INSECTS : THEIR EARS AND EYES. 167 



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 discern 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 matter, polished and trans- 

 parent as crystal. There are, according to the com- 

 putations 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. 



Every one of these 24,000 bodies represents a perfect 

 eye ; every one is furnished with all the apparatus and 

 combinations requisite for distinct 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 dissection. 



The glassy convex plate or facet in front of each 

 hexagon is a cornea or corneule, as it has been called. 

 Behind each cornea, instead of a crystalline lens, there 

 descends a slender transparent pyramid, whose base is 

 the cornea, and whose apex points towards the interior, 

 where it is received and embraced by a translucent cup, 

 answering to the vitreous humour. This, in its turn, is 

 surrounded by another cup, formed by the expansion of a 



