168 EVENINGS AT THE MICROSCOPE. 



nervous filament arising from the ganglion, or knot, 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. 



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, flattened 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 distinctly seen, on a proper 

 adjustment of the focus of the microscope, in every one 

 of the lenses whose line of axis admitted of it. The focus 

 of each cornea has been ascertained by similar experi- 

 ments to be exactly equal to the length of the pyramid 

 behind it, so that the image produced by the rays of light 

 proceeding from any external object, and refracted by the 

 convex cornea, will fall accurately upon the sensitive 

 termination of the optic nerve-filament there placed to 

 receive it. 



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 corner 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 



