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On The Generation Of Animals   

The same thing is shown also by the morbid affections of each kind
of sight. Cataract attacks the blue-eyed more, but what is called
'nyctalopia' the dark-eyed. Now cataract is a sort of dryness of the
eyes and therefore it is found more in the aged, for this part also
like the rest of the body gets dry towards old age; but is an excess
of liquidity and so is found more in the younger, for their brain is
more liquid.

The sight of the eye which is intermediate between too much and
too little liquid is the best, for it has neither too little so as
to be disturbed and hinder the movement of the colours, nor too much
so as to cause difficulty of movement.

Not only the above-mentioned facts are causes of seeing keenly or
the reverse, but also the nature of the skin upon what is called the
pupil. This ought to be transparent, and it is necessary that the
transparent should be thin and white and even, thin that the
movement coming from without may pass straight through it, even that
it may not cast a shade the liquid behind it by wrinkling (for this
also is a reason why old men have not keen sight, the skin of the
eye like the rest of the skin wrinkling and becoming thicker in old
age), and white because black is not transparent, for that is just
what is meant by 'black', what is not shone through, and that is why
lanterns cannot give light if they be made of black skin. It is for
these reasons then that the sight is not keen in old age nor in the
diseases in question, but it is because of the small amount of
liquid that the eyes of children appear blue at first.

And the reason why men especially and horses occasionally are
heteroglaucous is the same as the reason why man alone grows grey
and the horse is the only other animal whose hairs whiten visibly in
old age. For greyness is a weakness of the fluid in the brain and an
incapacity to concoct properly, and so is blueness of the eyes; excess
of thinness or of thickness produces the same effect, according as
this liquidity is too little or too much. Whenever then Nature
cannot make the eyes correspond exactly, either by concocting or by
not concocting the liquid in both, but concocts the one and not the
other, then the result is heteroglaucia.

The cause of some animals being keen-sighted and others not so is
not simple but double. For the word 'keen' has pretty much a double
sense (and this is the case in like manner with hearing and
smelling). In one sense keen sight means the power of seeing at a
distance, in another it means the power of distinguishing as
accurately as possible the objects seen. These two faculties are not
necessarily combined in the same individual. For the same person, if
he shades his eyes with his hand or look through a tube, does not
distinguish the differences of colour either more or less in any
way, but he will see further; in fact, men in pits or wells
sometimes see the stars. Therefore if any animal's brows project far
over the eye, but if the liquid in the pupil is not pure nor suited to
the movement coming from external objects and if the skin over the
surface is not thin, this animal will not distinguish accurately the
differences of the colours but it will be able to see from a long
distance (just as it can from a short one) better than those in
which the liquid and the covering membrane are pure but which have
no brows projecting over the eyes. For the cause of seeing keenly in
the sense of distinguishing the differences is in the eye itself; as
on a clean garment even small stains are visible, so also in a pure
sight even small movements are plain and cause sensation. But it is
the position of the eyes that is the cause of seeing things far off
and of the movements in the transparent medium coming to the eyes from
distant objects. A proof of this is that animals with prominent eyes

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