin’s head, on a plate of silver, or ona piece of silver 
leaf spread on glass. Warm it very gently and you will 
shortly see the particle become surrounded with a number of 
coloured rings, whose tints resemble those of Newton’s rings. 
Now, if these coloured rings are brought into the light a most 
singular phenomenon takes place, for the rings prove to be 
sensitive to the light, and their colours change, and after the 
lapse of a short time their original appearance is quite gone, 
and a new set of colours have arisen to occupy their places. 
These new colours are altogether unusual ones; they do not 
resembie anything in Newton’s scale, but seem to conform to 
a system of their own. For instance, the two first colours are, 
deep olive-green, and deep blue inclining to black, which is quite 
unlike the commencement of Newton’s scale. It will be un- 
derstood that the outermost ring is here accounted the first, 
being due to the thinnest stratum of iodide of silver, furthest 
from the central particle. The number of rings visible is 
sometimes considerable. In the centre of all, the silver leaf 
becomes white and semitransparent 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 state. The coloured rings seem to consist 
of iodide of silver in various stages of development. 
They have a further singular property, which is as follows: 
—It is well known that gold leaf is transparent, transmitting a 
bluish-green light, but no other metal has been ascribed as 
possessing coloured transparency, ‘These rings of iodide of 
silver however possess it, being slightly transparent, and trans- 
mitting light of different colours. In order to see this, a small 
portion of the film should be isolated, which is best done by 
viewing it through a microscope. . 
I then related another experiment, in which a particle of 
iodine was caused to diffuse its vapour over a surface of mer- 
cury. In order to this, a copper-plate was spread over with 
