PHYSIOLOGICAL PHYSICS. [Chap. xxvn. 



true for spherical mirrors that all the reflected rays 

 meet in one point. Similarly, it is not absolutely true 

 for lenses that the refracted rays meet in one point, 

 though it becomes more nearly true the smaller the 

 aperture (page 314) of the lens. The rays from the 

 circumference of the lens are focussed nearer to the 

 lens than rays from more central parts of the lens. 

 The result is, that when the centre of the image is well 

 defined the circumference is blurred, and vice versa, 

 because the focal points for the outer and inner rays 



do not correspond. This is 

 shown in Fig. 157, where A is 

 the focus for the outer rays a 

 c, and JB is the focus for the 

 central rays b. At the position 

 B the centre is in focus, but not 



Fig. 157. S herical AVer- , u . ,, mi . , 



ration. the circumterence. Ihis aber- 



ration is easily rectified by cut- 

 ting off the external rays. In front of the lens, there- 

 fore, a diaphragm or stop is usually placed with an 

 opening in the centre. In photography the diaphragm 

 is of the utmost consequence. Every photographer 

 prefers to have such illumination as will permit him 

 to use a diaphragm with a very small opening, since 

 this adds to the definition and sharpness of his image. 

 It, of course, at the same time diminishes the 

 amount of light. In the chapter on the eye it is 

 noted how the iris acts as diaphragm, and varies in the 

 size of its pupil with the amount of light. 



Spherical aberration can also be corrected by a 

 combination of lenses of suitable curvature. 



