72 RELATION OF POLARIZATION PART i. 



In perfectly regular crystals like the cube or die, the 

 octohedron, &c., there are three axes of symmetry and 

 of equal elasticity at right angles to one another. In 

 these regular crystals all the axes are optical, so that 

 they have no double refraction. 



Eight square prisms have two equal rectangular axes 

 of symmetry, two axes of equal elasticity, and one 

 optical axis. 



All crystals of the pyramidal and rhomboidal systems 

 have one axis of symmetry, two axes of elasticity, one 

 optical axis ; and form coloured circular rings traversed 

 by a black cross when viewed by depolarized light. 



Lastly oblique prismatic crystals which have three 

 unequal axes of symmetry have three axes of unequal 

 elasticity, two optical axes; and by depolarization give 

 coloured lamnescata, that is coloured figures having the 

 form of the figure 8 which are traversed by a black cross 

 in two opposite quadrants, and when the crystal is made 

 to revolve, the same figure, but in the complementary 

 colours and traversed by a white cross, appears in the 

 other two quadrants. 



The right and lefb-handed circular polarization of 

 quartz, according as certain facettes of the crystal are 

 turned to the right or left, and the property of double 

 refraction being exclusively possessed by crystals of the 

 phomboidal form, are striking instances of the connection 

 between the geometrical arrangement of the molecules 

 of matter and the optical and thermal forces, for the 

 polarization of heat and all its consequences are in 

 every respect analogous to those of light, and similar 

 phenomena would be seen were heat visible. 



Heat changes the position of the optical axes of 

 crystals. When applied to a crystal of sulphate of 

 lime, the two optical axes gradually approach to each 

 other and at last coincide ; if the heat be continued and 

 increased, the axes open again, but in a direction at 



