140 INTRODUCTION 
of vision, must fall upon one or several of them in the direction of their axes. 
A faceted eye, the surface of which frequently occupies more than a hemisphere, 
Pera] 
FIG. 57 a. Head of a Drone seen from the front, 
showing the compound eyes, the three ocelli, and the 
antennae. (After Swammerdam.) 
Fic. 57 & Three facets with retinulae from the 
compound eye of the Cockchafer: the pigment has 
been removed from two of them. J, corneal 
facet; A, crystalline cone; 7 pigment sheath; 
P’, pigment cells; P”, pigment cells of the second 
order; &, retinulae. (After Grenacher.) 
consists firstly of a number of hexagonal, 
transparent corneae, or facets, which, while 
moderately flat externally, frequently 
present lenticular prominences internally, 
and are separated from one another by 
shallow grooves. Behind each cornea 
there is a transparent refractive organ— 
the crystalline cone surrounded by a dark, 
funnel-like, pigment sheath. Internal to 
these two zones is the third and _ last 
layer, that of the vzsual rods. The narrow, 
stalk-like, internal end of the crystalline 
cone is enclosed by a funnel-shaped de- 
pression at the external end of the visual 
rod, ‘so that the two are directly con- 
nected. The large compound visual rods 
make up a hemispherical refima convex 
externally, and this extends to the bulbous 
expansion of the optic nerve, which 
receives external impressions and _trans- 
mits them to a ganglion corresponding 
to the brain of higher animals, where 
they result in sensations. 
Each facet, therefore, with its visual 
rod forms an independent eye, connected 
with others only by means of the common 
nerve-trunk. If, now, a reversed and 
reduced image of the surroundings is 
formed behind each facet (Gottsche), 
which is convex internally, this image is 
remote from the irritable part of the 
visual rod, and only its vertically incident 
axial ray (strengthened by refraction) can 
be perceived, as all the lateral rays are 
absorbed by the pigment. The im- 
pressions produced by such axial rays, 
the number of which corresponds to that 
of the individual nerve-rods, consequently 
form a kind of mosaic, which repeats 
upon the retina the arrangement of the 
points of the external object from which 
light is received. ‘The picture thus pro- 
jected, however, is deficient in brightness and detail (Claus, ‘Lehrbuch der. Zoologie,’ 
5th ed., 1891, p. 84). 
