AND ON THE ACTION OF LIGHT ON BODIES. 433 
musical ear to determine at once a tone as comnonly designated, 
while the eye encounters no such didiculty ia the determination 
of the colours. We should be much more inclined to compare 
the depth or height of a tone with the intensity of one particular 
colour, while the different colours themselves might be likened 
to the quality (dimbre) of the tones. The perceptions of these two 
organs of sense are therefore very different when similar out- 
ward causes are acting, and do not allow of much comparison ; 
if, however, this must be made, it then only appears that the act 
of vision must be quite different from that of hearing, and that 
while in the one case thousands of oscillations in a second are 
counted, in the other hundreds of billions probably cannot be. 
A more important objection might be made in the following 
manner. If, for the sake of simplicity, we leave out of considera- 
tion the green and yellow rays, which exert a peculiar influence 
on the iodide of silver, and only take notice of the violet, blue, 
red and orange, they all appear to have the same effect on the 
iodide; they reduce it to such a state that it is capable of con- 
densing mercurial vapours. All matters sensitive with regard 
to the action of light, with which we are acquainted, exhibit in 
the same manner a similarity in the actions of several colours. 
From this we might be induced to ask why it is they all act 
differently on the retina, to which they appear as various modi- 
fications? We might perhaps here be allowed to hint that our 
knowledge of the action of light on iodide of silver is too scanty, 
that we have only lately learnt the use of mercurial vapours as 
a test for it, and that there may probably be other means which, 
when employed, would show whether the action exerted has 
proceeded from one colour or the other. Moreover, it is not at 
all necessary that that which is correct for iodide of silver should 
also be so for all other bodies, as for instance, the retina; and 
that guaiacum exhibits a sensible difference between the action 
of blue and violet rays, inasmuch as the former turns it blueish- 
green, the latter blue. But even here it is not necessary to seek 
for an excuse in our want of knowledge. Seebeck found that 
chloride of silver is variously coloured in the different rays of the 
spectrum, and every observer who has made experiments on the 
action of light on any substance whatever, will have been as- 
tounded at the number of tints presented to his view under cer- 
tain circumstances. 
_ These tints are proofs of so many modifications which the sub- 
