THE SENSE OF SIGHT 717 



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 

 of this cone are shown in Fig. 306, 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 



FIG. 306. REFRACTION BY AN ASTIGMATIC SURFACE. (Hansell and Sweet.} 



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 

 eye is accommodated for one set of rays, it is not for another, and the result is a 

 fringe of colors around the image. This defect in the normal eye is so slight 

 that the mind fails to take cognizance of it. That the eye is incapable simul- 

 taneously of focalizing rays of widely different refrangiblity, as those which 

 give rise to the blue and red colors, is shown by the following experiment: 

 The eye being directed to a luminous point, a plate of cobalt-glass is 

 placed between the light and the observer close to the eye. This substance 

 has the property of intercepting all rays but the red and the blue and hence 

 these alone will be seen. The center of the image produced will be 



