6 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



to any desired position in front of the rays, or a new face can be ex- 

 posed when this is necessary, without loss of time. In the preliminary 

 examination, a number of small bits of lluorite were mounted on the 

 revolving table, near the edge, and these could then be brought one 

 after the other into the kathode rays, and their spectra studied with 

 a hand spectroscope. During the entire research the kathode stream 

 was controlled by means of a permanent magnet, and with it the 

 brightest luminescence could be brought out near the slit ; or, if the 

 crystal had been mounted a little too low or too high, the kathode 

 stream was brought into the most favorable position for bright lumi- 

 nescence by means of the magnet. 



Figure A. 



The large aperture spectroscope already described ^ was used for the 

 photography of the spectra, and Cramer Tri -chromatic plates were found 

 to give a fairly flat spectrum down as far as wave-length 6000. 



It was found that the time of exposure could not be increased beyond 

 a certain point with any advantage. The well-known phenomenon 

 of discoloration of the crystal faces takes place, and before long the 

 layer of color becomes so dense that practically no more kathode excita- 

 tion gets through it, and the luminescence stops. About half an hour 

 is the limit of profitable exposure for a single crystal face under the 

 conditions of excitation used in this work, and if the intensity of the 

 kathode stream is greatly increased, this time is reduced to a few min- 

 utes. The time varies with different crystals, and some of them remain 

 unattacked for a much longer period than others. When a longer 

 exposure than half an hour was found necessary, the crystal was simply 



8 The Fluorescence Spectrum of Fluorite, Astropliysical Journal, 21, 83 

 (Mar. 1905) ; Studies on Fluorite, I. Tliese Proceedings, 41, 587 (Mar. 1906). 



