16 THE ROYAL SOCIETY OF CANADA 



shortage of fluorite suitable not only for spectroscopic trains, but also 

 for achromatic combinations as well. 



Source of Samples of Fluorite Tested for Transparency 



Quite recently the ^ Adam Hilger Co., Limited, obtained some 

 samples of fluorspar from South Africa. These deposits which occur 

 to the south of Ottoshoop, in the Zeerust district of the Transvaal, 

 have apparently been worked for some time in connection with the 

 supply of fluorspar for gold refineries and for steel making plants. 

 The Geological formation appears to be of the nature of a large pipe 

 in the dolomite of the Transvaal system. The spar is colourless and 

 has been shewn by chemical analysis to be of great purity. 



Experimental Arrangements for Testing 



The fluorite spectrograph specially constructed for vacuum work 

 by McLennan, Ainslie and Fuller' was used to test samples of this 

 fluorite over the spectrum range available which was to below 1400 

 A.U. The source of light used was the vacuum carbon arc in the type 

 of lamp developed by McLennan, Ainslie and Fuller. The only differ- 

 ence in the experimental details adopted was that a small absorption 

 chamber to contain the fluorite samples was inserted between the arc 

 and the slit of the spectrograph. The whole apparatus could then be 

 evacuated and spectrograms taken of the light transmitted by each 

 sample. The time of exposure was from 30-45 minutes, a steady 

 carbon arc being maintained by a current of 10 amperes at 100 volts. 

 At frequent intervals spectrograms were taken with no fluorite in 

 the absorption chamber in order to test the light from the source. In 

 every case these spectrograms showed the carbon bands at A = 1464 A.U 

 and at A = 1430 A.U. Schumann plates prepared by the Adam Hilger 

 Co. were used throughout the experiments. 



The following table is a summary of the tests. 



^ Mcl^ennan, Ainslie and Fuller, Proc. Roy. Soc. Jan., 1919. 



