Dr. W. E. Sumpner on the Diffusion of Light. 91 



stance to be tested, the apparent absorption of this material 



^A (14) 



IS 



and the true absorption coefficient « is given by 



«=(i-v) 



(15) 



where rj is the previously found reflexion coefficient. 



When an inner envelope is used to produce a light which 

 radiates equally well in all directions, it is necessary for the 

 outer envelope to be large compared with it, since otherwise 

 the formula for the increase of illumination due to internal 

 reflexion cannot be applied. 



The following table gives the values of the absorbing 

 coefficients, expressed as percentages, for the four substances 

 at the head of Table I., from which the corresponding values 

 of rj used in applying (15) have been taken. 



Table II. 







Apparent Absorption. 



Real Absorption. 



White Blotting-paper 



77 per cent. 

 61 „ 

 23 

 9 



13*8 per cent. 



122 



150 



7-0 „ 



White Cartridge-pnper 



Tracing-cloth 



Tracing-paper 





Three large glass globes, made for arc-lamps, were also 

 tested. As the reflective powers of the globes could not 

 easily be found, only the apparent absorption was measured. 

 One globe was of opal glass (almost transparent), and the 

 ratio of apparent absorption was 15 per cent. A second 

 globe was of ground glass and absorbed 42 per cent. The 

 remaining one was of opal glass, too opaque to allow any 

 bright object placed within it to be distinguished. This 

 absorbed 39 per cent. 



Measurements of Transmitting -Power. 



The amount of light transmitted through the surfaces 

 above mentioned was measured in a very similar manner to 

 that in which their reflective powers were determined ; the 



