PHYSIOLOGICAL OPTICS. 



141 



behind the mirror P', the position of which is found by projecting 

 the lines of the reflected rays AC and BD back until they meet. 

 Since ALL rays from the point P on the object appear after re- 

 flection to come from one point on the image, P', the place from 

 which two of the rays seem to come, must be the apparent source of 

 all the others, in other words, P' must be the image of the point P. 

 The construction for Q is similar, Q' being its image. When the 



FIG. 38. To illustrate the formation of an image by a concave mirror. 



reflected rays only appear to come from the image, as in these 

 cases, and do not actually pass through it, the image is called a 

 VIRTUAL one. It will be noted that it is erect. 



Fig. 38 shows the construction for reflection of a similar object by 

 a concave mirror. C is the centre of curvature of the mirror; any 

 line from C to the surface is perpendicular to it, since it lies on one 

 of the radii. One ray from P, passing through C on its way to the 



