ASTIGMATISM. 



795 



another normal deviation from mathematical exactness in the refraction of rays by the 

 cornea and the crystalline lens, which is of considerable importance. If we place before 

 the eyes two threads crossing each other at right angles in the same plane, one of these 

 threads being vertical, and the other, horizontal, when the optical apparatus is adjusted 

 so that one line is seen with perfect distinctness, the other is not well defined. In other 

 words, when we accommodate for the vertical thread, the horizontal is indistinct, and 

 vice versa. If the horizontal line be seen distinctly, in order to see the vertical without 

 modifying the accommodation, it must be removed to a greater distance. This depends 

 chiefly upon a difference in the vertical and the horizontal curvatures of the cornea, so 

 that the horizontal meridian has a focus slightly different from the focus of the vertical 

 meridian. A condition opposite to that observed in the cornea usually exists in the 

 crystalline lens ; that is, the difference which exists between the curvatures of the lens 

 in the vertical and the horizontal meridians is such that the deepest curvature in the lens 

 is situated in the meridian of the shallowest curvature of the cornea. In this way, in 

 normal eyes, the aberration of the lens has a tendency to correct the aberration in the 

 cornea ; but this correction 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 sufficiently 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 

 ^5- or more, it is to be considered abnormal ; 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 dis- 

 tinctness 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 interfere 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 deformation, opaque spots, etc., 

 in the cornea, depends upon 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, we have irregular 

 variations in the curvatures of different sectors of the lens. As a consequence of this, 

 when the irregularities are very great, there is impairment 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 to a very 

 moderate degree in normal eyes ; but it is so slight as not to interfere with ordinary vision. 

 In what is called normal, irregular astigmatism, the irregularity depends entirely upon 

 the crystalline lens. If we place before the eye a card with a very small opening, ami 

 move this before the lens, so that the pencil of light falls successively upon diffcivnt 

 tors, it can be shown that the focal distance is different for different portions. The radi- 

 ating 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 bo perfectly corrected 

 by placing cylindrical glasses before the eyes, it is impossible, in the great majority of 

 cases, to construct glasses which will remedy the irregular form. 



