The Book of Household Management by Mrs. Beeton

necessary to their condition. As they pass a great portion of their lives among thickets and hedges, they are provided for the defence of their eyes from external injuries, as well as from the effects of the light, when flying in opposition to the rays of the sun, with a nictating or winking membrane, which can, at pleasure, be drawn over the whole eye like a curtain. This covering is neither opaque nor wholly pellucid, but is somewhat transparent; and it is by its means that the eagle is said to be able to gaze at the sun. "In birds," says a writer on this subject, "we find that the sight is much more piercing, extensive, and exact, than in the other orders of animals. The eye is much larger in proportion to the bulk of the head, than in any of these. This is a superiority conferred upon them not without a corresponding utility: it seems even indispensable to their safety and subsistence. Were this organ in birds dull, or in the least degree opaque, they would be in danger, from the rapidity of their motion, of striking against various objects in their flight. In this case their celerity, instead of being an advantage, would become an evil, and their flight be restrained by the danger resulting from it. Indeed we may consider the velocity with which an animal moves, as a sure indication of the perfection of its vision. Among the quadrupeds, the sloth has its sight greatly limited; whilst the hawk, as it hovers in the air, can espy a lark sitting on a clod, perhaps at twenty times the distance at which a man or a dog could perceive it."