XIV INSECTA 209 



These compound eyes project, one on each side of the head, 

 as sessile, convex, immovable structures, the surface of which, 

 when examined with a lens, is found to be marked off in a 

 very large number of little hexagonal or square areas or 

 facets. A microscopical examination of a section of the eye 

 (Fig. 140) reveals the fact that underneath each facet there 

 lies a series of structures, forming a long, narrow, pyramidal 

 body, which is in itself a complete organ of vision, though, 

 owing to its minute size and structure, it has a very limited 

 range, and needs, therefore, to be supplemented by the other 

 similar visual elements surrounding it. 



The exposed facet (Fig. 141, /) is the outer end of a 



glassy columnar body, =-e=_ 



the fens, beneath which ~feB XZI^^^^^= ■,..;'"■'"":: 

 is a transparent corm j[''l|{M'l -i-^^^^^J — "ZT'""^^ 



(Fig. 141, c), and c-T^^^^^^=^^~'^' 

 beneath this, again, a ,^j^^ , ^ | ~.^^^^ ^^.. 



ro<?-like structure, K il <^ I* '^ ^ 



which is surrounded piQ. 141._Three visual elements of the Eye 



by a delicate sheath enlarged. 



in connection with a /, Exposed facet of the element ; d, corneal lens ; 



branch of the optic «> ™'>«'' '"■ ""^'' P' P'gment shown by black 



rp,, . . , -1 dots ; 71, nerve cell. 



nerve. Ihis sheath 



acts as a retina, and receives the impressions of the rays of 

 light which enter the facets and pass down the cones and 

 rods. Each such set of parts in the eye is separated from 

 the adjacent parts by a sheath of dark- coloured pigment, 

 which is specially concentrated just at the bases of the 

 cones (Fig. 141, ^), and again at the bases of the rods. 



Many thousands of such elements as those described go to 

 make one compound eye, the number of them varying in 

 different insects. The mod,e of vision of such an eye is not 

 very certainly understood. It seems clear, however, that only 

 those rays of light which enter each facet at right angles 

 to its surface reach the base of the eye, for all obliquely 

 entering rays are absorbed by the pigment sheath. Probably, 

 therefore, only a few rays from any one point of an object 

 enter any one lens in such a way that a reduced image of 

 it is formed on the sensitive retina ; the rays from an adjacent 

 point of the object enter an adjacent lens, and form the 

 corresponding image on an adjacent part of the retina. In 

 VOL. I P 



