SENSE OF SIGHT. 



573 



some portions will be blacker and more distinct than others, 

 while in the latter the lines will present the same differences. 



Spherical Aberration (Fig. 351). When rays of light are 

 refracted, those which are incident near the edge of the lens 

 are refracted more than those near the principal axis, and will, 

 therefore, come to a focus in front of them, and produce an 

 indistinctness of the image. This indefiniteness of focus is 

 spherical aberration. To reduce this, a diaphragm is used in 

 optical instruments, by which these marginal rays are excluded, 

 or, as in large telescopic lenses, the same result is accomplished 

 by diminishing the curvature of the lens at its margin. This 



FIG. 351. Diagram showing the effect of a diaphragm in reducing the amount of 



spherical aberration. 



defect exists in the eye, and is lessened by the iris, which serves 

 as a diaphragm to cut off the marginal rays, and also by the 

 diminished refractive power of the marginal portions of the lens 

 as compared with its center. 



Spherical aberration is more marked with divergent than with 

 parallel rays, and as rays are more divergent the nearer the object 

 from which they come is to the eye, this is corrected by the greater 

 contraction of the iris i. e., the greater diminution of the pupil 

 which results in cutting off more of the marginal incident rays. 

 In the human eye spherical aberration is not an important defect. 



Chromatic Aberration. White light being a mixture of rays 

 of different colors, and these differing in refrangibility, the red 

 being the least and the violet the most refrangible, when white 

 light passes through a lens it is broken up into its component rays 



