REFLECTION OF LIGHT. 427 



The five flames should be carefully trimmed, so as to be as nearly 

 as possible of equal size ; their magnitudes may be measured with a 

 piece of wire bent like a hair pin and used like a pair of compasses. 

 Careful attention to this point is needful to make the experiment 

 give approximately correct results. 



39. Reflection of Light. Mirrors. The surfaces of 

 solid and liquid bodies ' reflect ' rays of light which fall 

 upon them, and it is only by the light thus reflected 

 that bodies which are not self-luminous are ren- 

 dered visible. Reflection of light varies in degree, 

 according to the condition and nature of the reflect- 

 ing surfaces. Let an open book be held with its back 

 turned towards, and somewhat below, the flame of a 

 lamp which illuminates a room in the evening ; the 

 open page will be in the shadow, and the print can 

 scarcely be read. If now a flat bright body, such as a 

 piece of paper, be held a little farther away from the 

 lamp, and higher than the book, the print will appear 

 sufficiently bright, at any rate upon the upper portion 

 of the page, to be read easily. Even holding up the 

 hand in the place of the paper will sensibly increase 

 the light which falls upon the page. If instead of the 

 paper a small square piece of looking-glass is held in 

 the same position, a portion of the page will be ren- 

 dered much brighter than in the case of the paper ; 

 moreover, the portion illuminated will now have a 

 sharply-defined boundary which separates it from the 

 rest, which remains dark. Thus, a well-polished sur- 

 face like that of a mirror reflects light in a definite 



C 1 



direction, while rough and dull surfaces reflect light in 

 all directions. 



The curved side of the shallow semi-circular vessel re- 

 presented in fig. 243 is divided into eighteen equal parts, 



