16 
POPULAR SCIENCE REVIEW. 
interior of the eyeball. And it results from the action of the 
several curved surfaces of the cornea and lens, and from the 
specific power of refraction due to the density of their substance, 
that an inverted image of any illumined object in front of the 
eye must pass through the transparent retina, and be reflected 
back again from the inner surface of the choroid (which is 
darkened by pigment) upon the retinal elements in contact 
vdth it. An optical image is, therefore, received by the retina, 
and 'perceived as if it were the object itself. The action of the 
refractive media, by ^vhich this picture of external objects is 
formed on the inner spherical concave of the pigmented choroid 
coat, is commonly illustrated by likening the whole dioptric 
apparatus to a “ camera obscura,” where the images formed by 
the lenses fall on a prepared surface, or, as in a photograph 
camera, on a ground glass plate, or the chemically sensitive 
surface substituted for it when a photograph is taken. 
The type of construction of the compound eye is, on the 
contrary, not that of a globe filled with refractive media, nor is 
the retina of the compound eye spread out in membrane-like 
expansion over a vitreous humor. The optic nerve at the 
bottom of the eye swells into a large solid mass by addition 
of fresh nerve matter (granules, nuclei, and medullary sub- 
stance) together with pigment, connective tissue, blood vessels, 
tracheae, and even muscle fibrils. And this mass, known as 
“ optic ganglion,” fills the space at the bottom of the eye, pre- 
senting, as it is continued forwards to the centre of the eye, the 
form of a solid cone, widening towards the front. The peri- 
pheral surface of this optic ganglion is covered with a thick 
layer of pigment, which appears to intercept all passage of light 
from the front to the back of the eye. But, through this pig- 
ment layer, numerous nerve fibres (enclosed in sheaths of 
investing membrane) pass onwards in direct lines towards the 
cornea, but terminate in peculiar-shaped bodies situated im- 
mediately behind it (these will afterwards be more particularly 
described). 
Thus, there is neither a central vitreous humor nor a lens 
answering to the crystalline lens of the vertebrate eye ; nor is 
tliere apparently any retina interposed at the focal plane of a 
dio[>tric apparatus to receive and perceive images. And since 
it is impossible that, in an eye thus constructed, images could 
be formed by tlie passage of collective rays of light through its 
intf*rior, it was supposed that the sensation of light was produced 
by (lirr*ct contact of rays, which, falling on the cornea and pass- 
in j' through without refraction, met the nerve fibres behind . hut 
ih re hi ' ng no focal convergence^ no optical image 'was formed. 
Sufh an hypothesis, however, failed to show how external objects 
‘•oubl ])e seen in definite form and with distinct detail. Ac- 
