t,
all the varied dimensions relations, positions, and visible qualities of
external objects.

The number, position, and perfection of the eyes, vary remarkably in
different orders, in many instances corresponding to the mode of life,
habitation, and food of the animal. A skillful anatomist may ascertain
by the peculiar formation of the eye, without reference to the general
physical structure, in what element the animal lives. Sight is one of
the most perfect of the senses, and reveals to man the beauties of
creation. The aesthetic sentiment is acknowledged to be the most
refining element of civilized life. Painting, sculpture, architecture,
and all the scenes of nature, from a tiny way-side flower to a Niagara,
are subjects in which the poet's eye sees rare beauties to mirror forth
in the rhythm of immortal verse.

In the vertebrates, the organs of vision are supplied with filaments
from the second pair of cranial nerves. In mammalia, the eyes are
limited to two in number, which in man are placed in circular cavities
of the skull, beneath the anterior lobes of the cerebrum. Three
membranes form the lining of this inner sphere of the eye, called
respectively the Sclerotic, Choroid, and Retina.

The _Sclerotic_, or outer covering, is the white, firm membrane, which
forms the larger visible portion of the eyeball. It is covered in front
by a colorless, transparent segment, termed the _cornea_, which gives
the eye its lustrous appearance. Within the sclerotic, and lining it
throughout, is a thin, dark membrane termed the _Choroid_. Behind the
cornea it forms a curtain, called the _iris_, which gives to the eye its
color. The muscles of the iris contract or relax according to the amount
of light received, thus enlarging or diminishing the size of the
circular opening called the _pupil_. The _Retina_ is formed by the optic
nerve, which penetrates the sclerotic and choroid and spreads out into a
delicate, grayish, semi-transparent membrane. The retina is one of the
most _essential_ organs of vision, and consists of two layers. A
spheroidal, transparent body, termed the _crystalline lens_, is situated
directly behind the pupil. It varies in density, increasing from without
inward, and forms a perfect refractor of the light received. The space
in front of the crystalline lens is separated by the iris into two
compartments called respectively the _anterior_ and _posterior
chambers_. The fluid contained within them, termed the