148 



THE MICROSCOPE. 



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

 phenomena 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. 



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 refraction, 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 



oj|!!' 'iiii'i. .; i|i " ~ ~ r ~~ ~ : 



FIG. 87. Darter's Selcnite Films and Stage. 



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 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. 88 and S8a. 



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. 88 ; on revolving the nitre, it gradually 



