Change of Sight in Birds.
Birds destined to move in the medium of a very rare atmosphere and which has but little tendency to refract the rays of the sun, have a great quantity of aqueous humor, in order that the light, strongly refracted in entering their eyes, may bring distinct images. Thus birds at heights where they appear to us only as points, perceive the smallest reptile concealed in the grass. But, as presbyte birds do not distinguish objects when brought near, nature has provided for this difficulty, which occurs when they descend from the heights of the air to seize their prey. To provide for this emergency, they have a membrane, by means of which they remove the crystalline lens from the retina; and thus changing the power of the eye by changing the focal distance of objects, as we do with spectacles, they never lose sight of their prey, whether in the air or on the ground.