CHAPTER XI. 

 ON FLUORESCENCE ABSORPTION. 1 



In a paper published in 1904 2 we used the term fluorescence absorption 

 in referring to the increase of the absorbing power of a fluorescent substance 

 which results from fluorescence. Such an effect was first observed by Burke, 3 

 who found a considerable increase in the absorption of uranium glass when 

 the glass was excited to fluorescence. The present writers observed the 

 same effect in solutions of fluorescein and eosin. The increase in the absorp- 

 tion appeared to be greater for those wave-lengths which corresponded 

 to the brightest regions of the fluorescence spectrum. Camichel, 4 upon 

 repeating these experiments, was unable to detect any change of absorbing 

 power during fluorescence either in the uranium glass used by Burke or in 

 the fluorescent solutions tested by us. The question was again attacked in 

 1907 by Miss Wick, 5 who made a detailed study of the phenomenon in the 

 case of an alcoholic solution of resorufin. Her results consistently showed an 

 increase in the absorbing power of the solution during fluorescence, and 

 were in complete agreement with the results obtained by us with fluores- 

 cein and eosin. More recently a method of detecting the effect, if it exist, 

 has been suggested by Wood, 6 and a few trials of the method by him led to 

 negative results. The most recent experimenter in this field is Houstoun, 7 

 whose very careful experiments also fail to give any indication of a change 

 in absorption due to fluorescence. 8 



The results obtained by ourselves, and especially those obtained by Miss 

 Wick, were so definite and positive that until recently we have been of the 

 opinion that the failure of others to observe the effect was due to the fact 

 that they had not chosen suitable conditions for the experiment. We were 

 led to suspect the existence of some systematic error, however, by the 

 results of a careful study of the collimator slit of the spectrophotometer 

 used in our experiments upon the distribution of energy in fluorescence 

 spectra. 9 It was found that the screw was a very accurate one and that the 

 opening of the slit was very closely proportional to the reading of the micro- 

 meter screw. To test this point the slit was mounted in a lantern and the 

 enlarged image was measured for a large number of different settings. The 

 results are shown in Fig. 163. 



We then tested the amount of light passing through the slit at different 

 widths by balancing two acetylene flames against each other, the adjustment 

 being made by varying the slit width in one case and by varying the dis- 



1 Nichols and Merritt, Physical Review, xxxi, p. 500, 1910. 

 2 Nichols and Merritt, Physical Review, xix, p. 397, 1904. 

 8 Burke, Philosophical Transactions, 191a, p. 87, 1898. 

 ^Camichel, Comptes Rendus, vol. 140, p. 139. 

 'Frances G. Wick, Physical Review, xxiv, p. 407, 1907. 

 R. W. Wood, Phil. Mag., 16, p. 940, 1908. 



'R. A. Houstoun, Proc. Royal Society of Edinburgh, 29, p. 401, 1909. 



8 Since the work described in this chapter was first published, still another article on the subject has 

 appeared, in which the results were also negative. 

 See Chapter XII. 



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