122 On the Use of Fluor-Spar in Optical Instruments. 



an achromatic lens, consisting of flint glass and fluor-spar, 

 having faces of lesser curvature than would be requisite for a 

 lens made of crown-glass with the same sort of flint. This is 

 obviously a great advantage, as both the spherical aberration 

 and the chromatic differences of the spherical aberration are 

 reduced when the curvatures are lessened. Hence the use of 

 fluor-spar for the construction of the improved achromatic 

 lenses may be regarded as an application of this material of 

 considerable importance. 



1 have now the honour of announcing a third application 

 of fluor-spar; namely to the construction of direct- vision 

 prisms for spectroscopic purposes. The very same con- 

 siderations which render fluor superior to crown-glass for 

 achromatizing lenses render it, by an inverse use of the 

 material, superior to crown-glass for the purpose of procuring, 

 along with prisms of dense flint, a wide dispersion. A direct- 

 vision prism made of one wide-angled prism of dense flint- 

 glass between two prisms of fluor has as great a dispersion- 

 power as an ordinary five-prism combination of alternate 

 crowns and flints. Such a prism, constructed for me by Mr. 

 C. D. Ahrens, a few months ago, shows extremely good 

 definition. It is of course shorter than an ordinary crown- 

 flint combination of equal power, and requires fewer surfaces 

 to be worked optically true. 



When talking over this prism with Mr. Ahrens, he made 

 the remark that it would now be possible to construct a direct- 

 vision prism wholly without glass. This suggestion has since 

 borne fruit ; and enables me to present to the Section a small 

 piece of apparatus which I believe is an entire optical novelty, 

 namely a spectroscope which also polarizes the light. It 

 consists simply of three prisms cemented together, like an 

 ordinary direct-vision prism ; the two end prisms being of 

 fluor-spar, and the middle prism of Iceland spar. The latter 

 is cut (as in the improved Nicol prisms which I have on two 

 former occasions described at meetings of the British Asso- 

 ciation) so that the crystallographic axis of the spar lies at 

 right angles to the line of vision. The combination, which 

 was constructed by Mr. Ahrens, acts therefore both as a Nicol 

 prism and as a direct-vision prism. It is intended to be 

 employed in spectrophotometric measurements. 



For the purpose of direct-vision prisms the fluor-spar 

 employed must be of the colourless kind, as a considerable 

 thickness of it is necessarily interposed in the path of the 

 light. That which Mr. Ahrens has cut for me is almost 

 colourless, having a feeble green tinge. The combined prism 



