Plate I
ANATOMY OF THE EYE
The upper illustration shows the six muscles attached to the eye. The Superior Rectus Muscle pulls and directs the eye upward; the Inferior Rectus, downward; the External and Internal Rectus Muscles pull the eye to the right and left; the Oblique Muscles move the eye slantwise in any direction.
Lack of balance of these muscles, and especially inability to focus both eyes on a near object without effort, constitute "eye-strain."
The lower cut illustrates the relation of the crystalline lens to sight. Lens Nearsight Focus shows the lens bulging forward and very convex; Lens Farsight Focus shows it flat and less convex.
This adjustment of the shape of the crystalline lens is called "accommodation"; it is effected by a small muscle in the eyeball.
In the normal eye, the rays of light from an object pass through the lens, adjusted for the proper distance, and focus on the retina.
In the nearsighted eye, these rays focus at a point in front of the retina; while in the farsighted eye these rays focus behind the retina; the nearsighted eye being elongated, and the farsighted eye being shortened.
PLATE I