DIOPTKIC MECHANISM OF THE EYE 



30*7 



FIG. 140. To illustrate the easiest 

 method of detecting corneal 

 astigmatism in the horizontal 

 or the vertical meridian. 



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

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

 most 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 

 objects. 



Various simple expedients have been 

 proposed to facilitate subjective percep- 

 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 



or moved away, or when 



the degree of accommo- 

 dation is altered. A FIG. 142. -Illustrates how 



. -, . j n best to distinguish 



tnim Way OI testing differences in corneal 

 different parts ap p nTY1Trmf j fl HnTi iri nnp'q astigmatism in the 



accommodation 111 One S different meridians. 



own eyes is shown in 



Fig. 142, which consists of a number of concentric radii. If the 

 figure is placed at such a distance from 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 



FI.C. 141. To demonstrate astig- 



