INSECTS : THEIE EAKS AND EYES. 193 



liant, or a larger example, than is presented by this fine 

 Dragon-fly (-(^sAwa), which I just now caught as it was 

 hawking to and fro in my garden. How gorgeously 

 beautiful are these two great hemispheres that almost 

 compose the head, each shining with a soft satiny lustre 

 of azure hue, surrounded by olive-green, and marked 

 with undefined black spots, which change their place 

 as you move the insect round ! 



Each of these hemispheres is a compound eye. I 

 put the insect in the stage-forceps, and bring a low 

 power to bear upon it with reflected light. You see 

 an infinite number of hexagons, of the most accurate 

 symmetry and regularity of arrangement. Into those 

 which are in the centre of the field of view, the eye 

 can penetrate far down, and you perceive that they are 

 tubes ; of those which recede from the centre, you dis= 

 cern more and more of the sides ; while, by delicate 

 adjustment of the focus, you can see that each tube is 

 not open, but is covered with a convex arch, of some 

 glassy medium polished and transparent as crystal. 

 There are, according to the computations of accurate 

 naturalists, not fewer than 24,000 of these convex 

 lenses in the two eyes of such a large species of Dragon- 

 fly as this. 



Evesry one of these 24,000 bodies represents a per- 

 fect eye ; every one is furnished with all the apparatus 

 and combinations requisite for distract vision ; and 

 there is no doubt that the Dragon-fly looks through 

 them all. In order to explain this, I must enter into 

 a little technical explanation of the anatomy of the 

 organs, as they have been demonstrated by careful dis- 

 section. 



The glassy convex plate or facet in front of each 

 9 



