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PROCEEDINGS OF THE AMERICAN ACADEMY. 



the fluorescence spectra makes this result necessary as far as that 

 method of excitation is concerned. The fluorescence spectrum of a crys- 

 tal of fluorite is a function of the exciting source, and changes completely 

 when the exciting wave-lengths are changed. It is therefore improbable 

 that any one of the fluorescence spectra should show more than approx- 

 imate or accidental coincidences with many lines excited by either heat 

 or kathode luminescence. There are lines which appear in the fluores- 

 cence spectra of a crystal under excitation by several different sources, 



