ON FLUORESCENCE ABSORPTION. 



171 



It was suspected that the phenomenon of fluorescence absorption appa- 

 rently demonstrated by Miss Wiek and ourselves might indicate not an 

 increase in the absorbing power of the solution but rather a decrease in its 

 power of fluorescing. The results obtained by us might be interpreted 

 equally well in either of these two ways. The fact that fluorescence absorp- 

 tion seemed to be in proportion to intensity of fluorescence in different parts 

 of the spectrum lent strength to this view. If the effect is a diminution of 

 fluorescence and not an increase in absorption the failure of our photo- 

 graphic tests could also be explained in an obvious manner. 



If the fluorescence is diminished we should expect the diminution to be 

 observable not only along the line of the transmitted light but in other 

 directions. To test this matter we set up two large fluorescent cells F\ 

 and F 2 (Fig. 167) covered with black paper except on the sides toward the 

 exciting mercury arc, and having two openings O x and 2 in the bottom 

 through which the light of the tungsten lamps 7\ and To, after reflection 

 from mirrors, might pass up into the cell. The balance having been ob- 

 tained with fluorescence alone, the lamp T\ was then turned on and we 



T, 

 O 



-0, 



n 



Mercury arc 



01 



f 2 



T d 



-Green glass 



Mercury arc 

 Blue glass 



-O; 







A 



Plane glaso 



<lTT] 



MC0 3 



O 



Fig. 167. Fig. 168. 



tried to determine whether there was any change in the balance resulting 

 from extinguishing 7\ and at the same time lighting T 2 . The only effect 

 was a slight one, opposite in sign to that which would be indicated in fluores- 

 cence absorption, and due undoubtedly to a small amount of stray light 

 entering the slit after reflection from the walls of the cell. 



It seemed to us possible that the effect might be analogous to the effect 

 of infra-red rays in suppressing fluorescence, and that possibly it might be 

 produced not by the visible rays which caused this annoyance through 

 stray reflection, but by the red or infra-red rays. We therefore interposed 

 ruby glass in the path of 7\ and T 2 , thus eliminating disturbances due to 

 stray light. Under these circumstances no disturbance of balance resulted 

 from extinguishing one lamp and lighting the other. 



It seemed possible that while fluorescence absorption might really be the 

 result of a diminution in the intensity of fluorescence this decrease might 

 not be the same in all directions, but might be greatest in the direction in 

 which transmitted light proceeded. The arrangement of apparatus shown 

 in Fig. 168 was intended to test for the effect in directions nearly but not 

 quite the same as the direction of transmitted light. The light from the 

 tungsten lamp Ti passes through the cell Fi so as not to fall in the slit of the 



