NERVES — SENSES '. SIGHT. 301 



less distinct conical cylinder, running towards the centre of 

 the eye; beneath this external membrane is a layer of 

 coloured matter, often of a blackish violet hue, but some- 

 times green, red, or banded, pierced with as many holes as 

 there are facets or pupils ; beneath this is a varnish of a 

 black colour, wdthin which is another belt, the inclosed space 

 receiving the optic nerve, \^Tith its numberless ramifications, 

 a branch probably going to each facet. The structure and 

 functions of the eyes of insects have been investigated with 

 great accuracy by Miiller, who has proved that the refractive 

 powers of the ocelli must be very great, each ray of light 

 suffering a four-fold refraction : the first produced by the 

 convex cornea, the second by the anterior convex sm'face of 

 the lens, the third by the posterior convex surface of the 

 lens, and the fourth by the convex surface of the glassy body 

 itself; hence a very distinct short sight, suitable for small 

 objects, is possessed by these eyes. In the compound eyes 

 the effect is different : here, owing to the convexity of these 

 eyes, and the consequent obhquity of the lateral facets and 

 their cylinders, a much wider horizon must be embraced, 

 each individual facet surveying but a small space of the en- 

 tire field of vision : each thus contributing to the perception 

 of objects comprised within the view, those rays of light only 

 that fall in a right line upon a facet, which itself forms the 

 segment of a circle, can reach the optic nerve of this facet, 

 whereas all others are withheld by the pigment which sepa- 

 rates the individual glass cases from each other, and partly 

 circularly surrounds the margin of the crystalhne lens be- 

 neath the cornea. According to Miiller, each nervous fila- 

 ment conveys to the bulb of the optic nerve the impression 

 of the ray which it has individually received ; and as all the 

 nervous filaments at first separated by the pigment are at 

 length united together into one common and continuous 



