Astigmatism is corrected by a cylindrical lens, which is in fact a segment of a solid cylinder of glass. The axis of the cylindrical lens should be at right angles to the defective meridian of the eye, in order to correct the astigmatism. Eye-strain is caused by astigmatism in the same manner that it is brought about in the simple farsighted eye, i. e., by constant strain on the ciliary muscle, which regulates the convexity of the crystalline lens. For it is possible for the inequalities of the front surface of the eyeball or of the lens to be offset or counterbalanced by change in the convexity of the lens produced by the action of this muscle, and it is conceivable that the axis of the lens may be tilted one way or another by the same agency, and for the same purpose. But, as we have already pointed out, this continual muscular action entails great strain on the nerve centers which animate the muscle, and if constant near work is requisite, or the health is impaired, the nervous exhaustion becomes apparent. The lesser degrees of astigmatism often give more trouble than the greater. [Illustration: PLATE I =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.]