lens, like the burning glass, possesses the property
of converging the rays of light which fall upon it, and bringing them to
a focus. When this lens becomes so opaque as to obstruct the passage of
light, either partially or entirely, a person is said to have a
_cataract_. This can be cured only by a surgical operation. The
_vitreous_ humor, situated back of the other two, forms the principal
part of the globe of the eye. It differs from the aqueous in one
important particular. When that is discharged in extracting the
crystalline lens for cataract or otherwise, it will be restored again in
a few hours, and the eye will continue to perform its function. But if
this be discharged by accident, the eye is irrecoverably lost. This,
however, does not often occur; for, as we shall presently see, the eye
is admirably fortified.

The eye is a perfect optical instrument, infinitely surpassing all
specimens of human skill. This is true, view it in what light we may. It
not only possesses the power of so adjusting its parts as to adapt it to
the examination of objects at different distances, and in light of
different degrees of intensity, but we are enabled to direct it at will
to objects above, beneath, or around us.

The various motions of the eye are produced by six little muscles. These
are attached at one extremity to the immovable bones of the orbit, while
at the other extremity they are inserted into the sclerotic coat, four
of them near its junction with the cornea, by broad, thin tendons, which
give to the white of the eye its pearly appearance. These muscles are so
arranged by the matchless skill of the Architect as to enable the
beholder to direct the eye to any object he chooses, and to hold it
there for any length of time that is compatible with the laws by which
muscular exercise should be regulated. By the slight or intense action
of four of these, called the straight muscles, the eye is less or more
compressed, and the relative positions of its humors are by this means
so nicely adjusted as to enable us to view objects near by or at a
distance. The other two are called oblique muscles, one of which, with
its long tendon passing through a cartilaginous loop, acts upon the
principle of the fixed pulley, and turns the eye in a direction contrary
to its own action. When the external muscle becomes too short, the eye
turns out; but if the internal muscle is unduly contracted, the eye
turns inward, toward the nose. One