ABNORMALITIES IN THE REFRACTION OF THE EYE 



857 



rays to an equal degree of refraction. Accordingly, this eye obtains 

 first of all an image of those rays which have traversed the more highly 

 curved vertical meridian of the cornea (AB) and lastly, one of those 

 rays which have passed through its relatively flat horizontal plane 

 (CD). The first image (ab) must necessarily be a horizontal line and 

 the second a vertical line (cd). In between these two images are 



Fig. 455. — Astigmatic Chart. (Howell.) 



situated first a horizontal ellipse, then a circle and lastly, a vertical 

 ellipse. The reason for this is that the rays ah again diverge distally 

 to the horizontal image and henceforth intermingle with the still 

 convergent rays cd. 



To illustrate, let us fill a tall beaker with water, place it upon a table and 

 project a round beam of light through its central area. The image is a vertical 



Figs. 456, 457. — Lines for the Detection of Astigmatism. 



line, because the column of water acts in the manner of a cylindrical lens, the great- 

 est convexity of which is adjusted from side to side. If this beaker is now held 

 horizontally so that its greatest convexity lies in the vertical plane, the linear 

 image assumes a horizontal position. The same results may be obtained with a 

 cylindrical lens. By means of two equally strong cylindrical lenses superimposed 

 upon another at right angles, these linear lines may be reconverted into a rounded 

 image. 



