664 DEMONSTRATING REFRACTIVE EYE DEFECTS [CH. XV 



a different position of the object. The intermediate lines cannot 

 be made perfectly sharp in any position. 



With the eye when it is accommodated for vertical lines, the 

 horizontal lines will be blurred, and when the horizontal lines are 

 sharp and clear the vertical lines will be blurred. All the inter- 

 mediate lines will be more or less blurred all the time. 



To correct astigmatism it is necessary to do away with the 

 inequality of the curvature of the refracting surface. This can be 

 done either by increasing the lesser curvature or by reducing the 

 greater curvature sufficiently to make the refracting surface 

 uniform. 



To demonstrate astigmatism there are needed: 



(1) A convex lens of 3 or of 4 diopters 

 (fig. 390). 



(2) A convex cylindrical lens of 0.5 

 diopter. 



(3) A concave cylindrical lens of 0.5 

 diopter. 



(4) A lantern slide of the history of ^' 

 astigmatism (fig. 393). 



(5) A lantern slide of the radial lines (fig. 391). 



Put the 3 or the 4 diopter lens in the lens holder, and the lantern 

 slide of the radial lines (fig. 391) in the slide-carrier. Move the 

 slide until the radial lines are as sharp as possible on the screen. 



Put with the projecting lens the 0.5 diopter convex cylinder and 

 turn it so that the axis of the cylinder is vertical. The horizontal 

 lines will remain sharp, and the vertical lines will be most blurred. 



The addition of the 0.5 convex cylinder produced anunsymmet- 

 rically curved refracting surface. Along the axis of the cylinder 

 no change is made in the refractive power, hence light rays, from 

 points in the horizontal lines, which are in planes parallel with the 

 axis of the cylinder, are brought to a focus at the same distance as 

 if the cylinder were absent, but rays in any plane oblique to this 

 axis are affected by the curvature of the cylinder, and are 

 not brought to the same focus, hence only horizontal lines appear 

 sharp. 



