PREFACE. XV 



As a collateral pursuit, that of Optics is perhaps one of 

 the most beautiful and productive, as seems to be evinced 

 by the rich harvest of discoveries made by BREWSTER in 

 relation to the polarization of light and double refraction, 

 discoveries which are fruitful in new and highly curious ex- 

 perimental phenomena, as well as useful in affording an un- 

 expected method for detecting in ambiguous cases, the pri- 

 mary forms of crystals. In addition to which mention may 

 be made of HERSCHEL'S ingenious explanation relative to 

 the deviation of the succession of colors, which many crys- 

 tals exhibit, from that scale of tints established by NEWTON, 

 by conceiving the axis of double refraction to be different 

 for different colors; and of another discovery of the same 

 philosopher, concerning the circular polarization of light to 

 the right or left, and the plagihedral crystallization of 

 Quartz; (see figures 371 and 373,) from which it appears 

 that right handed polarization always accompanies right 

 handed plagihedral faces, and left handed polarization, left 

 handed faces. Very interesting observations are likewise 

 connected with the origin oMhe various kinds of lustre as 

 dependent on structure, and the phenomena of colors in 

 Labradorite as the result of internal cavities having the 

 shape of the primary solid. 



Not the least curious developement is that made by SA- 

 VART, from which it appears that the acoustical phenomena 

 depending on the elasticity of the parts of the crystal lead 

 to information connected with the internal structure of min- 

 erals. The elasticity of all the diametral lines in transverse 

 plates cut from a common crystal of Quartz, parallel to the 

 axis, having similar optical properties, is equal ; but though 

 all the plates cut parallel to the axis have similar optical 

 relations, their acoustical properties have a relation to the 



