ed wall. The window frame may be plainly
seen for a moment.

Glance at the sun for a moment, close the eyes and the image of the sun
may be seen for a few seconds.

Experiment 152. Take a round piece of white cardboard the size of a
saucer, and paint it in alternate rings of red and yellow,--two primary
colors. Thrust a pin through the center and rotate it rapidly. The eye
perceives neither color, but orange,--the secondary color.

Experiment 153. To note the shadows cast upon the retina by opaque
matters in the vitreous humor (popularly known as floating specks, or
gossamer threads), look through a small pin-hole in a card at a bright
light covered by a ground-glass shade.

Experiment 154. _To illustrate accommodation_. Standing near a
source of light, close one eye, hold up both forefingers not quite in a
line, keeping one finger about six or seven inches from the other eye,
and the other forefinger about sixteen to eighteen inches from the eye.
Look at the _near_ finger; a distinct image is obtained of it, while the
far one is blurred or indistinct. Look at the far image; it becomes
distinct, while the near one becomes blurred. Observe that in
accommodating for the near object, one is conscious of a distinct
effort.

In many cases near-sightedness becomes a serious matter and demands
skillful advice and careful treatment. To remedy this defect, something
must be done to throw farther back the rays proceeding from an object so
that they will come to a focus exactly on the retina. This is done by
means of concave glasses, properly adjusted to meet the conditions of the
eyes. The selection of suitable glasses calls for great care, as much harm
may be done by using glasses not properly fitted to the eye.

[Illustration: Fig. 133.--Diagram illustrating the Myopic (near-sighted)
Eye.

The image P' of a distant object P falls in front of the retina even
without accommodation. By means of a concave lens (L) the image may be
made to fall on the retina (dotted lines). (To save space P is placed much
too near the eye).]

There is an optical condition of the eye known as astigmatism, in
which the cornea is usually at fault. In this defect of vision the
curvature of the cornea is greater in one meridian than in another. As a
result the rays from an object are not all brought to the same focus.
Objects appear distorted or are seen with unequal clearness. Glasses of a
peculiar shape