302 THE SPECIAL SENSES. 



myopic than in the emmetropic eye, since in the former the greater 

 length of the eyeball requires less accommodation in near vision and 

 the failure of the lens to refract is therefore not felt so soon What 

 is known as second-sight in the old may be brought about by 

 the late development of a myopic condition, that is, by a change 

 in the length of the eyeball or by a swelling of the crystalline 

 lens, and in such a case convex glasses for near work may be 

 dispensed with. 



Astigmatism. In a perfectly normal or ideal eye the refractive 

 surfaces, cornea, anterior and posterior surfaces of the lens, are 

 sections of true spheres, and, all the meridians being of equal 

 curvature, the refraction along these different meridians is equal. 

 Such an eye will bring the cone of rays proceeding from a luminous 

 point to a focal point on the retina, barring the disturbing influence 

 of chromatic and spherical aberration. If, however, one or all of the 

 refractive surfaces have unequal curvatures along different merid- 

 ians, then it is obvious that the rays from a luminous point can not 

 be brought to a 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 designated as astigmatism (from a, not, and rfT^a, 

 point). The effect may be illustrated by the diagram in Fig. 126, 

 which represents the refraction of the rays from a luminous point by 

 a planoconvex lens whose curvature along the vertical meridian is 

 greater than along the horizontal meridian. 



The rays along the vertical meridian are brought to a focus 

 first at G, but those from the horizontal meridian are still converging; 

 so that a screen placed at this point will give the image of a horizontal' 

 line (a-a'). The rays along the horizontal meridian are brought to a 

 focus at B, but those from focus G have by this time spread out 

 in a vertical plane, so that a screen placed at this point will give 

 the image of a vertical line (b-c). In between the images will be 

 elliptical or circular, as represented in the diagram. In the eye 

 astigmatism may be due to an inequality in curvature of either the 

 cornea or the lens, and may be either regular or irregular. By 

 regular astigmatism is meant that condition in which while the 

 curvature along each individual meridian is equal throughout its 

 course, the curvatures of the different meridians vary and in such 

 a way that the meridians of greatest and least curvature are at 

 right angles to each other or approximately so. Ordinary astig- 

 matism is of the regular variety, and is usually attributed to a 

 defect in the curvature of the cornea. If the astigmatism is such 

 that the vertical meridian has the greatest curvature it is termed 

 "with the rule," since usually this meridian is slightly more 



