Dr. W. E. Sumpner on the Diffusion of Light. 89 



lished with regard to tlie diffuse reflexion of radiant heat, 

 since 82 per cent, is generally given as the reflective power 

 of white substances*. 



The degree of consistency of the results may be judged 

 as follows : — In a series of 10 determinations of the reflec- 

 tive power of white blotting-paper, made with values of x 

 (see fig. 2), varying from 40 cm. to 82 cm., the mean 

 reflecting power was found to he 82*4 per cent., and the 

 average error of a single determination from the mean was 

 1*4. With surfaces of lower reflecting- power (such as 

 tracing-cloth) the numbers obtained in successive experi- 

 ments were more consistent. The value of rj for white 

 blotting-paper was checked by comparing it directly with 

 that of a piece of common mirror. The Methven standard 

 at L (see fig. 2) was replaced by the mirror, arranged so 

 as to reflect the light from the glow-lamp along the line of 

 the bench. The reflective power of the white blotting-paper 

 placed on the screen OR was found to be i)8*5 per cent, of 

 that of the mirror. The value of rj for the mirror, for normal 

 rays, was separately determined and found to be 82 per cent., 

 and hence <n for the blotting-paper comes out as 80'8 per 

 cent. 



Several of the numbers in the above table were confirmed 

 by comparative measurements, using white paper as a standard 

 reflector, the Methven standard at L (fig. 2) being replaced 

 by a surface of white paper exposed to the rays of the glow- 

 lamp at P'o The reflective power of one of the walls tested 

 was measured in diffuse daylight by exposing the aperture of 

 a photometer to the radiation of the wall, and balancing the 

 illumination against that of a standard candle. The part of 

 the wall affecting the photometer was then covered with a 

 sheet of white blotting-paper, and the ratio of the two illumi- 

 nations at once gave the ratio of the reflecting-powers. 



Measurement of Absorption, 



The absorbing-powers of some of the preceding substances 

 were determined by measuring the candle-power of the light 

 from a glow-lamp, first when this was uncovered, and after- 

 wards when it was surrounded with a cylinder of the paper 

 under test. The cylinders were short, being just longer than 

 the height of the lamp, and were closed at top and bottom 

 with caps of the same paper, so that the lamp was completely 



* See Jamin et Bouty, Tome Troisieme, p. 140. Results by MM- 

 Goddard, De la Provostaye et Desains. 



