284 Memoir upon the covipnund 



ourselves that it is not a real movement. It seems only to take 

 place, because the point being ohvays fixed, and in the middle 

 of the eye, consequently answering to all the facets of the cornea, 

 on whatever side we observe each facet, the latter being capable 

 of making it what it is, there results that kind of movement 

 which seems to be in the black point or the eye-ball, but which 

 depends on that made by the observer. 



The size of the compound eyes is worthy of being noticed. 

 This size is variable when we compare it with that of the body, 

 but it seems always more considerable in this respect than in 

 the vertebral animals. We observe in fact, in insects, that the 

 greatest dimension of the comjjound eyes, in comparison with 

 the body, is as 4 to 1, the medium as 10 to 1, and the smallest 

 60 to 1 . I have found the latter proportion in two genera only, 

 and I consider it therefore as extremely rare. 



To conclude : the dimensions of which I have spoken, prove 

 liow large the eyes of insects are in proportion to their bodies. 

 If it is with difficulty that we can conceive how vision can be 

 effected in these insects, by means of their compound ej'cs, we 

 shall experience much more difficulty when we contemplate the 

 smooth eyes. These eyes arc still more simple than the latter; 

 they are even so simple, that we might fairly doubt if they per- 

 formed the function of eyes at all, if experience did not demon- 

 strate it in an evident manner. The greater number of the 

 smooth eyes present a convex form,Avhich is generally observed 

 in the coi-iiea of the compound eves also : sometimes, however, 

 the cornea of the smooth eves, instead of presenting this struc- 

 ture, is more or less concave. This last form may nevertheless 

 present some inconveniences in the sense of sight; for, if the light, 

 in passing from a rarer into a denser medium, meets a concave 

 surface, its raj s, however parallel they were, become divergent ; 

 a disposition not very favourable to a distinct vision. In fact, 

 80 far from the rays diverging too much, it would seem that the 

 optic nerve placed towards the centre of the cornea can scarcely 

 receive the impression of them. As to the plain surface which 

 some smooth eyes exhibit, although it is less favourable to a 

 greater affluence of the ravs of light, — it is not an obstacle to 

 vision, since the light passing from a rarer medium into a body, 

 which presents a piano-transparent surface, and a denser me- 

 dium, its rays will ap])roach the perpendicular at their point of 

 immersion. We may also observe that, the smaller the smooth 

 eyes are, tlie less plain are their surfaces : thus the mrmtes, for 

 instance, which have very small eyes, have them projecting in a 

 remarkable manner. The size of the smooth eyes is, besides, 

 very variable in the genera which are most known : thus, it is 

 scarcely possible to become acquainted with the connexion which 



exist* 



