Section III, 1919 [15] Trans. R.S.C. 



On the Optical Transparency of Certain Specimens of Fluorite 



By Mr. J. F. T. Young, M.A. and Mr. H. J. C. Ireton, M.A. 



Presented by Professor J. C. McLennan, F.R.S.^ 



(Read May Meeting, 1919.) 



Recent researches in spectroscopy have shown the increasing 

 importance of a complete study of the ultra-violet spectra of the 

 elements, and attempts have been made to carry out such work at 

 various times. Schumann, Handke and Wolff have employed the 

 prism spectrograph, the optical train of which consisted of white 

 colourless fluorite supplied by Zeiss. The range of this type of instru- 

 ment depends of course on the transparency of the fluorite which, for 

 thin plates, reaches a lower limit at about 1200 A.U. A second 

 method employed for studying the Schumann region by Lyman, 

 involves the use of a vacuum grating spectrograph. The grating must 

 be specially ruled to throw the maximum of energy into the ultra- 

 violet of one order of the spectrum and since its focal length is limited 

 by the size of the vacuum chamber and the necessity of keeping the 

 distance comparatively short to maintain intensity at the camera, it 

 has been found in practice that the dispersion of the grating spectro- 

 graph is not greatly superior to that of the 60° fluorite prism spectro- 

 graph. This smaller dispersion of the fluorite spectrograph is, however, 

 more than compensated by the increase in intensity which is always 

 secured by such an optical train. In vacuum work this has its decided 

 advantage, for the range of spectrum covered, in the study of weak 

 sources and in the photography of the weaker lines in spectra, as well 

 as in a shortening of the time of exposure to ordinary light sources. 



It will be seen therefore that it is most desirable for the require- 

 ments of spectroscopy to locate a source of supply of pure quality 

 colourless fluorite, the monopoly of which in pre-war days was held 

 by Zeiss. Lyman has recorded in his work on absorption of crystal- 

 line solids that certain samples of a green variety of fluorite, from New 

 Hampshire, U.S.A., were almost as transparent as that supplied by 

 Zeiss, whereas all the other coloured varieties began to absorb strongly 

 at about 1700 A.LT. However, this New Hampshire source does not 

 seem to have been developed, for at the present time there is a great 



^ Communicated by permission of the Admiralty by Professor J. C. McLennan, 

 F.R.S. 



