which impressions are sent to the brain.

The retina contains curious structures which can be seen only with the aid
of the microscope. For instance, a layer near the choroid is made up of
nerve cells arranged in innumerable cylinders called "rods and cones," and
packed together not unlike the seeds of a sunflower. These rods and cones
are to be regarded as the peculiar modes of termination of the nerve
filaments of the eye, just as the taste buds are the modes of termination
of the nerve of taste in the tongue, and just as the touch corpuscles are
the terminations of the nerves in the skin.

Experiment 148. Close one eye and look steadily at the small a in
the figure below. The other letters will also be visible at the same
time. If now the page be brought slowly nearer to the eye while the eye
is kept steadily looking at the small a, the large A will disappear at a
certain point, reappearing when the book is brought still nearer.

[Illustration: a oAx]

On the reappearance of the A it will be noted that it comes into view
from the inner side, the x being seen before it. If now we move the book
towards its original place, the A will again disappear, coming again
into view from the outer side when the o is seen before it.

328. Inner Structure of the Eye. Let us imagine an eyeball divided
through the middle from above downwards. Let us now start in front and
observe its parts (Fig. 127). We come first to the cornea, which has
just been described. The iris forms a sort of vertical partition,
dividing the cavity of the eyeball into two chambers.

[Illustration: Fig. 128.--Diagram illustrating the Manner in which the
Image of an Object is brought to a Focus on the Retina.]

The anterior chamber occupies the space between the cornea and the
iris, and is filled with a thin, watery fluid called the aqueous
humor.

The portion behind the iris forms the posterior chamber, and contains
the crystalline lens and a transparent, jelly-like fluid, the
vitreous humor. This fluid is never renewed, and its loss is
popularly described by the phrase, "when the eye runs out."

Experiment 149. The retina is not sensitive where the optic nerve
enters the eyeball. This is called the "blind spot." Put two ink-bottles
about two feet apart, on a table covered with white paper. Close the
left eye, and fix the right steadily on the left-hand inkstand,
gradually varying the distance from the eye to the ink-bo