enters the bottom of the eye, not in the centre or middle, but a little on one fide; not in the point where the axis of the eye meets the retina, but between that point and the nose.The difference which this makes is, that no part of an object is unperceived by both eyes at the fame time.

In confidering vifion as achieved by the means of an image formed at the bottom of the eye, we can never reflect without wonder upon the smalluefs, yet correctness, of the picture, the fubtility of the touch, the fineness of the lines. A landscape of five or fix fquare leagues is brought into a space of half an inch diameter; yet the multitude of objects which it contains are all preserved; are all difcriminated in their magnitudes, pofitions, figures, colours. The profpect from HampfteadHill is compreffed into the compass of a fixpence, yet circumftantially represented. stage coach travelling at its ordinary speed for half an hour, paffes, in the eye, only over onetwelfth of an inch, yet is this change of place in the image diftinctly perceived throughout its whole progrefs; for it is only by means of that perception that the motion of the coach itself is made fenfible to the eye.

A

If any

thing can abate our admiration of the smallnefs of the visual tablet compared with the extent of vision, it is a reflection, which the view of nature leads us, every hour, to make, viz. that, in the hands of the Creator, great and little are nothing.

Sturmius held, that the examination of the eye was a cure for atheism. Befide that conformity to optical principles which its internal conftitution displays, and which alone amounts to a manifestation of intelligence having been exerted in its ftructure; befide this, which forms, no doubt, the leading character of the organ, there is to be feen, in every thing belonging to it and about it, an extraordinary degree of care, an anxiety for its prefervation, due, if we may fo fpeak, to its value and its tenderness. It is lodged in a strong, deep, bony focket, compofed by the junction of feven different bones*, hollowed out at their edges. In some few species, as that of the coatimondi †, the orbit is not bony throughout; but whenever this is the cafe, the upper, which is the deficient part, is fupplied by a cartilaginous ligament; a fubftitution which fhews the fame care.

Within this focket it is imbed

*Heifter, fect. 89. Mem. R. Ac. Paris, p. 117.

ded in fat, of all animal fubftances the best adapted both to its repofe and motion. It is sheltered by the eyebrows, an arch of hair, which, like a thatched penthoufe, prevents the fweat and moisture of the forehead from running down into it.

But it is ftill better protected by its lid. Of the fuperficial parts of the animal frame, I know none which, in its office and ftructure, is more deserving of attention than the eyelid. It defends the eye; it wipes it; it closes it in fleep. Are there, in any work of art whatever, purposes more evident than those which this organ fulfills; or an apparatus for executing thofe purposes more intelligible, more appropriate, or more mechanical? If it be overlooked by the observer of nature, it can only be because it is obvious and familiar. This is a tendency to be guarded againft. We pafs by the plainest instances, whilst we are exploring those which are rare and curious; by which conduct of the understanding, we fometimes neglect the ftrongeft obfervations, being taken up with others, which, though more recondite and fcientific, are, as folid arguments, entitled to much lefs confideration. In order to keep the eye moift and clean,

6

which

which qualities are neceffary to its brightness and its use, a wash is conftantly supplied by a fecretion for the purpose; and the superfluous brine is conveyed to the nofe through a perforation in the bone as large as a goofe quill. When once the fluid has entered the nofe, it spreads itself upon the infide of the noftril, and is evaporated by the current of warm air, which, in the course of respiration, is continually paffing over it. Can any pipe or outlet for carrying off the wafte liquor from a dyehouse or a distillery, be more mechanical than this is? It is eafily perceived that the eye must want moisture; but could the want of the eye generate the gland which produces the tear, or bore the hole by which it is discharged-a hole through a bone?

It is obfervable that this provifion is not found in fish, the element in which they live supplying a conftant lotion to the eye.

It were, however, injuftice to dismiss the eye as a piece of mechanifm, without noticing that most exquifite of all contrivances, the niclitating membrane, which is found in the eyes of birds and of many quadrupeds. Its ufe is to fweep the eye, which it does in an inftant; to ipread over it the lachrymal humor; to defend it alfo from

D 3

from fudden injuries; yet not totally, when drawn upon the pupil, to fhut out the light. The commodioufnefs with which it lies folded up in the upper corner of the eye, ready for use and action, and the quickness with which it executes its purpose, are properties known and obvious to every obferver; but, what is equally admirable, though not quite fo obvious, is the combination of two different kinds of fubftance, mufcular and elaftic, and of two different kinds of action, by which the motion of this membrane is performed. It is not, as in ordinary cafes, by the action of two antagonist muscles, one pulling forward and the other backward, that a reciprocal change is effected; but it is thus: The membrane itself is an elaftic fubftance, capable of being drawn out by force like a piece of elaftic gum, and by its own elasticity returning, when the force is removed, to its former pofition. Such being its nature, in order to fit it up for its office it is connected by a tendon or thread with a mufcle in the back part of the eye: this tendon or thread, though ftrong, is so fine, as not to obftruct the fight, even when it paffes across it; and the muscle itself being placed in the back part of the eye, derives from its fituation the advantage, not

only