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."