distance. Can any thing be more decifive of contrivance than this is? The moft fecret laws of optics must have been known to the author of a structure endowed with fuch a capacity of change. It is, as though an optician, when he had a nearer object to view, should rectify his inftrument by putting in another glass, at the fame time drawing out alfo his tube to a different length.

Obferve a new-born child first lifting up its eyelids. What does the opening of the curtain discover? The anterior part of two pellucid globes, which, when they come to be examined, are found to be constructed upon ftrict optical principles; the self-fame principles upon which we ourfelves conftruct optical inftruments. We find them perfect for the purpose of forming an image by refraction; compofed of parts executing different offices; one part having fulfilled its office upon the pencil of light, delivering it over to the action of another part; that to a third, and fo onward the progreffive action depending for its fuccefs upon the niceft, and minutest adjuftment of the parts concerned; yet, these parts fo in fact adjusted, as to produce, not by a fimple action or effect, but by a combina

tion of actions and effects, the refult which is ultimately wanted. And forafmuch as this organ would have to operate under different circumftances, with ftrong degrees of light, and with weak degrees, upon near objects, and upon remote ones, and thefe differences demanded, according to the laws by which the tranfmiffion of light is regulated, a correfponding diversity of structure; that the aperture, for example, through which the light paffes, fhould be larger or lefs; 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 fay, being the cafe and the difficulty, to which the eye was to be adapted, we find its several parts capable of being occafionally 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 fet it; but is not altogether unlike Harrifon's contrivance for making a watch regulate itself, by inferting within it a machinery, which, by the artful ufe of the different expanfion of metals, preferves the equability of the motion under all the various temperatures of heat and cold in which

the

the inftrument may happen to be placed. The ingenuity of this laft contrivance has been juftly praised. Shall, therefore, a structure which differs from it, chiefly by furpaffing it, be accounted no contrivance at all? or, if it be a contrivance, that it is without a contriver?

But this, though much, is not the whole: by different fpecies of animals the faculty we are defcribing is poffeffed, in degrees fuited to the different range of vision which their mode of life, and of procuring their food, requires. Birds, for inftance, in general, procure their food by means of their beak; and the distance between the eye and the point of the beak being fmall, it becomes neceffary that they fhould have the power of feeing very near objects diftinctly. 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 affifting them in descrying their prey, a power of feeing at great distance; a power, of which, in birds of rapine, furprising 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

diftances depends. The one is a bony, yet, in most species, a flexible rim or hoop, furrounding the broadeft part of the eye; which, confining the action of the mufcles to that part, increases the effect of their lateral preffure upon the orb, by which preffure its axis is elongated for the purpose of looking at very near objects. The other is, an additional mufcle called the marfupium,to draw, upon occafion, the crystalline lense back, and fo fit the fame eye for the viewing of very diftant 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 alfo, compared with those of terreftrial animals, exhibit certain diftinctions of ftructure, adapted to their state and element. We have already obferved upon the figure of the crystalline compenfating by its roundness the density of the medium through which their light paffes. To which we have to add, that the eyes of fish, in their natural and indolent ftate, appear to be adjusted to near objects, in this respect differing from the human eye, as well as thofe of quadrupeds and birds. The ordinary shape of the fifh's eye being in

a much

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 alfo 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, which has to work its head through fand and gravel, the rougheft and harshest substances, there is placed before the eye, and at some distance from it, a tranfparent, horny, convex cafe or covering, which, without obftructing the fight, defends the organ. To fuch an animal, could any thing be more wanted, or more ufeful?

Thus, in comparing together the eyes of different kinds of animals, we fee, in their refemblances and diftinction, one general plan laid down, and that plan varied with the varying exigences to which it is to be applied.

There is one property, however, common, I believe, to all eyes, at leaft to all which have been examined *, namely, that the optic nerve

* The eye of the feal or fea calf, I understand, is an exception. Mem. Acad. Paris, 1701, p. 123.