94: SPECIAL SENSES. 



tendency to correct the aberration in the cornea ; but this cor- 

 rection is incomplete, and there still remains, in all degrees 

 of tension of accommodation, a marked difference in the 

 vision as regards vertical and horizontal lines. 



The condition just described is known under the name of 

 normal, regular astigmatism ; but the aberration is not suffi- 

 ciently great to interfere with distinct vision. The degree of 

 regular astigmatism presents normal variations in different 

 eyes. In some eyes there is no astigmatism ; but this is rare. 

 According to Bonders, if the astigmatism amount to -fa or 

 more, it is to -be considered abnormal ; 1 which simply means 

 that, beyond this point, the aberration interferes with distinct 

 vision. 



From the mere definition of regular astigmatism, it is 

 evident that this condition and the degree to which it exists 

 may easily be determined by noting the differences in the 

 foci for vertical and horizontal lines, and it may be exactly 

 corrected by the application of cylindrical glasses of proper 

 curvature. Indeed, the curvature of a cylindrical glass, which 

 will enable a person to distinguish vertical and horizontal 

 lines with perfect distinctness at the same time, is an exact 

 indication of the degree of aberration. Regular astigmatism, 

 such as we have described, may be so exaggerated as to inter- 

 fere very seriously with vision, when it becomes abnormal. 

 This kind of aberration, however, which is dependent upon 

 an abnormal condition of the cornea, is remediable by the 

 use of properly-adjusted cylindrical glasses. 



Irregular astigmatism, excluding cases of pathological de- 

 formation, opaque spots, etc., in the cornea, depends upon ir- 

 regularity in the different sectors of the crystalline lens. In- 

 stead of a simple and regular aberration, consisting in a 

 difference between the depth of the vertical and the horizon- 

 tal curvatures of the cornea and lens, we have irregular varia- 

 tions in the curvatures of different sectors of the lens. As a 



1 DONDERS, On the Anomalies of Accommodation and Refraction of the Eye, 

 The New Sydenham Society, London, 1864, p. 456. 



