DIOPTRICS OF THE EYE. 



303 



curved than the horizontal one. If, on the contrary, the cur- 

 vature along the horizontal meridian is greater, the astigmatism 

 is " against the rule." The meridians of greatest and least curva- 

 ture may not lie in the vertical and horizontal planes, but in some 

 of the oblique planes; but so long as they are at right angles the 

 astigmatism is regular. It is evident that such a condition may 

 be corrected by the use of cylindrical lenses, so chosen as to in- 

 crease the refraction along,, the meridian in which the cornea 

 has the least curvature, in which case a convex or plus cylinder is 

 used, or, on the other hand, to diminish appropriately the refraction 

 along the meridian of greatest curvature, in which case a concave 

 or minus cylinder is used. An eye that suffers from a marked 



Fig. 126. Schema to illustrate the paths of the rays of light in a cornea showing 

 regular astigmatism. (McKendrick.) The lower line of figures represents the section 01 

 the cone of light, or the images obtained at different distances. The image varies from a 

 horizontal to a vertical line, but at no place can a point be obtained at which rays along 

 all meridians are focused. 



degree of astigmatism cannot focus distinctly at the same time 

 lines that are at right angles to each other; hence the use of a 

 series of lines whose images are formed along the different meridians 

 of the eye, as shown in Fig. 127, will reveal this defect if it exists. 

 If the eye is directed to the center of intersection of the lines some 

 of the lines appear distinct while those at right angles to them 

 are blurred. A normal eye can be thrown into an astigmatic con- 

 dition by approximating the eyelids closely. In this position the 

 tears make a concave cylindrical lens, which alters the curvature 

 along the vertical meridian. What is known as irregular astig- 

 matism is due to the fact that the meridians of greatest and least 

 curvature are not at approximately right angles, or, as is more 



