DEVICES FOR SEEING 291 



evidently just the reverse will be true, and his image will appear 

 broader than he is (Fig. 137). 



Consider next the case of a concave mirror whose surface 

 is the segment of a sphere. If one looks for the image of a 

 candle flame in such a mirror, there are three possible positions 

 which the candle flame may occupy: it may be (i) at the focus 

 of the mirror, (2) outside the focus, (3) within the focus. If 

 such a source of light should be at the center of curvature of 

 the mirror, all the rays will be reflected back to the same point, 

 since they move out along the 

 radii of the curved surface, 

 which radii are perpendicular 

 to that surface. If parallel rays 

 of light strike such a mirror, 

 they will all meet after reflec- 

 tion in a point known as the 

 focus, and this point must be 

 halfway between the mirror and 



,. T . T , FIG. 138. An object and its image 



its center of curvature. Light formed by a concave spherical mirror> 

 emanating from the focal point 



will be reflected evidently as parallel rays, while rays emanating 

 from a source nearer the mirror than the focus will be reflected 

 as divergent rays. 



Rays coming from a source farther from the mirror surface 

 than the focus will meet at a point. These two points, the one 

 from which the rays emanate, the other the one to which they 

 converge, are known as conjugate foci. 



If an object like a candle flame is at ab (Fig. 138), the mirror 

 will form an inverted image of it at a'b', which image may readily 

 be seen on the screen at this position. If, however, the candle 

 flame were at a'b f (turn the figure upside down), the image 

 would evidently be at ab. 



If the object is nearer the mirror than is the focus, no actual 

 image will be formed; but if the eye catches reflections in the 

 mirror from such an object, the object will appear magnified. 



