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produce, not by a simple action or effect, but by a combination of actions and effects, the result which is ultimately wanted. And forasmuch as this organ would have to operate under different circumstances, with strong degrees of light and with weak degrees, upon near objects, and upon remote ones, and these differences demanded, according to the laws by which the transmission of light is regulated, a corresponding diversity of structure; that the aperture, for example, through which the light passes, should be larger or less; the lenses rounder or flatter, or that their distance from the tablet, upon which the picture is delineated, should be shortened or lengthened: this, I say, being the case, and the difficulty to which the eye was to be adapted, we find its several parts capable of being occasionally changed, and a most artificial apparatus provided to produce that change. This is far beyond the common regulator of a watch, which requires the touch of a foreign hand to set it; but it is not altogether unlike Harrison's contrivance for making a watch regulate itself, by inserting within it a machinery, which, by the artful use of the different expansion of metals, preserves the equability of the motion under all the various temperatures of heat and cold in which the instrument may happen to be placed. The ingenuity of this last contrivance has been justly praised. Shall, therefore, a structure which differs from it, chiefly by surpassing it, be accounted no contrivance at all 2 or, if it be a contrivance, that it is without a contriver? But this, though much, is not the whole: by different species of animals the faculty we are describing is possessed, in degrees suited to the different range of vision which their mode of life, and of procuring their food, requires. Birds, for instance, in general, procure their food by means of their beak; and, the distance between the eye and the point of the beak being small, it becomes necessary that they should have the power of seeing very near objects distinctly. On the other hand, from being often elevated much above the ground, living in air, and moving through it with great velocity, they require, for their safety, as well as for assisting them in descrying their prey, a power of seeing at a great distance; a power, of which, in birds of rapine, surprising examples are given. The fact accordingly is, that two peculiarities are found in the eyes of birds, both tending to facilitate the change upon which the adjustment of the eye to different distances depends. The one is a bony, yet, in most species, a flexible rim or
hoop,” [Plate III. fig. 1, 2.] surrounding the broadest part of the eye; which, confining the action of the muscles to that part, increases the effect of their lateral pressure upon the orb, by which pressure its axis is elongated for the purpose of looking at very, near objects. The other is an additional muscle, called the marsupium, [Plate III. fig. 3, 4, 6.] to draw, upon occasion, the crystalline lens back, and to fit the same eye for the viewing of very distant objects By these means, the eyes of birds can pass from one extreme to another of their scale of adjustment, with more ease and readiness than the eyes of other animals. The eyes of fishes also, compared with those of terrestrial animals, exhibit certain distinctions of structure adapted to their state and element. We have already observed upon the figure of the crystalline compensating by its roundness the density of the medium through which their light passes. To which we have to add, that the eyes of fish, in their natural and indolent state, appear to be adjusted to near objects, in this respect differing from the human eye, as well as those of quadrupeds and birds. The ordinary shape of the fish's eye being in a much higher degree convex than that of land animals, a corresponding difference attends its muscular conformation, viz. that it is throughout calculated for flattening the eye. The iris also in the eyes of fish does not admit of contraction. This is a great difference, of which the probable reason is, that the diminished light in water is never too strong for the retina. In the eel, [Plate III. fig. 5.] which has to work its head through sand and gravel, the roughest and harshest substances, there is placed before the eye, and at some distance from it, a transparent, horny, convex case or covering, which, without obstructing the sight, defends the organ. To such an animal, could anything be more wanted, or more useful? Thus, in comparing the eyes of different kinds of animals, we see, in their resemblances and distinctions, one general plan laid down, and that plan varied with the varying exigencies to which it is to be applied. There is one property, however, common, I believe, to all eyes, at least to all which have been examined,’ namely, that the optic nerve enters the bottom of the eye, not in the Centre Or Illi (1(lie 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 same 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 subtilty 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, colors. The prospect from Hampstead-hill 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 one-twelfth 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 anything 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 he hands of the Creator, great and little are nothing. Sturmius held, that the examination of the eye was a cure for atheism. Besides that conformity to optical principles which its internal constitution displays, and which alone amounts to a manifestation of intelligence hav ing been exerted in the structure; besides this, which forms
* The flexible rim, or hoop, consists of bony plates, which in all birds occupy the front of the sclerotic; lying close together and overlapping each other. These bony plates in general form a slightly convex ring, Fig. 1, but in the accipitres they form a concave ring, as in Fig. 2, the bony rim of a hawk. It is a principle in optics, that the rays of light, ing through a lens, will be refracted to a point or focus beyond the o and this focus will be less distant in proportion as the lens approaches to a sphere in shape. This principle is very naturally applied to the explanation of the use of this apparatus. These scales partly lying over each other, so as to allow of motion, will, on the contraction of the straight muscles inserted into and covering them, move over each other, and diminish the circle of the sclerotica; and thus the cornea, which is immediately within the circle made by these scales, must be pressed forwards and rendered more convex, from the focus of the eye becoming altered, oy its axis being elongated. This consequent convexity of the cornea renders small objects near the animal very distinct. Without this structure a bird would be continually liable to dash itself against tree" when lying in a thick forest, and would be unable to see the minot: objects on which it sometimes feeds.-Parton.
sut dis Ove! he of Inted
no doubt, the leading character of the organ, there is to be
* Heister, sect. 89. t Mem. of the R. Ac. Paris, p. 117.
# The muscles which accomplish these actions are seen in TAB. XIV. Fig. 1, 2. 'The eyelids also moderate the force of a too brilliant light, and exclude, by a partial closure, that excess of it which would offend the eye. The eyelashes have a similar office, that of regulating the quantity of light: and it is believed, that they protect the eye from the small particles of dust that float in the air.—Parton.
tinually passing over it. Can any pipe or outlet for carry ing off the waste liquor from a dye-house 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 exquisite of all contrivances, the nictitating membrane, which is found in the eyes of birds and of many quadrupeds. [Plate IV. fig. 2.] Its use is to sweep the eye, which it does in an instant; to spread over it the lachrymal humour; to defend it also 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 inner 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 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 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, [Plate IV. fig. 3, 4, and 5,] derives from its situation the advantage, not only of being secure, but of being out of the way; which it would hardly have been in any position that could be assigned to it in the anterior part of the orb, where its function lies. When the muscle behind the eye contracts, the membrane, by means of the communicating thread, is instantly drawn over the fore-part of it. When the muscular contraction (which is a positive, and, most probably, a voluntary effort,) ceases to be exerted, the elasticity alone of the membrane brings it back again