INSECTS : THEDB EAES AND EYES. 195 



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 cornese on the rounded surface of the compound 

 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 combined action 

 of the whole 24,000 lenses can present to the sensorium 

 but the idea of a single, xmdistorted, 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 colour 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 cor- 

 necB. 



Tou 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, you 

 will see, just in front of the point where they sep- 

 arate and their front outlines diverge, a minute cres- 

 cent-shaped cushion of a pale-green colour, at each 

 angle of which is a minute antenna. Close to the base 



