20 
rOPULAR SCIENCE REVIEW. 
reall}" a projection of the inner corneal lamella?. : an excrescence, 
so to speak, of corneal substance : in fact, an exaggerated form 
of the inner curve of the corneule of the compound eye. M. 
Dujardin has well proved this in a memoir published in Annales 
des Sciences,” 1867 ; and Leydig has given figures showing it 
(figs. 7, 8). The cornea lenses therefore of the simple and 
compound eye are not exactly the same as the crystalline lens 
of the vertebrate eye. But they perform the same function 
and rank as analogous parts. The crystalline lens of the verte- 
brate e}"e is indeed developed from cuticular cells, and though 
a more perfectly differentiated structure, and separated from the 
cornea (which is also a cuticular mass metamorphosed into 
chitin in the insect eye, while in the vertebrate eye it retains 
traces of its cellular origin), is homologically almost idepitical 
with the corneal lenses of insects. Thus, one of the most 
striking differences between the two types is on closer examina- 
tion reduced to a variety of the same structural elements, the 
essential character of its functions being identical. 
The result of this variation in the disposition of corneal lenses 
is certainly remarkable : for in the compound eye a multiplica- 
tion of images is the consequence of its facetted arrangement, 
v.’hilst in the simple eye a single large corneal lens admits of the 
focal concentration of collective pencils of light upon the nerves 
behind it. AYhere, however, simple eyes (which are much 
smaller than the compound eye) are grouped together in one 
spot commanding the same held of vision, multiplication of 
images must occur so that the optical phenomena are similar. 
The reduction of multiple images into one mental picture is, 
however, a fact common to all animals, and not simply charac- 
teristic of the invertebrate eye. Single vision with two eyes is 
the most obvious and striking fact in our own experience. 
Bespecting the structure of the cornea little need be added. 
It is chitinised skin, or rather epidermis, its original cell 
elements being lost during the process of metamorphosis. The 
cornea shows nevertheless traces of a lamellar structure, as indi- 
cated by the hne horizontal lines running parallel with the 
surface (see figs. 3 and 6). Vertical lines more strongly marked 
(see same hgs.) divide the cornea into vertical segments, the 
anterior and posterior faces of which are bounded by the plane 
or curved facets, and each segment thus bounded is conveniently 
designated a corneule. Fig. 2 shows a surface view, and fig. 6 
a section in which the curve of the exterior facet is well seen : 
tlie interior facet is in tliis instance a plane surface, and therefore 
the wliole i)osterior surface of the cornea forms a continuous 
Kinooth curve. In different insects the thickness of the cornea 
varies greatly. Figs. 13 and 14 sliow a thick cornea with flat 
front facet and convex inner facet (plano-convex lens). Figs. 
