DIOPTRICS OF THE EYE. 



319. 



matism is of the regular variety, and is usually attributed to a 

 defect in the curvature of the cornea. If the astigmatism is such 

 that the vertical meridian has the greatest curvature it is termed 

 "with the rule," since usually this meridian is slightly more 

 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. 133. — 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 of 

 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 merid- 

 ians of the eye, as shown in Fig. 134, may be used to reveal this 

 defect if it exists. If one looks at such a chart through a convex 

 lens, e. g., a lens of 5 or 10 diopters, and, starting from a distance 

 at which the lines are all blurred, brings it gradually nearer to 

 the eye, it will be observed, if the eye is astigmatic, that the lines 

 do not all come into focus at once. If one meridian, for example, 

 that of 90°, comes into focus first, then at that distance the mer- 

 idian at right angles, 180°, will appear most blurred. A normal 



