282 Memoir upon the compound 



are so decided, that the eye appears externally as if formetl 

 by two distinct parts. Should this effect take place on account 

 of the great convexity of the cornea, and even of ti-fi position of 

 the eye of this s])ecies ? Foj- this position and this convexity 

 render the approach of the rays of light easier in the upper part- 

 than in the lower, because in this part the eye is perpendicular 

 with respect to the rays, whereas the lower is on the contrary 

 very oblique with respect to them. We also observe that the 

 upper part presents a red colour — a colour which has less re- 

 frangibility than the green of the lower part, and wliich also gives 

 less light. The lower part, to which the approach of the rays 

 of light was very difficult, aUo presents a green colour, which has 

 more refrangibility than red, and which also gives a greater in- 

 tensity of light, since, according to the elegant experiments of 

 Herschel, green rays are almost as brilliant as yellow. Thus, the 

 red, sending back rays less brilliant than the green, and being in 

 the point of the eye where the rays of light arrive most easily, 

 would have been placed in this part to absorb, by its greater 

 opacity, the excess of these very rays, which must have injured 

 by their too great excitability the optic nerves. The green, on 

 the contrary, presenting more refrangibility, and a greater in- 

 tensity of light, would augment the excitability of the optic 

 nerves in the lower part, and in this way correct the difficulty 

 ivhich the rays niiglit experience in reaching this part of the 

 eye. It is possible that what we have now said may be well 

 founded : however this may be, we can only regard the explana- 

 tion of this fact as proljable. 



Th.is arrangement is also visible in the locusta gigantea, iif 

 which the eye presents externally all its upper half of a reddish 

 brown, and its lower half of a very lively green. The centre of 

 the eye, i. e. the intermediate part between those two shades, is 

 of a bright red. These various shades are o^ying to the diversity 

 of colour in the tunic of the cornea. We may even remark, that 

 in proportion as the green tint approaches the lower part of 

 the eye, it takes a clearer shade, and verges more and more to- 

 wards the white. As to the smooth eyes, this species has only 

 two very small convex ones of a slight red, and situated on the 

 crown of the head lietween the compound eyes, and above and 

 in front of the antenna:. 



There are some cnleopterce, like the llaps, the pcdimis, and 

 certain tevehriones, which are completely lucifuge, and which^, 

 like the blatla, cannot support daylight. Thus, on exposing 

 the species of these difiereut geneva to a strong- light, they are 

 seen to shun it by lowering their heads. Insects being deprived 

 of the pupil, the explanation which has been given of this phie- 

 Bomenon in the nocturnal animals cannot be appllcai)le to them. 



Thus, 



