432 PROFESSOR LUDWIG MOSER ON VISION, 
with dry cotton, placed in the sun, and the shadow of a neigh- 
bouring body thrown upon it. In a short time the rest of the 
plate had become evidently dark, while the part where the sha- 
dow rested was light. The plate was now polished again, and 
placed in the sun with the same success. I operated on it thus 
eight times, and the experiment is not yet completed. After 
every polishing or rubbing it is seen that there is stili sensitive 
iodide of silver equally diffused over the whole plate. The result 
of this experiment is extremely curious, if we consider how ex- 
tremely minute the original layer of iodide is. Dumas states, 
without however explaining his method of measurement, that its 
thickness is less than one-millionth of a millimetre. Notwith- 
standing all the manipulations which are necessary to produce 
an image according to Daguerre’s plan, I have executed thirty 
images in succession on common plated copper; the plate then 
certainly began to show traces of projecting copper, but only 
single red spots, while the remainder of the plate was still in a 
perfect state. 
I will now mention certain objections which might be made 
to the proposed hypothesis concerning the action of light on the 
retina. They are objections which might certainly be raised, 
but which, when we observe them more narrowly, will be found 
to be rather confirmations, and are partly founded on phenomena 
which it was quite impossible to explain according to the usual 
view. 
In the first place, one might compare the sensations of the 
eye with those of the ear, and consider oscillations as the regular 
cause in both instances. But without considering that in the 
one case we have to deal with vibrations of ponderable matter, 
and in the other of imponderable, there is in point of fact 
no analogy between the impressions of the eye and those of 
the ear; for if we look only to the outward causes which ex- 
cite both the organs of sense, we should expect to find an ana- 
logy between the different colours and the deep and high tones, 
and yet there is nothing less analogous, at least as far as we can 
see. Colours produce a perfect impression (if we may be allowed 
the term), which cannot be confounded with each other, while 
the tones effect nothing of the kind. We certainly should not 
confound together very high and very deep tones, but this occurs 
very easily with an ordinary ear in the case of closely approxi- 
mated tones, and in apy case it requires a particularly fine and 
