SKCT. XXII. DISCOVERY OF POLARIZATION. 189 



spar has revolved through 90 ; because, in such posi- 

 tions of the spar as produce the colored images, only 

 two rays are visible at a time, the other two being re- 

 flected. When the analysis is accomplished by reflec- 

 tion, if two rays are transmitted by the mica, they are 

 polarized in planes at right angles to each other. And 

 if the plane of reflection of either of these rays be at 

 right angles to the plane of polarization, only one of 

 them will be reflected, and therefore no interference 

 can take place ; but in all other positions of the analy- 

 zing plate both rays will be reflected in the same plane, 

 and consequently will produce colored rings by their 

 interference. 



It is evident that a great deal of the light we see must 

 be polarized, since most bodies which have the power 

 of reflecting or refracting light also have the power of 

 polarizing it. The blue light of the sky is completely 

 polarized at an angle of 74 from the sun in a plane 

 passing through his center. 



A constellation of talent almost unrivaled at any 

 period in the history of science, has contributed to the 

 theory of polarization, though the original discovery of 

 that property of light was accidental, and arose from an 

 occurrence which like thousands of others would have 

 passed unnoticed, had it not happened to one of those 

 rare minds capable of drawing the most important in- 

 ferences from circumstances apparently trifling. In 

 1808, while M. Malus was accidently viewing with a 

 doubly-refracting prism a brilliant sunset reflected from 

 the windows of the Luxembourg palace in Paris, on 

 turning the prism slowly round, he was surprised to 

 see a very great difference in the intensity of the two 

 images, die most refracted alternately changing from 

 brightness to obscurity at each quadrant of revolution. 

 A phenomenon so unlocked for induced him to investi- 

 gate its cause, whence sprung one of the most elegant 

 and refined branches of physical optics. 



