504 THE HUMAN BODY, 



optical instruments, as it is impossible in practice to grind 

 lenses 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. Sup- 

 pose the vertical meridian to be the most curved; then the 

 rays proceeding from points along a vertical line will be 

 brought to a focus sooner than those from points on a hori- 

 zontal 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 pheno- 

 menon is most obvious, however, when a series of concentric- 

 circles (Fig. 134) is looked at: then when the lines appeal- 

 sharp along some sectors, 

 they are dim along the 

 rest. When this defect, 

 known as astigmatism, is 

 marked it causes serious 

 troubles of vision and re- 

 quires peculiarly shaped 

 glasses to counteract it. 



4. Opaque Bodies in 

 the Refracting Media. 

 In diseased eyes the lens 

 may be opaque (cataract] 

 and need removal; or 

 FIG. 131 opacities from ulcers or 



wounds may exist on the cornea. But even in the best 

 eye there are apt to be small opaque bodies in the vitreous 

 humor causing muscce volit antes; that is, the appearance of 



