60 



PHYSICAL PROPERTIES OF MINERALS. 



In calc spar it is less, it diminishing from 1*654 to 1*483 

 In quartz it is greater, it increasing from 1*5484 to 1*5582. 

 The former is said to have a negative axis, the latter a 

 'positive. 



This property of double refraction belongs to such of the 

 fundamental forms as have unequal axes ; that is, to all except 

 those of the monometric system. Those forms in which the 

 lateral axes are equal, (the dimetric and hexagonal systems, 

 have one axis of double refraction ; and those in which they 

 are unequal, (the trimetric, monoclinic and triclinic sys 

 terns,) have two axes of double refraction.* 



Both rays in the latter are rays of extraordinary refraction. 

 In niter, the two axes are inclined about 5° to each other ; 

 in arragonite, 18° 18 ; in topaz, 65°. The positions of the 

 axes thus vary widely in different minerals. 



Polarization. — The extraordinary ray exhibits a pecu- 

 liar property of light, termed polarization. Viewed by means 

 of another doubly-refracting crystal, or crystalline plate, 

 (called from this use of it an analyzing plate,) the ray of light 

 becomes alternately visible and invisible as the latter plate 

 is revolved. If the polarized light be made to pass through 

 a crystal possessed of double refraction, and then be viewed 

 in the manner stated, rings of prismatic colors are developed, 

 93 94 95 96 



and on revolving the analyzing plate, the colored rings and 



What is meant by positive and negative double refraction 1 What 

 crystalline forms exhibit double refraction 1 which have one and which 

 two axes of double refraction ? What are the effects due to polarization I 



* The figures in the note to page 42, represent the form of the mole 

 cules corresponding to these three conditions : 1, a sphere ; 2, an ellip 

 soid with equal transverse axes; 3, an ellipsoid with unequal latera 



