228 THE HUMAN BODY 



proceeding from a point in front, even when these are all of the 

 same refrangibility. Convex lenses whose surfaces are segments 

 of spheres, as are those of the eye, bring to a focus sooner the rays 

 which pass through their marginal than those passing through their 

 central parts. If rays proceeding from a point and traversing the 

 lateral part of a lens be brought to a focus at any point, then those 

 passing through the center of the lens will not meet until a little 

 beyond that point. If the retina receive the image formed by the 

 peripheral rays the others will form around this a small luminous 

 circle of light such as would be formed by sections of the cones 

 of converging rays in Fig. 78, taken a little in front of r r. This 

 defect exists in all glass lenses, as it is found impossible in practice 

 to grind them of the non-spherical curvatures necessary to avoid 

 it. In our eyes its effect is to a large extent corrected in the 

 following ways: (a) The opaque iris cuts off many of the ex- 

 ternal and more strongly refracted rays, preventing them from 

 reaching the retina. (6) The outer layers of the lens are less re- 

 fracting than the central; hence the rays passing through its 

 peripheral parts are less refracted than those passing nearer its 

 axis. 



3. Irregularities in Curvature. The refracting surfaces of our 

 eyes are not even truly spherical; this is especially the case with 



the cornea, which is very rarely 

 curved to the same extent in its 

 vertical and horizontal, diameters. 

 Suppose the vertical meridian to 

 be the most curved ; then the rays 

 proceeding from points along a ver- 

 tical line will be brought to a 

 focus sooner than those from points 

 on a horizontal line. If the eye is 

 accommodated to see distinctly 

 the vertical line, it will see indis- 

 F g . tinctly the horizontal and vice 



versa. Few people therefore see 



equally clearly at once two lines crossing one another at right 

 angles. The phenomenon is most obvious, however, when a series 

 of concentric circles (Fig. 91) is looked at: then when the lines 

 appear sharp along some sectors, they are dim along the rest. 



