168 Experimental Philosophy. [Lecture 12. 



exactly in the two lines D c and c B. The rays, 

 therefore, going from D to c 9 will seem to have 

 proceeded to A, and consequently the picture 

 will be there. For, as the rays have diverged in 

 going from the object at DD, and diffused them- 

 selves upon the surface of the glass, they will be 

 again converged into an equal focus, by the time 

 they arrive at B 5, and they will therefore paint 

 the object at A A. 



Hence we may learn, that if a man sees his 

 whole image in a plane looking glass, the part of 

 the glass that reflects his image, need be but one- 

 half as long, and one half as broad as the man. 

 For the image is seen under an angle, as large as 

 the life ; the reflecting mirror is exactly half-way 

 between the image and the eye, and therefore 

 need be but half as large as the object, to sub- 

 tend an angle as large as the image ; or, in other 

 words, it is just half as large as the image, which 

 is of the same size with the man. Thus the man 

 AB, (see fig. 56) will see the whole of his own 

 image in the glass CD, which is but half as large 

 as himself. His eye, at A, will see the eye of 

 the image at an equal distance behind the glass 

 at E. His foot at B, will send its rays to D ; 

 these will be reflected at an equal angle, and the 

 ray will therefore seem to have proceeded in the 

 direction of FDA, so that the man will see his 

 foot at F ; that is, he will see his whole figure 

 atEF. 



It is thus that plane mirrors reflect. The 



