112 BIOPHYSICAL CHARACTERISTICS OF THE EYE 



so that the chromatic aberration increases ; on the other hand, the diffrac- 

 tion decreases. The two changes, however, practically compensate 

 each other, leaving the definition of the image unchanged, but the depth 

 of focus decreases. 



Astigmatism 



In an ideal eye the refractive surfaces are sections of true spheres, 

 and, all the meridians being of equal curvature, the refraction along 

 these different meridians is equal. Such an eye with a small pupil will 

 refract a cone of monochromatic light, issuing from a luminous point, to 

 a focal point on the retina and will exclude the disturbing contributions 

 of spherical aberration. If one or all of the refractive surfaces, how- 

 ever, have unequal curvatures along different meridians, then the rays 



Rays due to greater curvature 



• ^ — o I I 1 



Primary a • 



line focus t Secondary 



linp fnrnQ 



Closest approach 

 to a point image 



line focus 



Fig. III-8. An astigmatic lens, showing the shape of the astigmatic ray bundles. 



from a luminous monochromatic point source cannot be brought to a 

 single focal point, since the rays along the meridian of greater curvature 

 will be brought to a focus first and begin to diverge before the rays 

 along the lesser curvature are focused. Such a condition is called 

 astigmatism (not a point). The effect can be illustrated by the diagram 

 of Fig. Ill— 8, which represents the refraction of the rays from a luminous 

 point by a cornea whose curvature along the vertical meridian is greater 

 than the curvature along the horizontal meridian. The lower line of 

 figures represents the section of the cone of light, or the images obtained 

 at different distances. The images vary from a horizontal line to a 

 vertical line, but at no place can a point be obtained at which rays 

 along all meridians are focused. At B the rays along the vertical 

 meridian are in focus; at A the rays along the horizontal meridian are 

 brought to a focus. 



Ordinary astigmatism is usually attributed to a defect in the curva- 

 ture of the cornea. The condition illustrated by Fig. Ill— 8 can be cor- 



