Fluor-Spar in Optical Instruments. 121 



unfitted for use in optical instruments. Like these sub- 

 stances also, fluor-spar is optically isotropic, belonging to the 

 cubic system of crystals. 



The crystals of fluor-spar most usually occurrent in this 

 country are by no means colourless, being generally purplish 

 in tint, and exhibiting at the surface nearest to the light a 

 fine fluorescence. Another variety, known as chlorophane, is 

 of a pale green tint. Pure colourless crystals have also been 

 found in Cornwall and in Switzerland ; but these are now 

 rather rare. 



Lenses and prisms of fluor-spar have been from time to 

 time used, instead of those made of rock-salt, for experiments 

 on radiant heat; and, indeed, until recently this was the only 

 optical application of this material. 



Recently, however, fresh attention has been drawn to the 

 subject by the announcement from the pen of Prof. Abbe, of 

 Jena, that it is by the use of fluor-spar in combination with 

 various sorts of glass that the well-known firm of Zeiss have 

 been enabled to construct the remarkable microscope-objectives 

 known as " apochromatic " lenses. These lenses were for 

 Jong supposed — in spite of much scepticism on the part of 

 opticians — to owe their excellent resolving power to the use 

 of the new kinds of optical glass produced in Jena by Dr 

 Schott. The secret, which was remarkably well kept for 

 several years, is now revealed. Triplet lenses, somewhat like 

 a Steinheil lens, with a fluor cemented between two outer 

 lenses of glass, are used to correct the aberrations of the front 

 lenses of glass. The peculiar value of the fluor lens for this 

 purpose lies in its possession, alluded to above, of a singularly 

 low dispersive power. According to Abbe the refractive 

 index of the colourless fluor for sodium light is 1*4338, and 

 for the hydrogen lines C and F the respective indices differ 

 only by 0*004:55. For the sake of comparison with other 

 materials, it is convenient (following Abbe) to consider "the 

 quantity 



as a measure of the specific power of any material to produce 

 dispersion. Now, for ordinary crown-glasses, this quantity 

 is about 0*0166 ; for dense flint as much as 0*019 ; for the 

 special phosphate crown-glasses made at Jena it is as low as 

 0*0143 ; while for fluor-spar it is only 0*0104. As Abbe 

 points out, the possession of this property enables the instru- 

 ment-maker, by substituting fluor for crown-glass, to produce 



