524 



VISION 



first focal line; when it is adjusted for the least refractive meridian the 

 retinal images are drawn out in the direction of the second focal line in 

 both cases, therefore, distorted. To prevent this the eye is adjusted so that 

 some point between the two focal lines falls on the retina. The distortion 

 of objects is thereby rendered less, but the distinctness of the image is more 

 or less reduced. 



Astigmatism may be demonstrated subjectively by the use of a chart (like 

 that in Fig. 220) composed of several radii, all of the same width and depth of 



FIG. 219. Refraction of the light rays in regular astigmatism, showing the form of a beam at 

 different cross sections, after Fuchs, v v, , vertical meridian of the cornea; h, h the horizontal 

 meridian. The focus for the horizontal meridian is at /,; that for the vertical meridian at /; 

 the image of a point is, therefore, not a point but a dispersion circle. The shape of the circle, 

 however, is determined by the spot at which the retina is situated. At the position 2, the 

 image of a point would be a vertical line, at 4 a circle, at 6 a horizontal line, etc. 



color. If this chart be held before the eye at such a distance that only one 

 meridian can be seen distinctly, this meridian corresponds in direction to the 

 most refractive meridian of the eye, and its image on the retina to the second 

 focal line. If now the chart be brought as close to the eye as possible, again 

 only one meridian is distinct. In regular astigmatism this meridian is at right 

 angles to the first: it gives the direction of the least refractive meridian of the 

 eye and its image corresponds to the first focal line. 



A certain degree of astigmatism occurs in all eyes, although, as a rule, it 

 is so slight as to have no practical importance. The astigmatism which causes 

 a noticeable distortion of images is caused mainly by the asymmetrical struc- 

 ture of the cornea. 



