INTRODUCTION. 3 



the part of the crystalline, or lens, in the eyes of animals, They 

 are straight and parallel with each other. A pigment fills all the 

 spaces between the cones, and between the nervous filaments, 

 and covers the underside of each cornea, except at the centre. 

 This pigment varies much in colour. There are almost always 

 two layers, of which the exterior one is the more brilliant. In 

 fact, these eyes often sparkle with fire, like precious stones. 



M. Lacordaire, in his "Introduction a rEntomologie," from 

 which we borrow the greater part of this information, has summed 

 up as follows, the manner in which, according to Miiller, the 

 visual organs of insects operate : 



"Each facette with its lens and nervous filament, separated 

 from those surrounding them by the pigment in which they are 

 enclosed, form an isolated apparatus, impenetrable to all rays of 

 light, except those which fall perpendicularly on the centre of 

 the facette, which alone is devoid of pigment. All rays falling 

 obliquely are absorbed by that which surrounds the gelatinous 

 cone. It results partly from this and partly from the immobility 

 of the eye that the field of vision of each facette is very limited, 

 and that there are as many objects reflected on the optic filaments 

 as there are corneae. The extent, then, of the field of vision will be 

 determined, not by the diameter of these last, but by the diameter 

 of the entire eye, and will be in proportion to its size and con- 

 vexity. But whatever may be the size of the eyes, like their fields 

 of vision, they are independent of each other ; there is always a 

 space, greater or less, between them ; and the insect cannot see 

 objects in front of this space without turning its head. What a 

 peculiar sensation must result from the multiplicity of images 

 on the optic filaments ! This is not more easily explained than 

 that which happens with animals which, having two eyes, see 

 only one image ; and probably the same is the case with insects. 

 But these eyes usually look in opposite directions, and should see 

 two images, as in the chameleon, whose eyes move independently 

 of each other. The clearness and length of vision will depend, 

 continues M. Miiller, on the diameter of the sphere of which 

 the entire eye forms a segment, on the number and size of the 

 facettes, and the length of the cones or lenses. The larger each 

 facette, taken separately, and the more brilliant the pigment placed 



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