88 Dr. W. E. Sumpner on the Di fusion of Light. 



This equation was used for the great majority of the re- 

 flexion tests. With feebly diffusing surfaces such as black 

 cloth, the distance x had to be so much reduced that the area 

 A subtended a greater angle at the photometer screen than 

 the aperture of the photometer itself. In such cases the 

 angle <j> in (11) was calculated from the solid angle sub- 

 tended by the aperture of the photometer at the centre of 

 the photometer screen. With diffusing surfaces which 

 appeared to shine slightly under the action of light the 

 effect of regular reflexion had to be separated from that of 

 diffused reflexion, by experiments made with the same 

 surface for different values of x. In such cases, which 

 will be alluded to subsequently, the true reflecting-power 

 is not given by formula (13). The areas A of the reflecting 

 surfaces used in the experiments were never circular, as 

 assumed in the above proof, but as they were always ap- 

 proximately square, with the central portion at (fig. 2), 

 any error in (12) and (13) arising in this way must be quite 

 negligible. The ratio kfK of the two lights was frequently 

 tested in the course of the experiments and was found very 

 constant during every set of tests. 



The results obtained are given in the accompanying table. 

 In the majority of cases the numbers given are approximate 

 only, as there seemed no object in aiming at great accuracy. 

 The first four surfaces referred to in the table, viz., thick 

 white blotting-paper, white (rough) cartridge-paper, tracing- 

 paper, and tracing -cloth, were, however, carefully tested, 

 and the numbers obtained represent the mean of many 

 observations. 



Table I. — Reflecting-Powers. 





per cent. 



White blotting-paper 82 



White cartridge-paper 80 



Tracing-cloth 35 



Tracing-paper 22 



Ordinary foolscap 70 



Newspapers ........... 50 to 70 



Tissue-paper (one thickness) ... 40 



,, (two thicknesses) . 55 



Yellow wall-paper 40 



Blue paper , , , 25 



Park brown paper 13 



Deep chocolate paper 4 



per cent. 



Plane deal (clean) 40 to 50 



(dirty) 20 



Yellow cardboard 30 



Parchment (one thickness) ... 22 



„ (two thicknesses) ... 35 



Yellow painted wall (dirty) ... 20 



,, „ „ (clean) ... 40 



Black cloth 12 



Black velvet 04 



Zollner found white surfaces to reflect from 70 to 78 per 

 cent, of the light incident upon them. The numbers given 

 at the head of the above table are slightly higher. They, 

 however, agree very well with results which have been pub- 





