AND ON THE ACTION OF LIGHT ON BODIES. 431 
ened iodide to its former state, and that this black body cannot 
well be a substance differing in any very great degree from the 
common coloured iodide of silver. As these experiments require 
the entire use of the instruments, I have not been able to carry 
them out in this manner; moreover, I was prevented from so 
doing by the suspicion that even after exposing them for months 
to this treatment I should not be able’to determine which image 
the plate really did possess. 
In order to arrive at a conclusion concerning this subject in 
a more convenient manner, I exposed iodized plates to daylight, 
~and whenever possible to strong sunlight. The plates became 
4 “first black, then lighter, and lastly greenish. After a lapse of 
~ fourteen days they appeared to become dark, and after a still 
longer time light again, and I have observed these alternations 
take place ae or six times. In order to observe the changes 
more easily I placed strips of dark paper on the plates from time 
to time, by which certain parts were protected from the influ- 
ence of the light; moreover, I always observed the plates in the 
same position with regard to reflected light, because otherwise 
light often takes the place of dark, as may be seen in the well- 
_ executed images of Daguerre. But the result of these experi- 
r ments eens more convincing to me by facts to be mentioned 
hereafter. I have no doubt that a well-conducted experiment 
will exhibit all these alternations in the effect of light: it will, 
however, be necessary to protect the plates from dust and moist- 
ure better than was the case in my trials, where they appeared 
to exert a deteriorating influence. An experiment made by 
_ Draper (Philos. Mag., Sept. 1841) is greatly in favour of the 
_ idea that light does not drive off the iodine from the plate. He 
‘saturated paper with a solution of starch and laid it on an iodized 
_ silyer plate. When the sun shone through, the plate became 
dark green, but the paper did not evince a trace of a blue colour- 
ing. From all these facts I consider it as fully proved, or at 
Teast as highly probable, that light produces no decomposition 
of the iodide of silver, and refer to the classes of phenomena 
which will be hereafter described. 
mt I will now return to the proof of my third assumption, that 
ie longest continued action of light only affects the most ex- 
nal surface of the iodide of ae although the whole layer is 
excessively thin. A yellow prepared plate was exposed for 
o months to daylight and direct sunlight; it was then rubbed 
