LEPIDOPTEROUS INSECTS. 91 
When the facetted eye of a butterfly is examined 
a little closely, it will be found to have the appear-. 
ance of a multiplying glass, the sides, or facets, 
nearly resembling a brilliant cut diamond. 
Tn the experiments performed by Mr Herschel, he 
describes the impulses received by the eyes of insects 
as analogous to those of sound, as given by Wollaston. 
He says,—* Although any kind of impulse or 
motions, regulated hy any law, may be transferred 
from a molecule in an elastie medium ; yet in the 
undulating theory of light, it is supposed that 
only such primary impulses as occur according to 
regular periodical laws, at equal intervals of time, 
and repeated many times in succession, can affect our 
organs with the sensation of light. To put in motion 
themolecules of the nerves of our retina with sufficient 
efficacy, it is necessary that the almost infinitely 
minute impulse of the adjacent etherial molecules 
should be often and regularly repeated, so as to 
multiply, and, as it were, concentrate their effect. 
Thus, asa great pendulum may be set in swing by a 
very minute force often applied, at intervals exactly 
equal to its time of oscillation ; or, as one elastic solid 
body can be set in vibration, by the vibration of 
another at a distance, propagated through the air, if 
in exact unison ; even so may we conceive the gross 
fibres of the nerves of the retina to be thrown into 
motion, by the continual repetition of the etherial 
pulses ; and such only will be thus agitated, as from 
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