856 



THE SENSE OF SIGHT 



because slight differences between the curvatures of its vertical and 

 horizontal meridians are not uncommon. Most generally, however, 

 this defect is overcome functionally, so that an appreciable disturbance 

 in vision can only result when these differences exceed a certain physio- 

 logical limit. Astigmatism is classified as regular and irregular, the 

 former term being applied to it when the meridian of maximal curva- 

 ture lies at right angles to that of minimal curvature. Accordingly, 

 irregular astigmatism may be defined as an improper curvature of the 

 cornea along meridians which do not lie at right angles to each other. 

 This variety is most commonly produced by an injury and subsequent 

 formation of a scar in the course of a single meridian; hence, it is re- 

 stricted to a relatively narrow region of the cornea. We also make use 

 of the terms "with the rule" and "against the rule" astigmatism. The 



*--*<=> 6 



FIG. 454. DIAGRAM TO ILLUSTRATE THE CORNEA OF THE RAYS IN "WITH THE RULE" 



ASTIGMATISM. 



AB, being the plane of greater curvature, its rays are brought to a focus nearer the 

 lens than those traversing plane CD. 



former implies that the cornea is more highly curved along its vertical 

 meridian, while the latter signifies that its horizontal curvature is 

 greater .than its vertical. Ordinarily, astigmatism is of the regular 

 variety, presenting itself, therefore, as an excessive curvature along 

 its vertical plane. 



The functional result of these corneal inequalities is not difficult to 

 understand, if it is remembered that the more convex surface converges 

 the rays of light more strongly than the less convex and hence, 

 focalizes them more quickly than the flatter surface. We are dealing 

 here with planes, i.e., with linear refraction (Fig. 454). Consequently, 

 an eye which has been rendered ametropic by "with the rule" astig- 

 matism, converges those rays of light in a greater degree which traverse 

 the vertical plane of its cornea (AB). Although the lens receives these 

 rays in a more convergent form than those which have passed through 

 the horizontal plane of the cornea (CB), it subjects both lines of light 



