MORSE. — STUDIES ON FLUORITE. 597 



atures. In the research of Pearsall 20 a long list of the various colors 

 emitted hy different fluorites at various temperatures and times of heating 

 is given. In this same research the matter of the regeneration by the 

 electric spark is taken up fully, but of course without any use of spectro- 

 scopic methods. 



As has been stated, the chlorophane begins to glow at a low temper- 

 ature. It reaches its maximum at a point not much above 350°, while 

 the other fluorite requires a higher temperature to start the luminescence, 

 and the temperature must be raised to a slightly higher point to completely 

 exhaust the power of luminescence. 



The Photography of the Ther mo-luminescent Spectra. 



Experimentally, the photography of spectra of luminescences of the 

 intensity of these offers no difficulty whatever. Patience is necessary, 

 since a large amount of material must be allowed to luminesce in front of 

 the slit of the spectroscope, and the time during which a single crystal of 

 fluorite glows brightly is rather short. Exposures of about four hours 

 were required to yield measurable plates. Longer exposures could of 

 course have been made, but the advantage gained would be slight, since 

 the continuous background of diffuse light is so strong that the sharp lines 

 are not much more clearly differentiated than with the shorter exposures. 

 A great advantage would be gained by the use of greater dispersion, as 

 ■the sharp lines could in this way be brought out while the continuous 

 background remained weak. For this preliminary study, however, the 

 spectrum as a whole is of interest, and the dispersion employed is well 

 suited for the purpose. The spectrograph employed was the wide-angle 

 one which had already done good service in the examination of the fluo- 

 rescence spectra of these same fluorites. 21 



After discarding mechanical arrangements of the nature of hot revolving 

 plates and dishes, the simplest possible method of exposure was employed. 

 The material was heated in a test tube out of range of the slit until it 

 began to glow. It was then held in front of the slit by hand until the 

 glow from the first heating had begun to grow weak, when it was reheated 

 and brought back in front of the slit, and so on until the luminescence was 

 exhausted. Fresh material was then put through the same treatment, and 

 the process continued throughout the time of the exposure. Later, a small 

 electric furnace was used in the same way ; the only advantage which it 



20 Journ. of Royal Institution, 1, 267 (1830). 



21 Astropliys. Journ., 21, 83 (1905). 



