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 are required to counteract this defect.

333.  The Movements of the Eyes.  In order that our eyes may be efficient instruments of vision, it is necessary that they have the power of moving independently of the head.  The mechanical arrangement by which the eyeballs are moved in different directions is quite simple.  It is done by six little muscles, arranged in three pairs, which, with one exception, originate in the back of the cavity in which the eye rests.  Four of these muscles run a straight course and are called the recti.  The remaining two muscles bend in their course and are called oblique.  The cooerdination of these tiny muscles is marvellous in its delicacy, accuracy, and rapidity of action.