30 



SPECIAL SENSES. 



natural transition to the compound eyes of insects, to which 

 we now give our attention. 



88. Compound eyes have the same general form as 

 simple eyes ; they are placed either on the sides of the head, 

 as in insects, or supported on pedestals, as in the crabs. 

 But if we examine an eye of this kind by a magnifying lens, 

 we find its surface to be composed of an infinite number of 

 angular, usually six-sided faces. If these fa9ettes are re- 

 moved, we find beneath, a corresponding number of cones (c), 

 side by side, five or six times as long as they are broad, 

 and arranged like rays around the optic nerve, from 

 which each one receives a little filament, so as to 

 present, according to Miiller, the following disposition. 



(Fig. 18). The cones are per- 

 fectly transparent, but sepa- 

 rated from each other by 

 walls of pigment, in such a 

 manner, that only those rays 

 which are parallel to the 

 axes, can reach the retina A ; 

 all those which enter ob- 

 liquely, are lost ; so that of 

 all the rays which proceed 

 from the points a and Z>, only the central ones in each 

 pencil will arrive at the optic nerve (rf) ; the others will 

 strike against the walls of the cones. To compensate 

 for the disadvantage of such an arrangement, and for the 

 want of motion, the number of fafettes is greatly multi- 

 plied, so that no less than 25,000 have been counted in 

 a single eye. The image on the retina, in this case, may 

 be compared to a mosaic, composed of a great number of 

 small images, each of them representing a portion of the 

 figure. The entire picture is, of course, more perfect, 



18> 



