VISION. 



435 



of each of these bases ov facets, as they are called, 

 is that of a hexagon, a form which admits of their 

 uniform arrangement with the greatest economy of 

 space, Hke the cells of a honeycomb ; and tlie 

 hexagonal divisions of the surface are very plainly 

 discernible on viewing the surface of these eyes with 

 a microscope ; especially as there is a thin layer 

 of black pigment intervening between each, like 

 mortar between layers of brick. The appearance 

 they present in the 3Ielolontha, when highly mag- 

 nified, is shown in Fig. 422.* The internal struc- 

 ture of these eyes will be best understood from the 



423 



424 



422 



mmmmmmrt 





MMMM 



section of that of the Lihellula vidgata, or grey 

 Dragon-fly, shown in Fig. 424, aided by the highly 



* In the Phalence, and other tribes, they are arranged in squares 

 (as shown in Fig. 423), instead of hexagons, and frequently much 

 less regularly; as must necessarily happen, in many parts, from the 

 curvature of the spherical surface. 



Mr. Shuckard has observed, that there is often a considerable 

 difference in the size of the different facets, even in the same com- 

 pound eye. This peculiarity may be seen in the figure of the eye 

 of the Tabanus, given by Hooke, in his " Micrographia." Mr. 

 Shuckard found a similarly well marked line of separation between 

 the large facets above and the small ones below it, in the eyes of 

 the male Bibio. In the Pangonia, the male Astata, and male 

 domestic Bee, the decrease in the size of the facets is gradual, and 

 from above downwards: they decrease posteriorly in the Pipunculus, 



