AND ON THE ACTION OF LIGHT ON BODIES. 437 
sesses the same index of refraction as that of stars from which 
it is receding, led Arago to this theory a long time since*. We 
must thus conclude, that of the various rays of a luminous body, 
one particular system acts on one sensitive body and another 
system on a different body, and that the retina itself must be 
numbered among these sensitive matters. There is a certain 
system of rays which act upon it alone, and which produce the 
impression of colour. This system is not more extended than 
it is with respect to other substances, for we have seen above 
that there is an iodide of silver which is affected by all colours 
just as well as the retina. If this be the case we could not deny 
the possibility of the existence of non-luminous chemical rays ; 
they would be such as do not act on the retina, but exert an 
influence on other sensitive bodies. here is nothing @ priori 
objectionable in this. Ritter, as is well known, observed the 
existence of such dark rays in the action of light on chloride of 
silver, for he noticed a blackening above the violet of the spec- 
trum; and Wollaston, Seebeck, and others, have confirmed this 
statement. I myself have tried the experiment in a darkened 
room, without however arriving at any very great degree of cer- 
tainty. The limits of the violet cannot be determined so easily ; 
and there is no doubt that they would be different for different 
eyes, or for the same eye in different circumstances. Moreover, 
unless a heliostat is used we must keep moving the paper, which 
must diminish the confidence to be placed in the experiment. 
However, I did not take any very great care when making these 
experiments, for at that time they did not interest me so much, 
and therefore I will not on their account throw the least shadow 
of doubt on the existence of non-luminous and yet active rays, 
which, as above stated, is perfectly immaterial for my theory 
either one way or the other. Herschel+ considers it possible 
that some animals, viz. insects, do not receive. an impression 
from any of those colours which are visible to us, but arezaffected 
by a species of oscillations which lie beyond=the limits*of our 
senses. Wollaston{ adopts the same hypothesis with respect 
to their sense of hearing. 
I will now direct my attention to the very interesting question 
concerning the sensibility of the retina to the action of light, 
* Poisson, Traité de Mécanique, 2me édit. tom. i. § 168. 
+ On Light, § 567. 
¢ Phil. Trans. for 1820. London and Edinb. Phil. Journal, vol. vii. p- 158. 
