VISION. 



487 



of the Libellula viilgata, or grey Dragon-fly, 

 shown in Fig, 424, aided by the highly magni- 

 fied views of smaller portions given in the suc- 

 ceeding figures, in all of which the same 

 letters of reference are used to indicate the same 

 objects.* The whole outer layer (cc) of the 



422 



mwmMw 





423 



424 



^yy 



compound eye may be considered as corres- 

 ponding to the cornea : each separate division of 

 which has been termed a Conieide, being com- 

 posed of a horny and perfectly transparent 

 material. Each corneule (c) has the form of a 

 truncated pyramid, the length of which (l) is 

 between two and three times the diameter of the 

 base (b). The outer surface (b) is very convex ; 

 but the internal, or truncated end (d) is con- 

 cave ; and the concavity of the latter being 



* These figures, as well as the account of the anatomy of the 

 eye of the Libellula, are taken from the memoir of Duges, in the 

 Annates des Sciences Naturelles, xx. 341. 



