PHOSPHORESCENCE 61 



intervening dark rings successively change places. If crys* 

 talline plates, having one axis of double refraction, be viewed 

 in the direction of the aids, the rings are circles, and they 

 are crossed by a dark or light cross. Figure 93 shows the 

 position of the colored rings and cross in calc spar, and 

 figure 94, the same at intervals of 90° in the revolution of 

 the plate. With a crystal having two axes of double refrac- 

 tion, there are two series of elliptical rings, as in figures 95 

 96 ; these figures show the character of tjie rings in niter 

 iie latter alternating with the former in the revolution of th 

 plate. 



The same results are produced when the light is polarized 

 by other means. For example, if a ray of light be reflected 

 from a plate of glass at a certain angle, (56^ 45',) it is polar- 

 ized ; and on causing this ray to pass through crystals, as 

 above, similar rings are shown with the same succession of 

 changes on revolving the analyzing plate. 



There are some monometric 97 



crystals which have the property 

 of polarization. The accomriany- 

 ing figure of a crystal ofanajcime, 

 by Sir David Brewster, exhibits a 

 singular symmetrical arrangement 

 of lines of prismatic colors and 

 dark alternating lines ^ith cross 

 bands, producing a very brilliant 

 effect. An irregular polarization 

 has also been detected in some 

 diamonds. 



PHOSPHORESCENCE. 



Several minerals give out light either by friction or when 

 gently heated. This property of emitting light is called 

 phosphorescence. 



Two pieces of white sugar struck against one another give 

 a feeble light, which may be seen in a dark place The 

 same effect is obtained on striking together fragments of 

 quartz, and even the passing of a feather rapidly over some 

 specimens of zinc blende, is sufficient to elicit light. 



Fluor sp^,r is the most convenient mineral for showing 

 phosphorescence by heat. On powdering it, and throwing 



What is/4aid of the appearance of certain crystals in polarized light 

 What is phosphorescence? Mention examples explaining the differen 

 modes 'of exhibiting it. 



I I 6 



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