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 EYE. 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 in the body 

 from which they emanated. For the accomplishment of this 



