THE ORGANS OF SPECIAL SENSE. 31 



Behind every facet is a transparent cone, surrounded by dark 

 red pigment, which terminates posteriorly in a rod, connect- 

 ing it, through the medium of a layer of nerve cells, with u 

 clear distinct nerve filament. 



The convexity of the facets of the cornea is a most important 

 point, as the vision of insects has been supposed to derive its 

 distinctness from the number of separate facets. It has been 

 supposed that distinctness of vision is due in them to the 

 absorption of all the rays of light which have not the same 

 direction as the axes of the cones and rods of which the eye is 

 composed. Such is not the case, however, in the fly ; the 

 lenticular structure of the cornea is most distinct, and the focus 

 of each lens appears to correspond with the apex of the 

 transparent cone beneath it. The necessity of any adaptive 

 power to distances, it may be, is entirely obviated by the small- 

 ness of the lenses, as the rays of light emanating from a point 

 an inch distant from the eye could not possibly make a greater 

 angle than a minute, so that virtually all rays would be parallel 

 when the distance of objects from the eye exceeded an inch. 

 Yet it is far from certain that even adaptation to distance is not 

 effected by a special arrangement of tracheal tubes between the 

 rods and cones.* Perhaps nearer objects are perceived only by 

 the ocelli, which are specially adapted for near vision, for which 

 the comparative flatness of the lenses of the compound eyes seems 

 but little fitted. 



The great convexity and extent of the compound eyes is such 

 that it enables the insect to see all the objects in at least four- 

 fifths of the sphere which surround it; but as no two facets look 

 towards the same spot, no provision for stereoscopic vision 

 exists in these organs, even if the insect be able to perceive 

 near objects by them. These deficiencies seem to be supplied 

 by the ocelli, which, from the great convexity of their refracting 



* Page 88. 



