PROFESSOR LUDWIG MOSER ON VISION. 423 
In the first place the three following propositions will be 
proved:—1. The blue and violet rays of light are not alone 
those which act chemically; or if there are supposed to exist 
in light chemical and luminous rays, the chemical ones are not 
contained solely in the most refrangible rays of the spectrum. 
2. It is not necessary to suppose—and in those phenomena 
which have been best observed it certainly is not the case—that 
light produces a separation of chemically-combined bodies. The 
action of light is, as I hope to show, of such a kind that it may 
be imitated in a perfectly different manner, so that the idea of 
a chemical decomposition is fully refuted. 3. Even the most 
continued action of light appears to affect only the finest surface 
bodies, and by no means to penetrate the usually so extremely 
in layer of iodide of silver. 
It is evident that if these three statements can be proved, the 
ongest objections to the new view are put aside. Whiist all 
lours act on the eye, it is generally believed that only certain 
lours, principally blue and violet, exert their influence on those 
ibstances which are capable of being affected by light. 
_ Ishall not attempt to nullify this objection by the remark 
(which however it is as well to remember in other instances), that 
we at present know the action of light on extremely few sub- 
“stances, in which cases it can be made visible to the eye; that 
ight may act on many other bodies, perhaps on all (see the end of 
his chapter) ; and that among them there may probably be some 
hich are affected by all colours alike. This call upon our ig- 
jorance for assistance is in so far unnecessary, as iodide of silver 
really does undergo a certain action from all colours, and as‘there 
is, indeed, as I shall show, a state of this body which, if it has 
‘once acquired, all colours act on it equally. The second and 
third statements show us how small the changes produced by 
light are; by a continued action they might become more con- 
siderable, but that would be exactly in favour of the new hypo- 
thesis: for the continued fixing of an object, which is always 
so difficult for the eye, shows by its after-action that the changes 
produced on the retina cannot be so very inconsiderable sas it 
requires, according to circumstances, minutes, hours, even days 
and months to return to its normal condition. 
_ Before I proceed to the experiments I will say a few words on 
best manner of making them. It appears at first sight most 
onvenient to make use of sensitive papers and a darkened cham- 
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