90 Dr. W. E. Sumpner on the Diffusion of Light. 



enveloped by the paper tested. The ratio of the diminution 

 of candle-power to the original candle-power gave the appa- 

 rent absorption of the paper. If the candle-power in some 

 particular case was found to diminish 30 per cent., it would 

 have been erroneous to conclude that the envelope absorbed 

 as much as 30 per cent, of the light incident upon it, or that 

 70 per cent, was transmitted. If the reflecting, absorbing, 

 and transmitting powers of a material be respectively denoted 

 by 77, a, and t, there must exist between these quantities the 

 relation ^ + a + T= i„ 



Also if Q be the quantity of light given out per second by 

 the light-source witbin the envelope, the quantity of light 

 incident per second upon the surface of the envelope will, 

 owing to internal reflexion, be increased to Q', where 



Q'(1_,) = Q; 



the quantities of light absorbed and transmitted will respect- 

 ively be 



&Q' and tQ', 



a,nd the ratios these quantities bear to Q will similarly be 



and 



1-7/ I-77' 



the sum of course being unity. 



If the light-source can be assumed to radiate equally in all 

 directions, the ratio of its candle-power, after putting on the 

 envelope, to the original candle-power will not be t but 

 t/(1 — t?). The influence of internal reflexion is to increase 

 both the absorption and the transmission, and unless it is taken 

 into account large errors may be made in estimating the 

 coefficients. A very simple way of showing the effect of 

 internal reflexion consists in surrounding a glow-lamp with 

 a white paper cylinder, open at the top, and adjusting a pho- 

 tometer screen till the illumination caused by it is balanced 

 against that of some standard source of light. If now a piece 

 of white paper be placed on the top of the cylinder, so as to 

 shut in the vertical rays of the lamp, the candle-power in the 

 horizontal direction will be found to increase considerably. 

 In some of the tests, in order to avoid error caused by non- 

 uniform radiation of the lamp, this was first surrounded with 

 an envelope of tracing-cloth, or blotting-paper, and the com- 

 bination used as the light-source. If k Q be its candle-power 

 in this condition, and if k ± be the observed candle-power 

 after completely surrounding it with an envelope of the sub- 



