302 



PHYSIOLOGICAL PHYSICS. [Chap. xxiv. 



B 



they are reflected and received by an eye placed 

 as shown in the figure. Similarly reflected rays 

 from B enter the eye. and from each part of the arrow 

 reflected rays will meet the eye. Thus the eye will 

 perceive an image of AB. But the eye always refers 



the object from which rays 

 reach it straight outwards 

 in the direction of the rays. 

 Thus the eye will not seem 

 to see the arrow in its proper 

 position. Suppose the re- 

 flected rays from A to be pro- 

 longed in a straight line back- 

 Fig. 128.-Image formed by a Wards > the 7 wil1 meet at tlie 

 Plane Mirror. point a behind the mirror, and 



in the line of the perpen- 

 dicular let fall from A on the mirror. The prolonga- 

 tion backwards of the reflected rays from B will meet at 

 b, and similarly the prolongation backwards of reflected 

 rays from intermediate points between A and B will 

 meet as shown in the figure. The eye will then see 

 the arrow AB as if it were behind the mirror. It 

 can be shown that this image of the arrow will be of 

 the same size as the real arrow, and will seem to be as 

 far behind the mirror as AB is in front of it. Thus in 

 plane mirrors images are produced of the same form 

 and size as the objects, and seem to be situated the same 

 distance behind as the object is in front. As shown in 

 Fig. 128, the image is not inverted, but it is reversed, 

 that is, right appears left and left right. 



Spherical mirrors are those which form part of 

 the surface of a hollow sphere. Polishing the inner 

 surface forms a concave mirror, and the outer surface 

 a convex mirror. A point in the polished surface at 

 an equal distance from all parts of the circumference 

 is the centre of the figure, and a line joining this 

 point and the centre of the sphere of which the 



