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. * The marsupium arises from the back of the eye; proceeding apparently through a slit in the retina, it passes obliquely into the vitreous humour, and terminates in that part, as in the eagle, Fig. 3. a section of the eye of the falco chrysaëtos. In some species it reaches the lens, and is attached to it, Fig. 4. 6. In the plate the marsupium is marked with a *. 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 any thing 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 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 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 ob Fig. 5. The skin of the head of an eel is represented turned back; and as the transparent horny covering of the eye a, a, is a cuticular covering, it is separated with it. jects 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 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 any 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. Besides that conformity to optical principles which its internal constitution displays, and which alone amounts to a manifestation of intelligence having been exerted in the structure; besides 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 orbitary * The frontal, maxillary, malar, lachrymal, ethmoid, palatine, and sphenoïd bones. 1 le Tacwrk its bead through ALTERA MASTI. Convex case IAS I SIDEs te es rent kinds PS I ES il ville bave been No II The pic where the James in a bur between that |