INTRODUCTION. 3 
the part of the crystalline, or lens, in the eyes of animals. They 
are straight and parallel with each other. A pigment fills all the 
spaces between the cones, and between the nervous filaments, 
and covers the under side of each cornea, except at the centre. 
This pigment varies much in colour. There are almost always 
two layers, of which the exterior one is the more brilliant. In 
fact, these eyes often sparkle with fire, like precious stones. 
M. Lacordaire, in his “ Introduction 4 l’Entomologie,” from 
which we borrow the greater part of this information, has summed 
up as follows, the manner in which, according to Miller, the 
visual organs of insects operate :— 
“Hach facette with its lens and nervous filament, separated 
from those surrounding them by the pigment in which they are 
enclosed, form an isolated apparatus, impenetrable to all rays of 
light, except those which fall perpendicularly on the centre of 
the facette, which alone is devoid of pigment. All rays falling 
obliquely are absorbed by that which surrounds the gelatinous 
cone. It results partly from this and partly from the immobility 
of the eye that the field of vision of each facette is very limited, 
and that there are as many objects reflected on the optic filaments 
as there are corner. The extent, then, of the field of vision will be 
determined, not by the diameter of these last, but by the diameter 
of the entire eye, and will be in proportion to its size and con- 
vexity. But whatever may be the size of the eyes, like their fields 
of vision, they are independent of each other; there is always a 
space, greater or less, between them; and the insect cannot see 
objects in front of this space without turning its head. What a 
peculiar sensation must result from the multiplicity of images 
on the optic filaments! This is not more easily explained than 
that which happens with animals which, having two eyes, see 
only one image ; and probably the same is the case with insects. 
But these eyes usually look in opposite directions, and should see 
two images, as in the chameleon, whose eyes move independently 
of each other. The clearness and length of vision will depend, 
continues M. Miller, on the diameter of the sphere of which 
the entire eye forms a segment, on the number and size of the 
facettes, and the length of the cones or lenses. The larger each 
facette, taken separately, and the more brilliant the pigment placed 
B2 
