450 PROFESSOR LUDWIG MOSER ON VISION, 
as when light acts on them. This identity between two causes 
apparently so different might have been supposed from the for- 
mer experiments on breathing on surfaces. I will now adduce ~ 
striking proofs of it. 
It has often been stated that there is a natural relation between 
light and shade in Daguerre’s pictures. Nothing can be more 
incorrect than this assumption, and that may be learnt without 
seeing many such images. The following is the true state of the 
case. If an iodized silver plate be allowed to remain too short » 
a time in the camera obscura, it afterwards exhibits no image 
when exposed to the vapours of mercury ; a light film of mercury 
is deposited over the whole plate, which is not only the case with 
this, but also with a plate of the pure metal and the blackened 
iodide of silver. If the plate remain a Jonger time in the camera 
obscura a picture certainly is produced, but in which only the 
brightest parts are depicted, and, which is here of importance, 
the light parts are of a white colour, 7. e. they condense the most 
mercury. If the plate remain still longer in the camera obscura, 
a picture with all its details is formed, but the bright parts have 
lost a portion of their whiteness, and appear gray, 2. e. they do 
not condense so much of the mercurial vapour. If it be left still 
longer, on taking it out no picture at all is to be seen; if now 
inserted into the mercurial vapours, a negative image is produced, 
or, in other words, these bright parts do not condense any mer- 
cury ; consequently, as a general proposition, we cannot speak 
of a correct relation between light and shade. 
If, therefore, light acts on iodide of silver, it imparts to it the 
power of condensing mercurial vapours in an increasing ratio ; 
but if it acts beyond a certain time, it then diminishes this 
power, and at length takes it away altogether, and this happens 
before the yellow iodide has changed its colour. The vapours 
of mercury have been seen to do the same, and if in the last-de- 
scribed phenomenon they produced a negative image, it is only 
what light would have done if it had been allowed to act still 
longer. This identity between the action of light and mercurial 
vapours is so very remarkable that I will adduce other proofs 
of it. 
An iodized plate of silver is allowed to remain the proper time 
in a camera obscura, and then exposed to the sun under a yel- 
low glass. As I have already stated, a negative image is first 
produced, which vanishes after a time and is replaced by a posi- 
