SENSE OP SIGHT. 



847 



cornea of higher animals, are found at the tips of the tentacula. Evi- 

 dences of a choroid and of a nervous net-work representing the retina 

 are also found. Such organs are termed ocelli. It is, therefore, seen 

 that two classes of visual apparatus may be recognized, the simple and 

 the compound; the former consisting simply of a mass of pigment in 

 connection with the nerve-fibres, and the latter of a cornea and of other 

 accessory organs. In many insects and in some crustaceans both 

 species of eyes are found. The simple eyes may be three or more in 

 number and are most usually placed on the summit of the head. In the 

 articulates, such as insects and crustaceans, are found eyes of a special 

 type of construction— what may be termed composite eyes— consisting of 

 a collection of a considerable number of diverging radiating,- tubes" or 

 cones, terminating on the surface in the shape of polygonal opacities and 

 inclosing in their interior a fluid analogous to the vitreous body, while 

 their deep extremity is continuous with the nerve-iilament and their 



Fig. 366.— Section of the Eye of the Cockciiafek (Melolontha 

 vulgaris). (Carprntpv.) 



A. A, facets of the cornea; E, transparent pyramids surrounded with pigment; c. fibres of the optic nerve; 

 d, trunk of the optic nerve. The same description applies toB, which is a portion of the eye more highly magnified! 



interior is lined with pigment. Each one of these two eyes, which are 

 frequently but a few millimeters in diameter, often incloses from ten to 

 twenty thousand of these little tubes ; while the inclosed membrane, 

 which is analogous to the choroid, is impregnated with pigment over the 

 greater part of its extent, except at the centre, where a transparent 

 opening is present through which the light passes (Fig. 365). These e\-es 

 are capable of forming distinct images, but each of the diverging cones, 

 disposed like the rays of a segment of a sphere (Fig. 366), is only 

 capable of transmitting the ray of light which coincides with its long 

 axis; all the other rays, striking more or less obliquely on the internal 

 walls lined with pigment, are absorbed ; as a consequence, in such an 

 eye the image is formed by the co-ordination of the rays coming from 

 corresponding isolated points of the object. While, nevertheless, objects 

 may be clearly appreciated by such an eye, a large quantity of light 



