OF INSECTS. 
187 
a cornea ; beneath this there is a layer of opaque 
matter, varying in colour, hut most commonly black, 
deep violet, red, or green, which produces the brilliant 
spots and bauds on the eyes of Tabanidse and other 
flies ; lying below this a dark-coloured varnish, which 
may be considered as a choroid : numerous air vessels 
are supplied to the last mentioned part, and there is 
a space beneath for receiving some of the ramifications 
of the optic nerve. According to Muller, who has 
been most successful in explaining the structure of 
these organs, each individual facet can survey but a 
small space of the entire field of vision, so that each 
contributes to tho perception of all the objects within 
the field ; but each separate one does not at the same 
time see all such objects, whence the insect must 
receive as many forms of objects in its eye, as there 
are individual facets to the eye. This consequence 
of a common and yet subsidiary vision of these facets, 
springs partly from the immobility of the eyes, and 
partly from the circumstance that only those rays of 
light which fall in a right line upon a facet of the 
eye, which itself forms the segment of a circle, can 
reach the optic nerve of this facet, whereas all others 
are withheld by the pigment which partly separates 
the individual glass lenses from each other, and partly 
surrounds the margin of the chrystalline lens, beneath 
the cornea. From this it follows, that the nearer 
the object is the more obliquely do all, but the per- 
pendicular rays of light, fall upon the facet ; and, 
therefore, contribute so much the less to the produc- 
tion of the image; the object consequently is most 
