to be constructed upon strict optical principles; the self-same principles upon which we ourselves construct optical instruments. We find them perfect for the purpose of forming an image by refraction; composed 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 so onward: the progressive action depending for its success upon the nicest and minutest adjustment of the parts concerned: yet these parts so in fact adjusted as to 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? 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 the 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, 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, to draw, on 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, 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 dis

tinctions one general plan laid down, and that plan varied with the varying exigencies to which it is to be applied1o.

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In viewing the structure of the eye as adjusted to the condition of fishes, we may remark the peculiar thickness of the sclerotic coat in the whale. Although he breathes the atmosphere, and lies out on the surface of the water; to escape his enemies he will plunge some hundred fathoms deep. The pressure therefore must be very great upon his surface, and on the surface of the eye. If a cork be knocked into the mouth of a bottle, so that it resists all further pressure that we can make upon it, and if this bottle be carried, by being attached to the sounding-lead, to a great depth in the sea, the pressure of the water will force in the cork, and fill the bottle; for the cork is pressed with a force equal to the weight of the column of water above it, of which it is the base. It is pressed in all directions equally, so that a commonsized cork is reduced to the size of that of a phial bottle.

“A creature living at the depth of 100 feet would sustain a pressure, including that of the atmosphere, of about 60 pounds on the square inch; while one at 4000 feet, a depth by no means considerable, would be exposed to a pressure of about 1830 pounds upon the square inch."-De La Beche, Theor. Geol. p. 243.

We can therefore comprehend how it shall happen, that on the foundering of a ship at sea, though its timbers part, not a spar floats to the surface; everything is swallowed up; for, if the hull has sunk to a great depth,