Fig. 1. The crystalline lens of a fish; it is proportionably larger than in other animals, and perfectly spherical.
Fig. 2. A section of the human eye. It is formed of various coats, or membranes, containing pellucid humours of different degrees of density, and calculated for collecting the rays of light into a focus, upon the nerve situated at the bottom of the eye-ball.
The external membrane, called sclerotic, is strong and firm, and is the support of the spherical figure of the eye: it is deficient in the centre, but that part is supplied by the cornea, which is transparent and projects like the segment of a small globe from one of a larger size. The interior of the sclerotic is lined by the choroid, which is covered by a dark mucous secretion, termed pigmentum nigrum, intended to absorb the superfluous rays of light.
The choroid is represented in the plate by the black line. The third and inner membrane, which is marked by the white line, is the retina, the expanded optic nerve.
Within these coats of the eye, are the humours. a, the aqueous humour, a thin fluid like water; b, the crystalline lens, of a dense texture; c, the vitreous humour, a very delicate gelatinous substance, named from its resemblance to melted glass. Thus the crystalline is more dense than the vitreous, and the vitreous more dense than the aqueous humour: they are all perfectly transparent, and together make a compound lens, which refracts the rays of light issuing from an object, d, and delineates its figure e, in the focus upon the retina, inverted.
Fig. 3. The lens of the telescope.
Fig. 4. The crystalline lens, or, as it has been called, the crystal line humour, of the eye.
Fig. 5, 6. A plan of the circular and radiated fibres which the iris is supposed to possess; the former contracts, the latter dilates the pupil, or aperture formed by the inner margin of the iris.
Fig. 7. a, a, a, a, the four straight muscles, arising from the bottom of the orbit, where they surround, c, the optic nerve; and are inserted by broad, thin tendons at the fore part of the globe of the eye into the tunica sclerotica.