682 SPECIAL SENSES 



in all degrees of accommodation, a certain difference in 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 sufficient 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. When the astigmatism amounts to -fa or more, it is to 

 be considered abnormal ; which simply means that beyond this point the 

 aberration interferes with distinct vision. 



From the simple definition of regular astigmatism, it is evident that 

 this condition and the degree in 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 that 

 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 just described, 

 may be so exaggerated as to interfere seriously with vision, when it 

 becomes abnormal. This kind of aberration, however, which usually is 

 dependent on an abnormal condition of the cornea, is remediable by the 

 use of properly-adjusted cylindrical glasses. 



Irregular astigmatism, excluding cases of pathological deformation, 

 opaque spots etc., in the cornea, depends usually on irregularity in the 

 different sectors of the crystalline lens. Instead of a simple and regular 

 aberration, consisting in a difference between the depth of the vertical 

 and the horizontal curvatures of the cornea and lens, there are irregular 

 variations in the curvatures of different sectors of the lens. As a con- 

 sequence of this, when the irregularities are very great, there is impair- 

 ment of the sharpness of vision. The circles of diffusion, which are 

 regular in normal vision, become irregularly radiated, and single points 

 appear multiple, an irregularity described under the name of polyopia 

 monocularis. Accurate observations have shown that this condition 

 exists in a moderate degree in normal eyes ; but it is so slight as not to 

 interfere with ordinary vision. In what is called normal irregular astig- 

 matism, the irregularity depends entirely on the crystalline lens. If a 

 card with a very small opening is placed before the eye and is moved in 

 front of the lens, so that the pencil of light falls successively on different 

 sectors, it can be shown that the focal distance is different for different 

 portions. The radiating lines of light observed in looking at remote 

 luminous points, as the fixed stars, are produced by this irregularity in 

 the curvatures of the different sectors of the lens. 



While regular astigmatism, both normal and abnormal, may be per- 



