INTRODUCTION, 3 



M. Lacordaire, in his " Introduction a I'Entomologie," from 

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

 up as follows, the manner in which, according to M. 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 pigment 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 convexity. But what- 

 ever 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 IM. 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 between the lenses, the more distinct 

 will be the image of objects at a distance, and the less distinct 

 that of objects near. With the latter the luminous rays diverge 

 considerably; while those from the former are more parallel. In 

 the first case, in traversing the pigment, they impinge obliquely 

 on the crystalline, and consequently confuse the vision; in the 

 second, they fall more perpendicularly on each facette. 



" Objects do not appear of the same size to each optic filament, 

 unless the eye is a perfect section of a sphere, and its convexity 

 concentric with that of the optic nerve. Whenever it is otherwise, 

 the image con'esponds more or less imperfectly with the size of 

 the object, and is more or less incorrect. Hence it follows, that 



B 2 



