18 
rOPULAR SCIENCE REVIEW. 
sought for ill the particular orgauisatiou and habits of the 
animal than in its place in the animal series. In the insect 
the com])oimd eye presents us with the solution of a truly 
wonderful problem : namely, the construction of an apparatus 
of vision surpassing in accuracy and perfection that of multi- 
tudes of creatures superior to it in other attributes, yet "with so 
little expenditure of material as not to burden its diminutive 
and buoyant body or interfere with its powers of flight. And 
when we remember tliat its most rapid motion is still guided by a 
sight so keen as to precede muscular action, we cannot avoid 
the conclusion that its faculty of seeing is adapted to its habits 
of life, without any reference to its position in our artificial 
classifications. 
If, then, it be admitted that our conception of a seeing 
faculty should be physiologically one and the same for all 
organs of siglit ; if, also, the variations of anatomical structure 
can be reduced to one fundamental scheme of construction, it 
follows that we shall best understand this by tracing the points 
of identit}* and similarity of parts and functions than by exag- 
gerating apparent differences, and finding in these a proof that 
Nature, in arranging an organ of sight, has departed from her 
usual singleness of aim and means. 
Now the fundamental principle maybe stated thus: the pro- 
duction by physical means of an optical image of external 
objects ; and the direct contact of percipient nerve elements 
with this image. And the problem which the anatomist has to 
solve, is to discover the constructive plan by which an optical 
image is produced and brought into contact with the percipient 
element. Whether the plane of contact be found at the back 
or front of the eye, the principle and the final result remain the 
same. The anatomical positions we take up are these. 1. The 
conipound eye of Articulata is the ground-type of visual organ 
in these animals. 2. The simple eye (found with the compound 
eye on the same animal, as in insects, spiders, &c.), is a variety 
of the compound eye, and not, as J. Muller believed, constructed 
on the so-called vertebrate type. 3. In neither kind of eye 
is the physiological signification of retinal or lens apparatus 
so essentially distinct as to justify the hypothesis of opposed 
principles of optical construction or visual function. 
In the iletails which follow we shall continue to employ the 
same (k^signations for homologous parts of the insect eye as are 
in common use in the description of the vertebrate eye. Thus 
the coats of the eye will still be called sclerotic, corneal, choroid 
f with its ap|»endage — iris). The dioptric structures (crystalline 
lens, vitreous humor), and the nerve structures (optic nerve 
trunk and fibres and retinal elements) will still receive the same 
flesignation wherever they are found, however modified. And 
first of tlie Cornea, 
