OF INSECTS. 187 



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 bands on the eyes of Tabanidse and other 

 lies ; lying below this a dark-coloured varnish, which 

 may be considered as a choroid : numerous air vessels 

 ire supplied to the last mentioned part, and there is 

 i space beneath for receiving some of the ramifications 

 the optic nerve. According to Miiller, who has 

 [>een 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 the 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 



