enters the bottom of the eye, not in the centre or middle, but a little on one side ; 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 considering vision as achieved by the means of an image formed at the bottom of the eye, we can never reflect without wonder upon the smallness, yet correctness, of the picture, the subtility of the touch, the fineness of the lines. A landscape of five or six square leagues is brought into a space of half an inch diameter; yet the multitude of objects which it contains are all preserved; are all discriminated in their magnitudes, positions, figures, colours. The prospect from HampsteadHill is compressed into the compass of a sixpence, yet circumstantially represented. A stage coach travelling at its ordinary speed for half an hour, passes, in the eye, only over onetwelfth of an inch, yet is this change of place in the image distinctly perceived throughout its whole progress; for it is only by means of that perception that the motion of the coach -itself is made sensible to the eye. If any -.> thing thing can abate our admiration of the smallness 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. Beside that conformity to optical principles which its internal constitution displays, and which alone amounts to a manisestation of intelligence having been exerted in its structure; beside this, which forms, no doubt, the leading character of the organ, there is to be seen, in every thing belonging to it and about it, an extraordinary degree of care, an anxiety for its preservation, due, if we may so speak, to its value and its tenderness. It is lodged in a strong, deep, bony socket, composed by the junction of seven different bones *, hollowed out at their edges. In some sewfpecics,as that of the coatimondi f, the orbit is not bony throughout; but whenever this is the case, the upper, which is the deficient part, is supplied by a cartilaginous ligament; a substitution which Ihews the fame care. Within this socket it is imbed

* leister, sect. 89. f Me«n. R. Ac. Paris, p. 117.

-' ll" D 2 ded ded in fat, of all animal substances the best adapted both to its repose and motion. It is sheltered by the eyebrows, an arch of hair, which, like a thatched penthouse, prevents the sweat and moisture of the forehead from running down into it.

But it is still better protected by its lid. Of the superficial parts of the animal frame, I know none which, in its office and structure, is more deserving of attention than the eyelid. It defends the eye; it wipes it; it closes it in sleep. Are there, in any work of art whatever, purposes more evident than those which this organ fulfills; or an apparatus for executing those 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 against. We pass by the plainest instances, whilst we are exploring those which are rare and curious; by which conduct of the understanding, we sometimes neglect the strongest observations, being taken up with others, which, though more recondite and scientific, are, as solid arguments, entitled to much less consideration.

In order to keep the eye moist and clean, 6 which which qualities are necessary to its brightness and its use, a wash is constantly supplied by a secretion for the purpose; and the superfluous brine is conveyed to the nose through a perforation in the hone as large as a goose quill. When once the fluid has entered the nose, it spreads itself upon the inside of the nostril, and is evaporated by the current of warm air, which, in the course of respiration, is continually passing over it. Can any pipe or outlet for carrying off the waste liquor from a dyehouse or a distillery, be more mechanical than this is? It is easily 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 observable that this provision is not found in fish, the element in which they live supplying a constant lotion to the eye.

It were, however, injustice to dismiss the eye as a piece of mechanism, without noticing that most exquisiteof all contrivances,the nictitating membrane, which is found in the eyes of birds and of many quadrupeds. Its use is to sweep the eye, which it does in an instant; to spread over it the lachrymal humor; to defend it also D 3 from from sudden injuries; yet not totally, when drawn upon the pupil, to shut out the light. The commodiousness 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 observer; but, what is equally admirable, though not quite so obvious, is the combination of two different kinds of substance, muscular and elastic, and of two different kinds of action,by which the motion of this membrane is performed. It is not, as in ordinary cases, by theaction of twoantagonist muscles, one pulling forward and the other backward, that a reciprocal change is effected; but it is thus: The membrane itself is an elastic substance, capable of being drawn out by force like a piece of elastic gum, and by its own elasticity returning, when the force is removed, to its former position. Such being its nature, in order to fit it up for its office it is connected by a tendon or thread with a muscle in the back part of the eye: this tendon or thread, though strong, is so fine, as not to obstruct the sight, even when it passes across it; and the muscle itself being placed in the back part of the eye, derives from its situation the advantage, not


« VorigeDoorgaan »