The successive zones in the insect eye modify the rays that pene-
trate the sight, passing by each facet, and by the centre of each con-
vergins cyunder radiating to the optic ganglion. The layer of pig-
ment <K)es nothing but diminish the quantity of light, and adjust it. It
is found in most if not all diurnal insects, and the iris being perforated
with as many holes as there are facets in the cornea, it is subjected to
multiplied modifications. As might be expected, this pigment is not
met with in any of the nocturnal insects.
Insects that fly re<^uire an ample field of vision. T|;ie combined cor-
neules become one large pupil. The multiplied facets render super-
fluous eyelids and muscles to move the eye. In consequence of the
vision being directed to the whole circumference, it comprehends, by
relative a^ustment, all objects around. A simpler eye occurs in the
grovelling insects that see only what is near with distmctness. In in-
sects which fly by night, like uie moths, there is, in place of the black
or colored pigment, a substance of a resplendent green, or silvery color,
serving not to absorb, but to reflect the rays of light, and enabling them
to see Dy a more obscure illumination than that of daylight. The eyes
of moths look always luminous, and appear as if they were phosphores-
cent, Grom this reflecting power. This organization gives a solution to
the reason why moths fiy to the candle. They lose all discernment in
the blaze of radiance that overwhelms them by reflection ; and they
perish in the flame into which they rush.
