196 EVENINGS AT THE MICROSCOPE 



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 con- 

 vex 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. 



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 hex- 

 agon 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 toward the interior, where it is 

 received and embraced by a translucent cup, answering 

 to the vitreous humor. This, in its turn, is surrounded by 

 another cup, formed by the expansion of a nervous fila- 

 ment arising from the ganglion on the extremity of the 

 optic nerve, a short distance from the brain. Each lens- 

 like pyramid, with its vitreous cup and nervous filament, 

 is completely surrounded and isolated by a coat (the 

 choroid) of dark pigment, except that there is a minute 

 orifice or pupil behind the cornea, where the rays of light 

 enter the pyramid, and one at the apex of the latter, where 

 they reach the fibres of the optic nerve. 



