The reason why : $b a careful collection of many hundreds of reasons for things which, though generally believed, are imperfectly understood. A book of condensed scientific knowledge for the million
John Stuart Mill · en
K. Fragment of the bone by which one of the muscles is fastened.
967. _Why do some persons squint?_
Because it sometimes happens that a muscle of the eye _acts too
powerfully_ for its companion muscle, and draws the eye too much on one
side.
968. _Why does the pupil of the eye look black?_
Because the pupil is an _opening_ through which the rays of light pass
into the chamber of the eye. There is, therefore, nothing in the pupil,
of the eye to reflect light.
[Verse: "Keep me as the apple of thine eye; hide me under the shadow
of thy wings."--PSALM XVII.]
969. _Why is the pupil of the eye larger sometimes than at others?_
Because the _iris_, a ring of extremely fine muscles which surround the
pupil, contracts when too much light falls upon the retina, and dilates
when the light is feeble. It therefore enlarges or diminishes the size
of the pupil to _regulate the admission of light_.
[Illustration: Fig. 62.--SECTION OF THE EYE SEEN FROM BEHIND.]
A. The _pupil_ of the eye through which the light enters.
B. The _iris_, which dilates or contracts, and thereby increases
or lessens the size of the _pupil_.
C. The three coats of the eye, called the _sclerotic_, _choroid_,
and _retina_.
D. The _ciliary processes_, or hair-like muscles, which have a
slight vibratory motion which they impart to the fluids of the eye.
E. The dark coat of the _choroid_, the coat forming the _retina_
removed.
970. _Why have we two eyes?_
Because the field of vision is thereby _much extended_; the _intensity_
of sight is also increased, the impressions upon the brain being
clearer and better defined, just as in a _stereoscope_ the effect of
vision is heightened by a double picture; the sense of sight being more
_constantly_ exercised than any other sense during our waking moments,
_one eye is frequently called upon to give rest to the other_; and
the important faculty of vision, being endangered by the necessary
exposure of some parts of the eye, and the equally necessary delicacy
of an organ formed to receive impressions from so ethereal an element
as light, is rendered the more secure to us, since though one eye may
become enfeebled, diseased, or wholly lost, _the other eye will retain
the blessing of sight_.
[Verse: "The eyes of the Lord are upon the righteous, and his ears
are open unto their cry."--PSALM XXXIV.]
971. _Why, having two eyes, and each eye receiving a reflection upon
its retina, does the brain experience only one impression of an object?_
Because, besides those optical laws which bring upon the two retinas
the exactly corresponding images of the same objects, the optic nerves
_meet_ before they reach the brain, _and blend the impulses which they
convey_.
972. _Why are the eyes provided with eyelids?_