NATURAL POLARIZATION. 145 



polarized light are due to atomic arrangement. By 

 altering the molecular arrangement of transparent bodies, 

 either by heat or by mere mechanical pressure, the un- 

 equal tension or strain of the particles is at once indi- 

 cated by means of the polarized ray of light and its 

 rings of colour. Differences in the chemical constitu- 

 tion of bodies, too slight to be discovered by any other 

 mode of analysis, can be most readily and certainly 

 detected by this luminous investigator of the molecular 

 forces.* 



Although we cannot enter into an examination of all 

 the conditions involved in the polarization of, and the 

 action of matter on, ordinary light, it will be readily con- 

 ceived, from what has beeen already stated, that some 

 most important properties are indicated, beyond those 

 which science has made known. 



Almost every substance in nature, in some definite 

 position, appears to have the power of producing this 

 change upon the solar ray, as may be satisfactorily shown 

 by examining them with a polarizing apparatus. f The 

 sky at all times furnishes polarized fight, which is most 

 intense where it is blue and unclouded, and the point of 

 maximum polarization is varied according to the relative 

 position of the sun and the observer. A knowledge of 

 this fact has led to the construction of a " Solar Clock,"! 



* On the Application of the Laws of Circular Polarization to the 

 Researches of Chemistry : by M. Blot. Nouvelles Annales du 

 Museum d'Histoire Naturelle, vol. iii., and Scientific Memoirs, 

 vol. i. p. 600. On Circular Palarization : by Dr. Leeson. 

 Memoirs of the Chemical Society. 



f In Sir David Brewster's Treatise On Optics, chap, xviii., On 

 Polarization^ the best arrangements for a polarizing apparatus will 

 be found described. 



; This beautiful application was recently made by Professor 

 Wheatstone, the particulars of which will be found in his interest- 

 ing communication. On a means of determining the apparent 

 Solar Time by the diurnal changes of the Plane of Polarization 

 at the Northern Pole of the Sky : Eeport of the Eighteenth 

 Meeting of the British Association. 



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