MORSE. — THE KATHODO-LUMINESCENCE OF FLUORITE. 5 



3. Fluorite from Westmoreland, N. H. This is a clear, light-green 

 fluorite, which shows no very strong fluorescence, but which is most 

 brilliant in thermo-luminescence, giving out a purple light, the spec- 

 trum of which has been fully described in a previous paper. 6 Its 

 kathodo-luminescence spectrum is in many respects very different from 

 all the others described. The details of this spectrum are given in 

 Table III, and a photograph of the spectrum is reproduced in Figure 3 

 of the plate. 



4. Fluorite from Hardin County, Ohio. This is a clear pink variety 

 of no very strong fluorescence or thermo-luminescence, but which 

 shows a fairly strong kathodo-luminescence. Its spectrum is shown in 

 Figure 4, and the detail of the lines is given in Table IV. 



5. Purple fluorite from Weardale, England. This locality has fur- 

 nished some of the most beautiful fluorspar crystals of the world, and 

 this particular crystal was cut from a large and perfect natural crystal. 

 It is the same crystal as No. 5 of the paper on the fluorescence of 

 fluorite, 7 and it is characterized by a fine series of layers of different 

 colors, in planes parallel to the natural faces of the crystal. (Table V 

 and Figure 5.) 



6. Green Weardale crystal. A deep green variety from the same 

 locality, showing a kathodo-luminescence spectrum very much like that 

 of the purple variety, but different in some strong lines. Table VI, of 

 wave-lengths, and Figure 6 of the plate, show its characteristics. 



7. Yellow Weardale crystal. From the same locality, but of deep 

 straw-yellow color. Not very strong in fluorescence or thermo-lumi- 

 nescence, but giving a fine purple kathodo-luminescence. Shown in 

 Figure 7 and described in Table VII. 



III. After the preliminary study of the method, exposure, condi- 

 tions for brightest luminescence, etc., the crystals described were cut 

 from the natural crystals and their faces polished. This treatment 

 permits of excluding the lines of gases in the tube as completely as 

 possible, and gives a field of light which is regular and smooth. The 

 crystals were then mounted in the vacuum tube so that one of the 

 polished faces was exposed directly to the kathode bombardment, 

 the spectroscope being so placed that it would take in all the light 

 possible from the polished face of the crystal. 



The form of tube shown in the figure (Figure ^4) is convenient 

 for this special purpose. The crystal is mounted on the little table 

 which forms the end of the stop-cock, and so mounted it can be turned 



6 Studies on Fluorite, II. These Proceedings, 41, 503 (Mar. 1906). 



7 The Fluorescence Spectrum of Fluorite, Astrophysical Journal, 21, 83 

 (Mar. 1005); Studies on Fluorite, I. These Proceedings, 41, 587 (Mar. 100(3). 



