light can enter the eye except through it. The
iris possesses the power of dilating and contracting, so as to admit
more or less light, as it may be needed. This change in the size of the
pupil is effected by two sets of muscular fibers. The first set converge
from the circumference of the iris to the circular margin of the pupil,
and constitute the _radiated muscle_. The outer ends of these fibers are
attached to the sclerotic coat, which is unyielding; hence, when they
contract, the pupil _enlarges_ to receive more light. The other set is
composed of circular fibers, which go round in the iris from the border
to the pupil, and constitute the _orbicular muscle_, the contraction of
which _diminishes_ the size of the pupil. When too much light enters the
eye, the excited and sensitive retina immediately gives warning of the
danger, and the nerves, which are plentifully distributed to the iris,
stimulate the orbicular muscle to contract, and the radiated one to
relax, by which the size of the pupil is lessened. But when the light
which enters the pupil is insufficient to transmit a distinct image of
objects to the brain, the orbicular muscle relaxes, and the radiated one
contracts, so as to enlarge the pupil. The contraction of the pupil is
readily seen when a person passes from a darkened room into a bright
sunlight, or when a light is first brought into a room in the twilight
of evening. Any person may notice this contraction in his own eye by
beholding himself in a glass immediately after passing from a dark to a
well-lighted room. So, also, when a person looks at an object near the
eye, the pupil contracts, but when he looks at an object more remote, it
dilates. The muscles of the iris are somewhat under the control of the
will; for most persons can contract or dilate the pupil, in some degree,
at pleasure. Some persons possess this faculty to a great extent.

The three _humors of the eye_ have been compared to the glasses of a
telescope, and the coats to the tube, which keeps them in their places.
The _aqueous_ humor is situated in the fore part of the eye, and is
divided by the iris into what are called the anterior and posterior
chambers of the eye. The _crystalline_ humor, or lens, is situated
immediately behind the aqueous humor, a short distance back of the
pupil, and is a perfectly transparent double convex lens, closely
resembling in shape the common burning glass. This resemblance does not
stop here; for this