528 THE HUMAN BODY. 



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 centre of the lens will not meet until a little beyond that 

 point. If the retina receive the image formed by the periph- 

 eral 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. 140, 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 external and more strongly 

 refracted rays, preventing them from reaching the retina. 

 (b) The outer layers of the lens are less refracting 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 pro- 

 ceeding from points along a vertical 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 indistinctly 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 obvi- 

 ous, however, when a series of 

 concentric circles (Fig. 153) is 

 looked at : then when the lines 

 appear sharp along some sec- 

 tors, they are dim along the 

 rest. When this defect, known 

 as astigmatism, is marked it 

 causes serious troubles of vis- 

 ion and requires peculiarly 

 shaped glasses to counteract 

 it. 



4. Opaque Bodies in the 

 FIG. 153. Refracting Media. In dis- 



eased eyes the lens may be opaque (cataract) and need 



