MORSE. STUDIES ON FLUORITE. 589 



period, which result in the emission of light. It would be compara- 

 tively easy to picture resonance of such a character that the absorbed and 

 emitted periods should be the same, but there does not appear to be any 

 experimental evidence that such a case exists. Even in sodium vapor, 

 which is nearest this ideal case, the excited fluorescence light has not 

 exactly the same wave-length under all conditions as the exciting light, 

 as may be seen immediately from Professor Wood's photographs. And 

 when we remember that very frequently luminescence spectra of very 

 nearly the same character are given out by a substance when it is 

 excited by light, heat, kathode bombardment, X-rays, friction, or radium, 

 it is evident that a very general property is involved of which excitation 

 by light vibrations is only a phase. 



It seems probable that neither chemical analysis nor spectroscopic 

 analysis in its present condition can serve as an adequate tool with which 

 to attack the fundamental problems of fluorescence, yet they are almost 

 the only tools available. Burke's investigation on the change of absorp- 

 tion during fluorescence, 4 and Nichols' and Merritt's work on the change 

 of electrical conductivity of solutions while fluorescing, 5 are suggestions 

 of possible variations in the method of attacking these phenomena. 



The present paper contains a brief summary of the spectroscopic 

 results obtained during a study of a single substance — fluorite. This 

 substance is remarkable because of the large number of methods of 

 excitation which may be employed with it and because of the bright- 

 ness of the luminescence light emitted under the influence of most of 

 these exciting sources. 



It is the author's intention to carry the study of light emission by 

 fluorite through the entire series of methods of excitation. The present 

 paper contains data on the fluorescence spectra only ; that is, on the 

 light emitted by fluorite when the exciting source is light. 6 



Experimental Data. 



The crystals examined were all from Weardale, England. Spectro- 

 scopic data on the results of the study of these crystals has already been 

 given in full in the Astrophysical Journal for March, 1905. It has 

 seemed advisable to submit a summary of these and of other results 

 obtained since that paper was published to the American Academy, 



* Phil. Trans., 191, 87 (1898). 



6 Pliys. Rev., 19, 396 (1904). 



6 The spectrum of the light emitted being examined during excitation. 



