_aqueous humor_,
is secreted by the cornea, iris, and ciliary processes. The space behind
the crystalline lens is occupied by a fluid, called the _vitreous
humor_. This humor is denser than the other fluids and has the
consistency of jelly, being perfectly transparent. "The function of the
crystalline lens is to produce distinct perception of form and
outline."[3] The transparent humors of the eye also contribute to the
same effect, but only act as auxiliaries to the lens.

[Illustration: Fig. 62.]

The figure on the next page represents the course of the rays of light
proceeding from an object _a b_, refracted by the lens, and forming the
inverted image _x y_ on the screen. All rays of light proceeding from
_b_ are concentrated at _y_, and those proceeding from _a_ converge at
_x_. Rays of light emanating from the center of the object _a b_ pursue
a parallel course, and form the center of the image. Rays of light
passing through a double convex lens converge at a point called the
_focus_. In the organ of vision, if perfect, the focus is on the retina,
which serves as a screen to receive the image or impression. We have a
distinct perception of the outline of a distant hill, and also of a book
lying before us. The rays of light we receive from these objects cannot
have the same focus. How, then, can we account for the evident
accommodation of the eye to the varying distances? Various theories have
been advanced to explain this adjustment; such as changes in the
curvature of the cornea and lens; a movement of the lens, or a general
change in the form of the eyeball, by which the axis may be lengthened
or shortened.

[Illustration: Fig. 63.]

Two facts comprise all the positive knowledge which we possess on this
subject. Every person is conscious of a muscular effort in directing the
eye to a near object" as a book, and of fatigue, if the attention is
prolonged. If, now, the eyes be directed to a distant object, there will
result a sense of rest, or passiveness. By various experiments it has
been proved that the accommodation or adjustment of the eye for near
objects requires a muscular effort, but for distant objects the muscles
are in an essentially passive condition. An increase in the convexity of
the crystalline lens is now admitted to be necessary for a distinct
perception of near objects. We may give two simple illustrations, cited
by Dr. Dalton in his recent edition of Human Physiology. If a candle be
held n