The organ of sight is, indeed, more highly developed in birds than in any other class of animals. The volume of the eye itself is large compared with the head. It includes an addition which seems to be confined to birds. This is a black membrane, with many folds, very rich in blood-vessels, and situated at the bottom of the ocular globe, and advancing towards the crystalline. Anatomy has failed to explain the use of this, but it is supposed that by advancing or withdrawing it, it gives to birds additional power of vision. Other parts of the eye, such as the choroïds, the thin membrane which covers the posterior part of the eye, the iris, the retina, present nothing remarkable. The white of the eye is surrounded by an osseous or cartilaginous matter, evidently placed there for protection of this delicate and useful organ. [Illustration: Fig. 69.] Besides the ordinary upper and lower pupils, birds possess a third. This consists of an extensive transparent membrane, disposed vertically, which covers the eye like a piece of network, protecting it from the effects of a blaze of light. It is this pupil, or nictating membrane, placed at the internal angle of the eye, between the orb and the external pupil, which the animal uses at will, which permits the Eagle to gaze at the sun, and prevents the nocturnal birds of prey from being dazzled when exposed to daylight. The perfection of the sight of birds seems to be proved from the Vulture, so distant from his prey as to appear a mere speck in the heavens, without deviation flying directly to it; or the Swallow, which perceives, while on rapid wing, the smallest insect on which it feeds. According to Spallanzani, the Swift has sight so piercing, that it can see only five lines in diameter at the distance of five hundred feet.