Memphis Daily Appeal

The eyes of all birds have a peculiarity of structure which enables them to see near and distant objects equally well, and this wonderful power is carried to the greatest perfection in the bird of prey. When we recollect that an eagle will ascend more than a mile in perpendicular height, and from that enormous elevation will perceive its unsuspecting prey, and pounce upon it with unerring certainty: and when we see the same bird scrutinizing with almost microscopic nicety, an object close at hand, we should at once perceive that he possesses power of accommodating his sight to distance in a manner to which our eye is unfitted, and of which it is totally incapable.

If we take a printed page, we shall find that there is some particular distance, probably ten inches, at which we can read the words and see each letter with perfect distinctness; but if we move a page to a distance of forty inches, we shall find it impossible to read it at all; a scientific man would therefore, call ten inches the focus or focal distance of our eyes. We cannot alter this focus except by the aid of spectacles.

But an eagle has the power of altering the focus of his eye just as he pleases: he has only to look at an object at the distance of two feet or two miles in order to see it with perfect distinctness. The ball of his eye is surrounded by fifteen little plates, called sclerotic bones. They form a complete ring, and their edges slightly overlap each other.

When he looks at a distant object, this little circle of bones expands, and the ball of the eye being relieved from the pressure, becomes flatter; and when he looks at a very near object, the little bones press together, and the ball of the eye is thus squeezed into a rounder or more convex form. The effect is very familiar to everybody. A person with very round eyes is near-sighted, and can only see clearly an object that is close to him, and a person with flat eyes, as in old age, can see nothing clearly except at a distance. The eagle, by the mere will, can make his eyes round or flat, and see with equal clearness at any distance.