112 Memoir upon the compound 



ascertained, on the difference in thickness and the various co- 

 lours of the humour which adheres to it. 



Under the cornea is seen a conduit or pipe, not very liquid, 

 not soluble in water, and strongly adhering to this membrane. 

 Its colour in general is between the darkest violet and the deepest 

 black. When this coating presents this colour, which is most 

 frequently the case, it is almost imjjossible to distinguish it from 

 the humour of the choroid. This is not the case in the eyes 

 the coating of which is red or green, or of both colours united. 

 We then see very distinctly the coating with its variegated 

 colours, and the humour of the dioroid with its black tinge 

 which never varies. This arrangement is very conspicuous in 

 the locusla gigantea, lilifolia, the UheUula vulgaris, and the 

 greater number of the tahani. It is also very striking in the 

 gryllus lineola, the eyes of which appear to be streaked with 

 brown and green bands, and which is indebted for this singu- 

 larity to the coating of tiie cornea being alternately brown and 

 green, and that by nearly parallel bands. 



It is therefore to the mixture of the tints of the tunic of the 

 cornea that the variegated colours presented to us by the eyes 

 of insects are owing, preciselv in the same way as from its va- 

 rious degrees of thickness and colouring the variegated stripes 

 and marble tint arise which appear at the exterior part of the 

 eye, and which might have been thought peculiar to the cornea. 

 Frequently, as a consequence of this arrangement, one and the 

 same eye presents spots and stripes of A'arious colours, or even 

 one side of a colour totally different from the other. 



The coating of the cornea, therefore, covers all the internal 

 surface of this membrane. Its thickness and consistence, like 

 its opacity, are very much subject to variation, as already ob- 

 served : but it woidd seem that, in general, the more this tunic 

 is opaque, the thicker and broader are the nervous threads which 

 pass through it, in order perhaps that its opacity may not be an 

 obstacle to vision. 



We are under the necessity of anticipating a little upon our 

 description, and of speaking here of the optic nerves, which, fur- 

 nished by the retina, pass through the choroid and its Irumour, as 

 does the coating of the cornea, in coming into correspondence with 

 the facets of the latter membrane. This arrangement is not the 

 same in the species which present bands or stripes, and in those 

 ■which want them. If \\c cavefully remove the cornea, and in such 

 a manner as to remove very little of its coating, we observe, in the 

 species in which the compound eye presents stripes externally, 

 this coating to be (composed of very distinct rays, one of which 

 is blackish and the other much deeper, and so on alternately. 

 These two colours are far from being uniform j and however 

 "^ slightly 



