RUMFORD'S PHOTOMETER. 423 



the candle Z. 2 , receives light from L only ; hence if both 

 shadows are of equal intensity, the illumination due to 

 each light must be the same, and it is only necessary to 

 measure the distance from the screen to each source of 

 light, and to square these distances, in order to find the 

 ratio of the illuminating powers. If the distance of the 

 lamp L r from the shadow S. 2 , which receives light from 

 it, is 56 cm , and the distance of the candle L% from the 

 other shadow S l is 16 cm , then the ratio of their illumi- 

 nating powers is 50x55 : 16x16, or 3136 : 256. 

 The intensity of the light of the lamp is to that of the 

 candle in the ratio of 3136 to 256, or in other words it 



Q 1 O/? 



is = 12 -25 times as great. 



256 



The screen may be opaque or semi-transparent. The vertical rod 

 of the retort- stand may serve as an opaque rod. Care should be taken 

 that the light from both sources falls very nearly perpendicular 

 upon the screen. Fig. 241, Z>, shows a plan of the arrangement ; in 

 both figures the lines L l S l and -L 2 $ 2 serve merely to indicate the 

 direction of the shadows ; the distances to be measured are, L^ 2 

 and Z/2/S'p As long as lamp and candle remain in their positions 

 their respective distances from the shadows cannot be conveniently 

 measured ; it is therefore best to draw a line with chalk round the 

 feet of the candlestick and lamp, then to remove both, and to 

 measure the distances from the screen to the centres of the two 

 circles. In making an actual experiment the distances are taken 

 considerably greater than in the above numerical example. 



An insurmountable difficulty in these experiments arises from 

 the fact, that two sources of light never emit light which is of equal 

 whiteness. The light of a candle is always somewhat more reddish 

 than that of a paraffine lamp ; hence the shadow projected by the 

 lamp /S\ "that is, the portion illuminated solely by the candle has a 

 more reddish tinge than the other shadow, and in consequence of 

 this difference in the colour of both shadows it is impossible to 

 decide whether both are equally illuminated. 



When a piece of paper, in the middle of which a faint 



