218 RECENT PROGRESS OF THE THEORY OF VISION. 



metrical curve, but is variously bent in different direc- 

 tions. I have also devised a method of testing the 

 4 centering ' of an eye during life, i.e. ascertaining whether 

 the cornea and the crystalline lens are symmetrically 

 placed with regard to their common axis. By this means 

 I discovered in the eyes I examined slight but distinct 

 deviations from accurate centering. ^ The result of these 

 two defects of construction is the condition called astig- 

 matism, which is found more or less in most human eyes, 

 and prevents our seeing vertical and horizontal lines at 

 the same distance perfectly clearly at once. If the degree 

 of astigmatism is excessive, it can be obviated by the use 

 of glasses with cylindrical surfaces, a circumstance which 

 has lately much attracted the attention of oculists. 



Nor is this all. A refracting surface which is im- 

 perfectly elliptical, an ill-centered telescope, does not 

 give a single illuminated point as the image of a star, 



FIG. 31. 



but, according to the surface and arrangement of the 

 refracting media, elliptic, circular, or linear images. Now 

 the images of an illuminated point, as the human eye 

 brings them to focus, are even more inaccurate : they are 

 irregularly radiated. The reason of this lies in the con- 



