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 _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