THE COMPOUND EYE OF INSECTS AND CRUSTACEA. 
15 
physical conditions of organic no after, and the physiological 
laws of animal sensation, which obtained on the surface of our 
globe at a time when no human eye had been formed ; and this 
with as much mathematical certainty as if we were drawing 
conclusions from the preparations of insect and crustacean eyes 
lying on our table, or studying diagrams from the latest work 
on physiological optics. 
The term compound,” as applied to the eyes of articulata, 
is at once significant of the most important modifications of 
structure met with in these animals, and of the most remark- 
able differences of opinion held by physiologists respecting 
the modus operandi of the organs in question. An entirely 
satisfactory definition of the compound eye is, therefore, scarcely 
possible, so long as its structure and function remain subject to 
dispute on all sides. As it is advisable to avoid entering into 
a controversy which still agitates the scientific world, and to 
limit our remarks as far as possible to the positive (i.e. the 
anatomical) side of our subject, we shall content ourselves with 
a very brief reference to the general doctrines which bear on 
the explanation of the several modes of vision. 
To place our readers properly en rapport with the doctrine 
at present held respecting the vision of animals possessing 
compound eyes, we must advert to the theory propounded by 
J. Muller, and still almost universally taught : namely, that 
the type of construction (and the optical principle on which it 
is based) differs radically from that of the eye of man and verte- 
brates generally. The vertebrate type is represented by a 
hollow globe formed by membranes commonly called tunics or 
coats of the eyeball. The interior of this globe is occupied by 
a large central mass of transparent substance (the vitreous 
humor), in front of which is placed a crystalline lens with a 
moveable diaphragm or iris. Light is admitted into the interior 
of the eye through the front transparent portion of its outer 
coat (cornea), and, passing through the crystalline lens, is con- 
verged to a focus near the centre of the eyeball ; but the rays 
cross at this point, and are transmitted through the vitreous 
humor in a diverging course coincident with the radii which fall 
upon the inner (concave) surface of the inner coat of the eye 
(choroid coat) from an imaginary centre, which closely corre- 
sponds with the focus of converging rays admitted through the 
cornea in front. At the back of the eye, where the inverted 
rays of light, after traversing the vitreous humor, fall on the 
inner coat, lies, interposed between the convex surface of the 
vitreous humor and the concave (inner) surface of the choroid 
coat, a membrane-like expansion of nerve fibres with associated 
elements composing the retina. This retina, therefore, is in 
direct contact with the rays of light that have traversed the 
