194 EVENINGS AT THE MICHOSCOPE. 



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 vil/reous humov/r. This, in its 

 turn, is surrounded by another cup, formed by the ex- 

 pansion of a nervous filament arising from the ganglion 

 on the extremity of the optic nerve, a short distance 

 from the brain. Each lens-like pyramid, with its vit- 

 reous cup and nervous filament, is completely sur- 

 rounded and isolated by a coat (the choroid) of dark 

 pigment, except that there is a minute orifice q^ pwpil 

 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. 



Each cornea is a lens with a perfect magnifying 

 power, as has been proved by separating the entire 

 compound eye by maceration, and then drying it, flat- 

 tened out by pressure, on a slip of glass. "When this 

 preparation was placed under the microscope, on any 

 small object, as the points of a forceps, being interposed 

 between the mirror and the stage, its image was dis- 

 tinctly seen, on a proper adjustment of the focus of the 

 microscope, in every one of the lenses wliose line of 

 axis admitted of it. The focus of each cornea has been 

 ascertained by similar experiments to be exactly equal 

 to the length of the pyramid behind it, so that tbe 

 image produced by the rays of light proceeding from 

 any external object, and refracted by the convex 

 cornea, will fall accurately upon the sensitive termina- 

 tion of the optic nerve-filament there placed to receive 

 it. 



