chap, xxix.] CORRECTION FOR ASTIGMATISM. 393 



divergence of c and D is now considerable \ at 5 F and 

 E have reached their focus, c and D have diverged to 

 the extent LM, the diffusion- image being a line. 



Thomas Young was the discoverer of astigmatism, 

 having observed it in his own eye. It appears that 

 the cornea has a different radius of curvature in 

 its several meridians. Generally the maximum cur- 

 vature is towards the vertical, and the minimum 

 towards the horizontal meridian. Indeed, it is 

 asserted that few eyes are absolutely without this 

 defect. This, one can test for himself in his own eyes, 

 by testing the farthest point of distinct vision for fine 

 vertical lines, and the farthest point for distinct vision 

 for fine horizontal lines. If both meridians were the 

 same in curvature, the distances ought to be equal, 

 but generally the distances are unequal. If two 

 threads intersecting one another, the one vertical, the 

 other horizontal, are not seen with equal distinctness 

 at the same time, the defect is present. 



The correction for astigmatism is secured, 

 if a lens be interposed in front of the eye which shall 

 either add to the curvature of the meridian with 

 the less curvature, or diminish the curvature of the 

 meridian of greater curvature, so that both meridians 

 have practically the same curvature. The former 

 procedure is that usually employed. It is effected by 

 cylindrical glasses. If one makes a section of a 

 cylinder in a plane parallel to the long axis of the 

 cylinder, it is seen that, if placed vertically, in the 

 vertical meridian the anterior and posterior surfaces 

 of the section are parallel to one another, so that rays 

 will pass through that meridian and issue in a direc- 

 tion parallel to that of entrance, i.e. they are not 

 converged, just like rays passing through a plate with 

 parallel faces (page 311). On the other hand, in the 

 horizontal meridian the surface is curved. If the 

 section be placed with long axis horizontal, then the 



