[Illustration:  FIG. 78.—­The eye.]

We learned in Section 114 that a change in the position of the object necessitated a change in the position of the screen, and that every time the object was moved the position of the screen had to be altered before a clear image of the object could be obtained.  The retina of the eye cannot be moved backward and forward, as the screen was, and the crystalline lens is permanently located directly back of the iris.  How, then, does it happen that we can see clearly both near and distant objects; that the printed page which is held in the hand is visible at one second, and that the church spire on the distant horizon is visible the instant the eyes are raised from the book?  How is it possible to obtain on an immovable screen by means of a simple lens two distinct images of objects at widely varying distances?

The answer to these questions is that the crystalline lens changes shape according to need.  The lens is attached to the eye by means of small muscles, m, and it is by the action of these muscles that the lens is able to become small and thick, or large and thin; that is, to become more or less curved.  When we look at near objects, the muscles act in such a way that the lens bulges out, and becomes thick in the middle and of the right curvature to focus the near object upon the screen.  When we look at an object several hundred feet away, the muscles change their pull on the lens and flatten it until it is of the proper curvature for the new distance.  The adjustment of the muscles is so quick and unconscious that we normally do not experience any difficulty in changing our range of view.  The ability of the eye to adjust itself to varying distances is called accommodation.  The power of adjustment in general decreases with age.

117.  Farsightedness and Nearsightedness.  A farsighted person is one who cannot see near objects so distinctly as far objects, and who in many cases cannot see near objects at all.  The eyeball of a farsighted person is very short, and the retina is too close to the crystalline lens.  Near objects are brought to a focus behind the retina instead of on it, and hence are not visible.  Even though the muscles of accommodation do their best to bulge and thicken the lens, the rays of light are not bent sufficiently to focus sharply on the retina.  In consequence objects look blurred.  Farsightedness can be remedied by convex glasses, since they bend the light and bring it to a closer focus.  Convex glasses, by bending the rays and bringing them to a nearer focus, overbalance a short eyeball with its tendency to focus objects behind the retina.

[Illustration:  FIG. 79.—­The farsighted eye.]

[Illustration:  FIG. 80.—­The defect is remedied by convex glasses.]