THE SENSE OF SIGHT. 



665 



Astigmatism. Astigmatism may be defined as a condition of the eye 

 characterized by an inquality of curvature of its refracting surfaces in con- 

 sequence of which not all of a homocentric bundle of rays are brought to 

 the 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 formation of the image is scarcely 

 noticeable. A transverse section 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 in the vertical plane will be more 

 sharply refracted and brought to a focus much sooner than the rays passing 

 through the horizontal plane. The result will be that the cone of light will 

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



FIG. 326. REFRACTION BY AN ASTIGMATIC SURFACE. (Hansell and Sweet.) 



of this cone are shown in Fig. 326, which represents the appearance pre- 

 sented on cross-section both before and after focalization of each set of rays. 

 Though the vertical plane has usually the sharper curvature, it not infre- 

 quently happens as illustrated in this figure, 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 astigmatism. 



Spheric Aberration. When the rays of light which emanate from a 

 point fall upon a spheric lens, they do not after passing through it reunite 

 at one point because of the fact that the more peripheral rays have a 

 shorter focus than the central rays. To this condition the term spheric 

 aberration is given. Spheric aberration can be demonstrated in the human 

 eye. 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 refracting media. 

 In widely dilated eyes the spheric aberration of the peripheral parts may 

 amount to as much as 4 or 5 dioptrics. 



Chromatic Aberration. When a beam of 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 refrac- 

 tion and those rays which give rise to one color are brought to a focus at a 

 point somewhat different from those which give rise to other colors. If the 



