CONCA VE MIRRORS. 



99 



from the mirror than the principal focus, or vice versd if the 

 luminous rays fall divergent upon the concave mirror, a 

 focus is obtained at another point called the conjugate focus. 

 In the first case the rays, instead of being parallel, will 

 converge towards L (Fig. 88), while in the second the focal 



FIG. 87. 



point of light will be removed further away from the mirror, 

 and the rays proceeding from a lamp may be brought to a 

 focus until the distance between the source of illumination 

 and the mirror has been lessened to the centre of curvature, 

 the rays being then reflected on to themselves. 



FIG. 88. 



FIG. 89. 



If the source of light be placed between the principal 

 focus and the mirror the reflected rays will be divergent, as 

 shown in Fig. 89. 



Let us now consider the action of lenses upon illuminating 

 rays. In a double convex lens the refracted rays from a 

 parallel pencil of light form a focus very near to the centre 

 of curvature of the lens, and conversely when a lamp is 

 placed in its principal focus a double convex lens may be 

 made to appear the source of light, as shown in Fig. 90. 



H 2 



