POLAEISING CKYSTALS. 147 



the microscope to reach the polariser, that all extraneous 

 light may be excluded. 



The concave mirror should be used with a bull's-eye 

 condenser by lamplight. The condenser may be dispensed 

 with by daylight. The above apparatus is furnished by 

 Messrs. Powell and Lealand. 



The crystals best adapted to show the phenomena of 

 rings round the optic axes, are : 



Quartz. A uniaxial crystal, one system of rings, no 

 entire cross of black, only the ends of it, the centre being 

 coloured, and as the tourmaline is revolved, the colour 

 gradually changing into all the colours of the spectrum, 

 one colour only displayed at once. 



Quartz. Cut so as to exhibit right-handed polarisation. 



Quartz. Cut so as to exhibit left-handed polarisation ; 



that is, the one shows the same phenomena when the 



tourmaline is turned to the right, as the other does when 



turned to the left. 



Quartz. Cut so as to exhibit straight lines. 

 Gale Spar. A uniaxial crystal, one system of rings, and 

 a black cross, which changes into a white cross on revolving 

 the tourmaline, and the colours of the rings into their 

 complementary colours, 



Topaz. A biaxial crystal, although it has two axes, only 

 exhibits one system of rings with one fringe, owing to the 

 wide separation of the axes. The fringe and colours 

 change on revolving the tourmaline ; this is the case in all 

 the crystals. 



Borax. A biaxial crystal; the colours more intense 

 than in topaz, but the rings not so complete, only one 

 set of rings taken in, from the same cause as topaz. 



Rochelle Salt. A biaxial crystal; the colours more 

 widely spread. Very beautiful. Only one set of rings 

 taken in. 



Carbonate of Lead. A biaxial crystal, axes not much 

 separated, both systems of rings exhibited, far more widely 

 spread than those of nitre. 



Aragonite. A biaxial crystal, axes widely separated ; but 

 both systems of rings exhibited, and decidedly the best 

 crystal for displaying the phenomena of biaxial crystals. 

 The field-lens of the eye-piece requires to be brought as 

 L 2 



