and it might be for this reason, perhaps, that he was induced to separate it from the eagles and class it with the owls. There can be no doubt Aristotle was in error in making this remark, as the distance from which it is observed to dart upon its prey proves its sight to be very acute rather than deficient. Aldrovand examined the eye of this bird with minute attention, in order to verify or disprove Aristotle's observations. He accordingly discovered, what Aristotle probably meant by a shade or cloud, that the opening of the pupil, commonly covered only by the transparent cornea, is in the osprey lined with a very delicate membrane, having the appearance of a small spot. Any apparent inconvenience of structure that might arise from this circumstance is compensated by the great transparency of the circle round the pupil,、 which in most other birds is dull and opaque. It does not therefore follow that its sight is fainter than that of other birds, because the light can pass easily and largely through the small circle which bounds the pupil, though it is probable the middle of the image will be marked with a small obscure spot, and that the lateral vision ought to be more distinct than the direct. It does not however appear that it sees worse than other birds. It may not perhaps soar quite so high as the golden eagle (Aquila chrysaëtas, ALDROVAND), nor descry and pursue its prey from such remote distances, nor have quite so acute vision; but it is not, like the owls, blinded by dazzling light, and it searches for victims as well by day as by night. Willughby thinks it "partly false or uncertain," that the golden eagle "doth so excel in quick-sightedness, soaring so high in the air that she can very hardly be discerned by us in all that light, yet she can espy a hare lying under a bush, or a little fish swimming in the water; though I grant," he adds, "that both the eagle and other rapacious birds are very sharp-sighted, yet do I not think that their eyes can reach the object at such distances*" We may remark, however, with all deference to the high authority of Willughby, that his scepticism is here carried too far, as the accounts he objects to are supported by undoubted facts. For though we should reject the authority of Homer, who, as Pope renders it, says― Endued with sharpest eye, The sacred eagle, from his walks above, Looks down and sees the distant thicket move, Then stoops, and sousing on the quivering hare, and though we should doubt the testimony of Aurelius Augustine, who says that "the eagle, when so high in the air as to be invisible to us, can perceive a hare lurking in an orchard, or a small fish swimming in the water;" yet we cannot refuse to admit as unquestionable facts the observations of such men as Wilson and Vaillant. Speaking of the white-headed eagle (Haliaëtus leucocephalus, SAVIGNY), Wilson says, "from the ethereal heights to which he soars, looking abroad, at one glance, on an immeasurable expanse of forests, fields, lakes, and ocean, deep below him;" and of the osprey (Pandion haliaëtus, SAVIGNY) he says, "down rapid as an arrow from heaven he descends, the roar of his wings reaching the ear as he disappears in the deep§." M. Vaillant again says of his vociferous 'eagle (Haliaëtus vocifer, SAVIGNY), that, "like the osprey and the white-tailed eagle (Haliaëtus albicilla, SAVIGNY), it dives rapidly from a great height in the air upon a fish which it *Ornithology, by Ray, p. 57. Apud Aldrovand, Ornith. i. 15. +Iliad, xvii. §Amer. Ornith. v. descries*." We have ourselves more than once seen the osprey dash down from a height of two or three hundred feet upon a fish of no considerable size, and which a man could with difficulty have perceived at the same distance t; but, in an instance elsewhere mentioned‡, we observed the golden eagle (Aquila chrysaëtas, ALDROVAND), at Mehlem on the Rhine, beating about among the orchards, as if it were on the look out for a hare or a rabbit; and if it were allowed to make a single instance like this overturn a general conclusion, we might oppose it to the testimonies already adduced. Ross, in his voyage to Baffin's Bay, proved that a man under favourable circumstances could see over the surface of the ocean to the extent of one hundred and fifty English miles. It is not probable that any animal exceeds this power of vision, though birds perhaps excel men and most quadrupeds in sharpness of sight. M. Schmidt threw, at a considerable distance from a thrush (Turdus musicus), a few small beetles, of a pale grey colour, which the unassisted human eye could not discover, yet the thrush observed them immediately and devoured them. The bottle-tit (Parus caudatus) flits with great quickness among the branches of trees, and finds on the very smooth bark its particular food, where nothing is perceptible to the naked eye, though insects can be detected there by the microscope. A very tame redbreast (Rhondella rubecula) discovered crumbs from the height of the branch where it usually sat, at the distance of eighteen feet from the ground, the instant they were thrown down, and this by bending its head to one side, and using, of course, only one eye. At the same distance a quail (Coturnix major, BRISSON) dis*Oiseaux d'Afrique, i. 18. † J. R. Architecture of Birds, p. 174. covered, with one eye, some poppy seeds, which are very small and inconspicuous*. In looking at objects which differ in their distances, the eye has been supposed to undergo some change of its parts or relations. It is supposed, in a word, to do for itself what spectacles or glasses do for those who are very long-sighted, or who are very nearsighted. What this change is, has been the subject of minute investigation and of learned discussion; but still there is little with regard to it certainly known. Sir Charles Bell is somewhat doubtful of the fact of the alleged change in the eye, or, at least, he thinks it much less than has been conjectured. He is therefore inclined to ascribe what takes place, in looking at near and distant objects, in a great measure to attention. We can attend at pleasure to a letter of a word, to the whole word, or to the page of a book, in the same way as we can attend to a distant object, while we overlook those which are nearly on a line with it but nearer t. The mechanical effect produced by thus directing the attention, Sir Charles Bell does not attempt to trace. In a note, however, on the iris, he states a fact of which he might have taken advantage. When a cat is roused to attention, as by the scratching of a mouse, the pupils of her eyes dilate; and the same thing occurs when she struggles to get loose from your hands ‡. Several ingenious experiments were made by Dr. T. Young to discover the alleged changes in the eyeball. He forced upon the ball of the eye the ring of a key, so as to cause by its pressure a luminous spot, and, looking at objects of different distance, he * Blicken en den Haushalt der Natur, p. 26, 1826. expected the spot would become greatly larger; but, on the contrary, it remained the same. In another experiment, he placed two candles, corresponding to the extent of nerve of the eye, and then made the highest change of its focus, expecting that, in consequence, the outer candle would appear to move away from him; but in this also he was disappointed *. On the assumption of a change in the eye, several suppositions have been made concerning the nature of that change, most, if not all, of which are liable to objections not easily repelled. We shall briefly advert to the chief of these opinions. They refer to a change in the globe of the eye,—in the cornea,—in the iris, -in the ciliary ring,—and in the crystalline. According to the first of these, the globe of the eye is compressed or relaxed by the surrounding muscles of the eyeball, in order to render the axis of the globe longer or shorter. But were this so the retina would be puckered up into folds; and, besides, we should be more conscious of the change, inasmuch as the muscles of the eye are voluntary t It was the opinion of Monro, that the change partly arose from the varied pressure of the eyelids upon the ball, and he made several experiments to prove this. He kept his eyelids wide asunder, and attempted to read a book while he held it so near that the letters were indistinct. He could not read it, in these circumstances; but, without moving his head or the book, and bringing his eyelids within a fourth of an inch from each other, he found he could read distinctly. Sir Charles Bell, however, on keeping the eyelids open, and using flat camel-hair pencils, as a substitute for the eyelashes, found the same effect; *Phil. Trans. 1793-1801; Medical Liter. p. 98-9. + Hosack, Phil. Trans. 1794, p. 196; Knox, Edinb. Trans. x. 50. Three Treatises on the Eye. |