SENSE OF SIGHT. 855 



the density of the lens at its central part, in order that it may act more 

 strongly on rays of light, by which the refracting power is increased 

 at that point ; this is accomplished in the ciystalline lens of the eye in 

 this manner since it is less dense at the circumference than in the centre : 

 or, spherical aberration may be diminished by placing a diaphragm 

 between the object of which the image is to be formed and the lens, so as 

 to cut off those rays which pass through the circumference and allow the 

 image to be formed only by the central rays. This method, also, is 

 adopted in the construction of the eye, where the movable diaphragm is 

 represented by the iris. 



Again, another point is to be mentioned : white light, as is well 

 known, is composed of seven colors, which vary in their different degrees 

 of refrangibility violet, indigo, blue, green, yellow, orange, and red. If 

 a beam of white light is passed through a triangular prism of glass it is 



FIG. 376. DIAGRAM ILLUSTRATING SPHERICAL ABERRATION. (Ganot.) 



The rays passing through the edges of the lens have a shorter focal distance than those passing nearer to 



the centre. 



decomposed into its constituent ra} T s, the violet ra}^s being refracted 

 most strongly and the red the least (Fig. 377). A white point on a black 

 ground does not form a simple image on the retina, but many colored points 

 appear after each other. If the eye is accommodated so as to focus to a 

 sharp image the violet rays are refracted most strongly ; the other colors 

 will form concentric diffusion circles, being most marked in the case of 

 the red rays. In the centre of all the colors a white point is produced by 

 their mixture, while around it are placed colored circles. Such an action, 

 of course, produces dimness of the object, and is known as chromatic 

 aberration. 



This, top, may be corrected in two ways : either by making use of 

 the diaphragm and cutting off the rays passing through the circumfer- 

 ence of the lens ; or in optical instruments by the use of two kinds of 

 glass, one of which has a different dispersing power from the other, but 



