EXPLANATION OF PLATE. 



The upper spectrum is that of the spark between cadmium terminals, and the 

 numbers indicate wave-lengths. 



The seven numbered spectra are kathodo-luminescence spectra of tlie fol- 

 lowing : 



1. Fluorite from Amelia Court-House, Virginia. 



2. Fluorite from Trumbull, Conn. 



3. Fluorite from Westmoreland, N. H. 



4. Fluorite from Hardin Co., Ohio. 



5. Purple fluorite from Weardale, England. 



6. Green fluorite from Weardale, England. 



7. Yellow fluorite from Weardale, England. 



