ABSORPTION PRODUCED BY FLUORESCENCE. 



89 



as briefly as clearness will permit, a short account of the general principles on which 

 the coefficients of absorption may be calculated. 



(fi) MODE OF DETERMINING ABSORPTION. 



Let us consider two cubes of the same fluorescent substance (fig. 1 ). 

 We shall, by comparing the light from B when fluorescing, which is transmitted 

 by A according as the latter is fluorescing or not, obtain 



E 2 = E.,/3, 



E being 1 the intensity of the light emitted by each cube separately and independently 

 of any light transmitted from the other. 



The quantities a. and ft are clearly the fractions of the incident light transmitted 

 by A according as it is or is not fluorescing. They may be called the coefficients of 

 transmission of the cube under these different circumstances. 



Similarly, if we denote the coefficients of absorption of the cube by a and b, and 

 that of reflection at the two faces by >', we have 



Hence 



a - 1 _ ( r + a ) ; 



&). 



E 1 = E [2-(r + a)J; 

 E a =E [l-(r+&)]. 



It has been found more convenient to have to deal with the transmission coefficients 

 rather than with those of absorption, equality in the former of course implying 

 equality in the latter, though, as we have seen, the convei'se is not necessarily true. 



Fl>S. 2. 



Let fig. 2 represent a plan of the plane horizontal surfaces of four similar and equal 

 cubical blocks A x , A z , A'^ A' 2 of some fluorescent substance such as uranium glass, 

 which is singly refracting. We shall put aside for the present all minor details. Let 

 P be a photometer furnished with two vertical slits, the breadths of which can be 

 varied, and rr a screen, whose plane is perpendicular to that of the paper, serving to 



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