THE COMPOUJ^D EYE OF INSECTS AND CHUSTACEA. 
19 
The general surface of the insect cornea is spherical and more 
or less complete according to the size of the eye. And the an- 
terior and posterior faces of the compound cornea are as a whole 
parallel (see Plate XXXVII. fig. 1, Plate XXXYIII. fig. 9). But 
in many insects (see figs. 3, 6, dragon-fly) the anterior surface is 
partitioned into a varying number of minute facets — four-sided 
in Crustacea, six-sided in insects (with unimportant exceptions). 
The number and shape of these are however of little signi- 
ficance, for four- and six-sided facets may be seen on the same 
cornea, whilst in beetles, butterflies and other insects a deposit of 
pigment at the angles formed by the sides leaves only a central 
circular clear space for transmission of light. A fact far more 
important is the convex lens shape of the anterior or posterior faces 
of the facetted cornea. This convexity is most strongly marked 
on the posterior faces. On the anterior face it is mostly slight 
(fig. 15, Hymenopterous insect, figs. 3, 6, dragon-fly, show it 
more distinctly). On the posterior surface corresponding to 
facets in front, the curve is often almost hemispherical (see figs. 
13, 14, Coleopterous insects) ; but it is not so strong in Diptera 
(common fly) or in Hemiptera (notonecta). In beetles an an- 
terior and posterior curve is found. In certain Crustacea 
(Herbstia, fig. 12, Ilia, Lambrus) the posterior curve is strong, 
but in the cray-fish (fig. 9) it is flat. Without multiplying 
examples, it may be stated that the corneal facets of almost all 
insects present either an inner or outer curve, sometimes both. 
The optical significance of this fact is all-important; for it fol- 
lows that every corneule * of a compound cornea produces a dis- 
tinct focal convergence of rays, just as a plano-convex or double 
convex lens does. That is to say, an optical image is formed by 
each corneule, and on looking through a piece of compound 
cornea we see as many separate images as there are facets. 
Thus, an old writer remarks, on looking at a man through a 
piece of insect cornea we see an army of dwarfs ! Under such 
circumstances mosaic vision ” (see ante) is simply impossible. 
But how does the case stand with the simple insect eye ? J. 
Muller was led by his investigations to conclude that the simple 
insect eye closely resembled the eye of a fish. He describes the 
corneal surfaces as being plain, with their outer and inner faces 
parallel : behind this a globular crystalline lens, which however 
he expressly states to be adherent to the posterior surface of the 
cornea. Later researches have shown this description to be 
erroneous. The large globular ^Hens” of the simple eye is 
* This term is employed to denote each small segment of corneal substance 
corresponding to a facet and lying between the anterior and posterior boun- 
daries of its thickness. The corneule is at once understood by looking at the 
section of a cornea. 
