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



only difference being that the glow-lamp at V x (see fig. 2) 

 was moved to the opposite side of the surface as in fig. 3. 



OR represents the screen of paper, pinned on a wooden 

 frame, and placed perpendicular to the optical bench Y^h. The 



Fig. 3. 





OP 1 =OP 2 =^ P 2 L=y. 



glow-lamp was at P 1? the photometer at P 2 , and the Methven 

 standard at L. The distances OP^ OP 2 were arranged to be 

 equal, and balance obtained by moving L along the bench. 

 On referring to equations (8) to (13) and the arguments 

 used in establishing them, it will be seen that they are all 

 applicable to the case now considered if only we substitute r, 

 the transmitting-power, for rj. 



When, however, tests were made with the paper surfaces 

 already referred to, it was soon found that the numbers 

 calculated from expression (13), 



^ ^ -v- V 



(16) 



were not constant for the same substance. They differed 

 from each other far more than could be accounted for by 

 errors of experiment ; thus, the values found for t by this 

 formula were too high, and frequently exceeded 100 per cent. 

 For any given surface the values were found to increase pro- 

 gressively with the value of x used in the tests. The reason 

 for this is easily seen when it is remembered that equations 

 (8) to (13) are only true on the assumption that the surfaces 

 considered are purely diffusive, and do not alter in appearance 

 as the eye changes its point of view. When light is trans- 

 mitted through a semitransparent substance such as tracing- 

 paper, or tracing-cloth, the brightest part of the surface is 

 always on the line joining the eye to the light, and visibly 

 moves over the surface as the point of view is changed. The 



