224 FACULTIES OF BInDS. 
the eye is spread over its inner angle, and when 
any dust has fallen upon the ball, it sticks to this 
and is carried into the corner of the eye by the 
membrane folding back. In birds, again, this eye- 
brush is much more extended, and is spread over 
the whole eye by means of two muscles, one form- 
ing a tendinous cylindrical canal, the other passing 
through this and working like a cord in a pulley. 
The membrane being translucent, when it is drawn 
over the eye like a curtain it is well calculated to 
prevent it from being dazzled by too much light. It 
is evidently, however, a mere conjecture that it is by 
means of this membrane the eagle can look at the 
sun; for we do not see how it could be proved by 
experiment, as it would be impossible to perceive 
through it the direction of the pupil. Aristotle was 
of opinion it was for the purpose of moistening the 
eye; but Aldrovand thinks we cannot well draw 
such an inference, as our own eye is kept suffi- 
ciently moist without such a provision. 
Willoughby describes the eye of the golden eagle 
as having ‘‘a certain thick tunicle stretched forth 
from below upward, covering the globe in nicta- 
tion. ‘Two eyelids, one above, the other beneath, 
although the lower alone extending upward is suf- 
ficient to cover the whole eye. The region of the 
eyebrows is very eminent, like that prominent part 
of houses called the eaves, under which the eyes 
lie hidden, as it were, in a deep cavity. The eyes 
are of a green colour (chlorops) of a fiery splendour, 
shining forth in a pale blue. The pupil is of a 
deep black. It is very admirable to observe what 
care nature hath taken, and what provision she 
hath made for the conservation of the eyes, than 
which there is no part in this animal more excel- 
lent; for not being content with one tegument, as 
is usual in other animals, she seemeth to have 
framed four several lids or covers for them. The 
membrane for nictation is the same thing, and af- 
