Fluorescence and Arrangement of Molecules. 475 



salt in the vessel G v and the fluorescent light from the latter 

 served as the standard with which the intensities of light 

 from the 23 solutions mentioned above were compared. 



i 



sj 



With the fluorescent light emitted from G l and G 2 in all 

 directions there was mingled a comparatively large quantity 

 of light from the exciting pencil, scattered at the sides of the 

 vessels Gr x and Gr 2 ; this had necessarily to be separated from 

 the fluorescent light itself. As this was only possible by a 

 spectroscopic method, the photometer chosen was that of 

 Vierordt (K), which is nothing more than a spectro- 

 scope with two slits, one immediately above the other, at S 3 . 

 In this case the slits were each covered with a totally reflect- 

 ing prism, so that one of them received light from the left, 

 the other from the right. In the figure only one of these 

 prisms can be shown. These prisms received the fluorescent 

 light from the two vessels Gr 1 and Gr 2 respectively. After the 

 widths of the slits had been adjusted until they gave, on looking 

 through the eyepiece, spectra of the fluorescent light of equal 

 brilliancy, ttfe ratio of these widths, which were measured by 

 micrometer-screws, gave directly the intensity of light from 

 the substance under examination in the vessel Gr 2 . 



The "fluorescibility," however, depends not only on the 

 intensity of the fluorescent light, but also on the quantity of 

 light absorbed ; and, moreover, it is evident that in the latter 

 term we must take into account the absorption of the 

 fluorescent light itself. Now there is a simple theory, for 

 which my original paper must be referred to, according 

 to which the fluorescibility /, as given by the experiment, is 



