304 PHYSIOLOGICAL PHYSICS. [Chap. xxiv. 



It is not strictly true for spherical mirrors that 

 all the reflected rays meet at one point. It becomes 

 more and more true, however, the smaller the aper- 

 ture of the mirror. It is strictly true for parabolic 

 mirrors. 



If the rays proceed from F, then, when reflected, 

 they will be parallel. 



To find the principal focus of a concave mirror, 

 expose it to the sun's rays and catch the reflection on 

 a screen. Move the screen nearer to, or farther away 

 from, the mirror, till the position is found where the 

 image is best. That is the principal focal distance 

 and half the radius of curvature. 



2. Suppose the rays are not parallel, but diverge 

 from a point /(Fig. 130) the angle of incidence is less 



than in the first case, 

 so also will be the 

 angle of reflection, and 

 the reflected rays will 

 consequently meet in 

 a point F' outside of 

 the principal focus 



Fig. 130. Coujug-ate Foci of Concave / i u . j 



Mirror. (which is represented 



by a dot) and between 



it and the centre of curvature. Should the source 

 of light be at F', then F'AC becomes the angle of in- 

 cidence, and CA/ the angle of reflection. Since they 

 remain equal to one another, then the reflected rays 

 will meet at f. f and F' are thus related to one 

 another, and this relation is expressed by saying they 

 are CONJUGATE FOCI. 



3. By reference to Fig. 130 it is readily seen that 

 the farther y is removed the larger grows the angle of 

 incidence, and the larger, consequently, the angle of re- 

 flection. As a result, the nearer will F' approach to the 

 principle focus. When f has reached an infinite dis- 

 tance, its rays become parallel, and when reflected 



