THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE, 



[FIFTH SERIES.] 



FEBRUARY 1893. 



XII. The Diffusion of Light. By W. E. SuMPNEE, D.Sc* 



LITTLE information appears to have been published about 

 the diffusing power of unpolished surfaces. The sub- 

 ject has been studied by optical measurements by Zollner and 

 others, chiefly for astronomical purposes ; and the radiation 

 from such surfaces has also been investigated with appa- 

 ratus designed for measuring radiant heat. But the in- 

 fluence of such diffusion in increasing the illumination of 

 rooms and open spaces, although well known, does not appear 

 to be appreciated to the extent that its importance deserves ; 

 and a few numerical determinations of the coefficients of 

 reflexion, absorption, and transmission of diffusing surfaces 

 may prove of interest. 



Terms in light are used vaguely, and it will not be deemed 

 out of place to define those which will be here needed. By 

 the reflecting power of a surface is meant the ratio of the 

 amount of light reflected by it to the total amount of light 

 incident upon it. The illumination of a surface is the amount 

 of incident light per unit area of the surface. The unit 

 quantity of light is the flux of radiation per second across a 

 unit area of a sphere of unit radius at whose centre a unit 

 light (of one candle) is placed. The amount of light radiated 

 by a source of k candle-power, within a solid angle Q, is kfl, 

 and the total quantity of light emitted by it is Airk. The 

 brightness of a diffusing surface is its candle-power per unit 



* Oommimicated by the Physical Society : read Dec. 9, 1802. 

 Phil Mag. S. 5. Vol. 35. No. 213. Feb. 1893. G 



