430 PROFESSOR LUDWIG MOSER ON VISION, 
amined, in order to allow us to suppose that the effect of light 
on the substance of the retina is not considered as a more con- 
siderable chemical metamorphosis than it really is. 
Tt has been noticed above, that the blackened iodide cannot 
possibly be metallic silver, inasmuch as we can produce an image | 
on it. I will now pursue this subject further. If we take an 
iodized plate of silver, and placing it in a camera obscura, allow 
it to remain there twenty-four hours, we obtain, as I have before 
stated, a positive image, which is therefore the second that has 
been formed upon the plate, for the first is a negative image. 
The positive one exhibits in reality only shades of gray, and I 
have never seen colours produced under these circumstances. — 
We may then naturally ask, whether this second image be the 
last, or whether a third negative, then a fourth, &c. can be 
formed, if we allow the light sufficient time to exert its full 
influence. 
For the elucidation of this point two plates were iodized, one 
of them was exposed to the vapours of chloride of iodine, and 
then each put into a camera obscura. ‘The weather was very 
unfavourable, it being winter, and the sun scarcely shining at 
all; each camera obscura was placed in a darkened chamber, in 
order to exclude all light from the sides, and was directed to- 
wards distant houses. The plates were taken out at the expira- 
tion of thirteen days, and both exhibited a correct positive image. 
That one which had been exposed to the chloride of iodine ex- 
hibited a most beautiful appearance; the light parts were of the 
brightest sky-blue, and the dark spots of a fiery red colour, The 
other image possessed the same colours, but not near so bright. 
I have not the slightest doubt that this was still the second 
image, for by such bad light a fourth could hardly be expected, 
even if it be ever formed. But this experiment allows us to go 
one step further. It teaches us that the blackened iodide of 
silver had changed into the coloured variety, solely by the con- 
tinued action of light. It was found that this iodide was similar 
to the original coloured salt in other respects. The original un- 
affected iodide of silver is easily dissolved by a solution of hypo- 
sulphite of soda, as Herschel has observed, but not so the 
blackened iodide. On introducing the above-mentioned plate 
into a solution of this salt, the coloured coating was rapidly dis- 
solved, and a negative image remained behind. This is a clear 
proof that the continued action of light brought back the black- 
