386 Prof. E. Wiedemann on the 



The radius of the aperture in the screen co is 0*15 millim., 

 consequently its section is 7rx (0*15) 2 = , 0706 sq. millim. 



If the surface B^were simply a reflecting surface, as in the 

 first experiments, then, since the rays from s fall parallel upon 

 B, only a pencil of parallel rays of section co would reach the 

 objective in the direction of the axis. But since the surface 

 B is self-luminous, rays reach the objective through co from 

 the whole surface /3; hence the ratio of the radiating surfaces 

 in the two cases is /jl/co (the projection yu- is, as we have seen 

 above, to be used instead of ft) . 



Of the rays radiated by each point of the surface B in all 

 directions, only those which pass through the opening co reach 

 the objective. The observed brightness is therefore 



~a — o-=7r77TK of the total radiation: 

 47T6 2 6400 



that is, it bears the same proportion to the total radiation as 

 the surface co to the surface of the cone with radius e. 



The observed brightness of the phosphorescent light was 

 1'7; hence the total brightness was 



T=l*7x 6400=10880. 



If light from the whole surface {3 reached the objective also 

 in the case of mirror-reflexion, the brightness would be /a/co 

 greater, and a quantity of light corresponding to this latter 

 brightness produces the phosphorescent light. 



We had observed a brightness of the exciting light of 

 13200 units; the quantity of light R, which has really ex- 

 cited the surface /3, is, however, 



R = (1-26/0-0706) x 13200 = 235000. 

 Of the incident energies R, therefore, we have transformed 

 into radiant energy 



T/R = 10880/235000 = 0'046, 



or, in round numbers, J2 °f ^ ne total incident energy is con- 

 verted into emitted light. 



In this, however, it is assumed in the first place that the 

 phosphorescent substance is transparent for the rays which 

 it emits, and further that the whole of the incident light 

 penetrates into it, whereas no doubt a part is lost by diffusion. 

 Hence the ratio T/R becomes greater. 



A somewhat large 'portion of the energy of the incident and 

 phosphorescence-exciting light is therefore transformed into the 

 energy of emitted light ; the remainder has either been absorbed 

 as heat or does not appear again as light in the motions which 

 occur in the rearrangement of the molecules. 



