VI 



DIOPTKIC MECHANISM OF THE EYK 



307 



astigmatism in Die horizontal 



or the vertical meridian. 



zontal, and in 13-4 per cent the oblique meridian. It is thus 

 proved that in the great majority of cases the vertical is the 

 must refractive corneal meridian, which bears out Gullstrand's 

 view that the cause lies in the unequal flattening of the cornea 

 by pressure of the eyelids. 



In most instances, owing to ordinary corneal astigmatism, it 

 is necessary, when looking fixedly with 

 one eye at two black lines which cross on 

 a white background, to bring the test- 

 object a little nearer, in order to see the 

 horizontal line distinctly, than is required 

 in focussing the vertical. The eye is there- 

 fore comparatively myopic for horizontally, 

 and hypermetropic for vertically placed 

 ol ijects. 



Various simple expedients have been 



Tvrrmnon/1 fn fanili rifp iil lipr-H VP -nprppn Fio. 140. To illustrate the easiest 



pioposeu to iacmcaLe sui>je< ;ive peicep- Tll ,. t hod of detecting c.,n,<a! 

 tion of astigmatism. One such, represented 

 in Fig. 140, consists of four contiguous 

 squares, two crossed by horizontal, two by vertical black lines at 

 uniform distances. When the eye is accommodated for distinct 

 vision of the horizontal lines, the vertical lines are less distinct ; 

 when it is accommodated for the vertical lines, the horizontal are 

 blurred. Another method is shown in Fig. 141, which consists of 

 equidistant, concentric circles. On looking at these circles 

 attentively with one eye it is impossible to see all the lines in 



the different sectors 



clearly at the same 



moment ; this is only 



possible in two opposite 



sectors, the position of 



which alters as the 



object is brought near 



( ii moved away, or when 



the degree of accommo- 

 dation is altered. A 



FIG. 141. To demonstrate astig- third Way of testing 



SKcSJ? ' '' accommodation in one's 



own eyes is shown in 

 Fig. 14'2, which consists of a number of concentric radii. If the 

 figure is placed at such a distance fr< mi the eye that only the hori- 

 zontal line is seen clearly, then on slowly bringing it nearer the 

 oblique lines become plainer till finally the vertical line alone is 

 seen distinctly, all the rest being more or less blurred. 



Physiological astigmatism causes no appreciable disturbance 

 of vision so long as it is slight, but when it exceeds certain 

 limits, and becomes abnormal, it alters the shape of the retinal 



FIG. 142. Illustrates how 

 beat to distinguish 

 ilillereuees in cornea] 

 astigmatism in the 

 ditlerent meridians. 



