POLARISED LIGHT. 147 



close as possible to the eye-lens, to see properly the pheno- 

 mena in quartz and aragonite ; it must be placed at an 

 intermediate distance for viewing topaz, borax, Rochelle 

 salt, and carbonate of lead; it must be drawn out to its 

 full extent to view nitre and calc spar. 



The powers of the micro-polariscope cannot be better 

 displayed than in the exhibition of the foregoing pheno' 

 mena; there is nothing more beautiful, and few studies 

 more interesting and enlarging to the mind than that of 

 light, whether common or polarised, which must be entered 

 upon if the phenomena are to be understood. 



The crystal eye-piece, with an artificial tourmaline as an 

 analyser, will be found very useful for polariscope objects 

 generally; there is some spherical aberration, but the 

 largeness of the field far more than compensates for the 

 same; it does best for those objects that require the two- 

 inch object glass. 



Mr. Darker of Lambeth, Messrs. Elliott of the Strand, 

 and Messrs. Home and Co., and M. Soliel of Paris, supply 

 the properly-cut crystals. 



It was long believed that all crystals had only one axis 

 of double refraction; but Brewster found that the great 

 body of crystals, which are either formed by art, or which 

 occur in the mineral kingdom, have two axes of double re- 

 fraction, or rather axes around which the double refraction 

 takes place; in the axes themselves there is no double 

 refraction. 



Nitre crystallises in six-sided prisms with angles of about 

 120.° It has two axes of double refraction, along which 

 a ray of light is not divided into two. These axes are each 

 inclined about 2 i° to the axes of the prism, and 5° to each 

 other. If, therefore, we cut off a piece from a prism of 

 nitre with a knife driven by a smart blow of a hammer, 

 and polish the two surfaces perpendicular to the axes of 

 the prism, so as to leave the thickness of the sixth or 

 eighth of an inch, and then transmit a ray of polarised 

 light along the axes of the prism, we shall see the double 

 system of rings shown in figs. 89 and 90. 



When the line connecting the two axes of the crystal is 

 inclined 45° to the plane of primitive polarisation, a cross 

 is seen as at fig. 89, on revolving the nitre, it gradually 



l2 



