Chap XXTX.] 



ASTIGMATISM. 



For the myopic eye the formula becomes 



1 



d 



10' 



Suppose the person reads at 8 inches distance. Then 



I 

 f 



1 

 8 



1 

 10" 



1 

 40' 



40 is the focal distance of the desired concave lens. 



AstigsiaaJisiii is an anomaly of refraction due to 

 an asymmetrical condition of the refracting media. 

 The condition is such that the focal length of the 

 different meridians of the refracting media are different. 

 The result of this is that rays of light passing 

 through the lens or system of lenses are not brought 



J o 



to a focus at the same 

 point. They are not 

 /tomocentric. Consider, 

 for example, the hori- 

 zontal and the vertical 

 meridians, and sup- 

 pose that the former 

 has a less curvature, 



i.e. a greater focal 

 length, than the latter, 

 then rays which pass 



Fig. 176. Astigmatism. 



through the vertical meri- 

 dian will reach their focus before rays which pass 

 through the horizontal meridian. Hence the name 

 astigmatism, a not, and 0-rLy/j.a a point. The effect of 

 such differences in the curvature is to produce diffusion 

 images of a particular sort, which will be understood 

 by referring to Fig. 176. 



Let ACD be a curved medium on which parallel 

 rays of light fall. They should all come to one focus 

 after passing through the medium. But let the verti- 

 cal meridian CAD have a greater curvature than the 



