INSECTS 1447 



nation of the optic nerve-filament placed there to 

 receive it. 



The rays which pass through the several pyramids 

 are prevented from mingling with each other by the 

 isolating sheath of dark pigment; and no rays except 

 those which pass along the axis of each pyramid can 

 reach the optic nerve ; all the rest being absorbed in 

 the pigment of the sides. Hence it is evident that as 

 no two corneae on the rounded surface of the com- 

 pound eye can have the same axis, no two can transmit 

 a ray of light from the very same point of any object 

 looked at; while, as each of the composite eyes is 

 immovable, except as the whole head moves, the com- 

 bined action of the whole 24,000 lenses can present 

 to the sensorium but the idea of a single, undistorted, 

 unconfused object, probably on somewhat of the same 

 principle by which the convergence of the rays of 

 light entering our two eyes gives us but a single 

 stereoscopic picture. 



The soft blue color of this dragon-fly's eyes as 

 also the rich golden reflections seen on the eyes of 

 other insects, as the whameflies, and many other Dip- 

 tera is not produced by the pigment which I have 

 alluded to, but is a prismatic reflection from the 

 corneae. 



You would suppose that, having 24,000 eyes, the 

 dragon-fly was pretty well furnished with organs of 

 vision and surely would need no more ; but you would 

 be mistaken. It has three other eyes of quite another 

 character. 



If you look at the commissure or line of junction of 

 the two compound eyes on the summit of the head, 



H VOL. IV. 



