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CHAPTER IX. 



DISCOVERY OF THE LAWS OF PHENOMENA OF 

 DIPOLARIZED LIGHT. 



BESIDES the above-mentioned perplexing cases 

 of colours produced by common light, cases of 

 periodical colours produced by polarized light began 

 to be discovered, and soon became numerous. In 

 August 1811, M. Arago communicated to the Insti- 

 tute of France an account of colours seen by passing 

 polarized light through mica, and analyzing 1 it with 

 a prism of Iceland spar. It is remarkable that the 

 light which produced the colours in this case was 

 the light polarized by the sky, a cause of polariza- 

 tion not previously known. The effect which the 

 mica thus produced was termed depolarization; 

 not a very happy term, since the effect is not the 

 destruction of the polarization, but the combination 

 of a new polarizing influence with the former. The 

 word dipolarization, which has since been proposed, 

 is a much more appropriate expression. Several 

 other curious phenomena of the same kind were 

 observed in quartz, and in flint-glass. M. Arago 

 was not able to reduce these phenomena to laws, 



1 The prism of Iceland spar produces the colours by separating 

 the transmitted rays according to the laws of double refraction. 

 Hence it is said to analyze the light. 



