PROCEEDINGS OF mi: AMERICA* ACADEMY. 



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



1 ed when this la necessary, without loss of time In the preliminary 

 examination, a number of BmaU bits of fluorite were mounted on the 



living table, near the edge, and these could then be brought "ne 

 r 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 broughl out near the slit ; or, if the 

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

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

 nescence by means of the magnet. 



Figure A. 



The large aperture spectroscope already described 8 was used fur the 

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

 to give a fairly Hat 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. A.bou1 half an hour 

 is the limit of prolitable 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 crystal-, and BOmeof 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, Astrophysical Journal, 21, 83 

 (Mar. 1005) ; Studieson Fluorite, I. These Proceedings, 41, 687 (Mar 1906). 



