182 PHILOSOPHY OF ZOOLOGY. 
optic nerve, which may be considered as the common reti- 
na. Nervous filaments proceed from this retina, penetrate 
the black membrane, which has been regarded as the 
choroides, and proceed to the concave surface of each facet, 
between which the opake matter only is interposed. We 
may consider the retina in these animals, in the light of a 
ganglion, and the individual filaments as the separate bran- 
ches of the retina. 
Some of the annulose animals have only these compound 
eyes, others only the simple ones, while many species are 
in possession of both kinds. 
2. Functions oF THE EyrE.—We come now to consi- 
der this important organ in action. 
Rays of light emanate from luminous bodies in all direc- 
tions, and the eye may be regarded as an optical instru- 
ment destined to act on these rays, and produce an impres- 
sion on the retina, indicative of their colour, intensity, and 
direction. 
These cones, or pencils of rays, falling upon the convex 
and transparent cornea, have their direction changed, and 
are made to converge. The effect is increased by the three 
humours of the eye through which the rays pass, so that 
they meet at a point beyond the vitreous humour, which, 
by opticians, is termed the Focus. Here the retina is 
spread out to receive the impression, and to communicate 
the same to the mind. 
No object is visible to the eye, unless the angle formed 
by its extreme points exceeds thirty-four seconds of'a degree. 
In order to render the impression distinct, it is necessary 
that all the rays which proceed from any one point of a 
body, should be collected in one point of the retina, and 
that all the points of union thus formed, should be disposed 
in that organ, in the same relative position as im the body 
from which they emanated. For the accomplishment of this 
