ON FLUORESCENCE ABSORPTION. 



173 



the slit or that at a distance could be screened from the exciting light, which 

 in this case was the mercury arc. The spectrophotometer was set for a 

 wave-length near the crest of the fluorescence spectrum and not overlapping 

 either of the mercury lines, so that errors due to stray light were excluded. 

 Upon screening that part of the cell lying nearest the slit the field was 

 illuminated by the fluorescence light of the distant part of the cell after 

 transmission through the rest of the cell T. Upon screening the distant 

 part of the cell the light received in the instrument was due to the fluo- 



Table 21. 



The numbers in the table give the width of the slit near F (Fig. 170) when set 

 to a balance with the standard in front of the other slit. 



rescence of the nearer part F. Upon removing both screens fluorescence 

 and transmission occurred at the same time C. If absorption is increased 

 by fluorescence we should have 



F+T>C 



The readings in one case are given in Table 22. 



Table 22. 



