SECT. XXII. DISCOVERY OF POLARIZATION. 195 



and no interference will happen. But, when two rays are trans- 

 mitted by the mica, they will be separated into four by the spar, 

 two of which will interfere to form one image, and the other two, 

 by their interference, will produce the complementary colours of 

 the other image when the spar has revolved through 90 ; be- 

 cause, in such positions of the spar as produce the coloured 

 images, only two rays are visible at a time, the other two being 

 reflected. When the analysis is accomplished by reflection, if 

 two rays are transmitted by the mica, they are polarized in 

 planes at right angles to each other. And, if the plane of reflec- 

 tion 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 

 analyzing plate both rays will be reflected in the same plane, and 

 consequently will produce coloured 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 centre. 



A constellation of talent almost unrivalled at any period in 

 the history of science has contributed to the theory of polariza- 

 tion, 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 

 inferences from circumstances apparently trifling. In 1808, 

 while M. Malus was accidentally 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 in- 

 tensity of the two images, the most refracted alternately changing 

 from brightness to obscurity at each quadrant of revolution. A 

 phenomenon so unlocked for induted him to investigate its 

 cause, whence sprung one of the most elegant and refined 

 branches of physical optics. 



Fluorescence, or the internal dispersion of light, though far from 

 possessing the beauty or extensive consequences of polarized light, 

 is scarcely less wonderful. A variety of substances, such as 

 canary-glass, a solution of sulphate of quinine, fluor-spar, and 



K 2 



