THE SENSE OF SIGHT. 



623 



same focus. The inequality may be either in the cornea or lens, or 

 both, though usually in the cornea. 



In the normal cornea the radius of curvature in the vertical 

 meridian is a trifle shorter, 7.6 mm., than that of the horizontal, 7.8 

 mm., and hence its focal distance is slightly shorter. The difference, 

 however, in the focal distances is so slight that the error in the forma- 

 tion of the image is scarcely noticeable. A transection of a cone of 

 light coming from the cornea is practically a circle. If, however, the 

 vertical curvature exceeds the normal to any marked extent, the rays 

 passing through this meridian will be more sharply refracted and 

 brought to a focus much sooner than the rays passing through the 

 horizontal meridian. The result will be that the cone of light will 

 be no longer circular, but more or less elliptic. The variations of the 

 shape of this cone are shown in Fig. 299, which represents the 

 appearances presented on cross-section both before and after focaliza- 

 tion of each set of rays. Though the vertical meridian has usually 



FIG. 299. REFRACTION BY AN ASTIGMATIC SURFACE. (Hansell and Sweet.) 



the sharper curvature, it not infrequently happens that the reverse 

 is true. For the reason that the rays from one point do not all 

 come to the same focus or point, the condition is termed astig- 

 matism. 



Spheric Aberration. On account of the difference in curvature 

 of successive portions of the surface of the lens, there is an unequal 

 refraction of the rays which pass through those portions between the 

 center and the periphery. This would give rise to many focal points 

 and hence an indistinctness of the image. That this condition is 

 present to but a slight extent in the normal eye is due to the presence 

 of the iris, which intercepts those rays which would otherwise pass 

 through the marginal portions of the lens. 



Chromatic Aberration. When a beam of white light is made 

 to pass through a prism, it is decomposed into the primary colors 

 owing to a difference in the refrangibility of the rays. In passing 

 through the refracting media of the eye the different rays composing 

 white light also undergo unequal refraction and those rays which 



