NINETEENTH CENTURY. PT. in. 



for the polarization of light is a very difficult subject ; but 

 this was the first step made in it. Fresnel afterwards 

 worked out accurately why, when light is reflected at a cer- 

 tain angle, the vibrations are all made to move in one plane, 

 and so the light is polarized, as Malus had found it to be 

 from the surface of the Luxembourg windows. He also 

 showed how in some crystals, as in quartz crystals, the waves 

 are made to act upon each other, so that, instead of moving 

 to and fro, they wind round and round like the wire of a 

 corkscrew. These and many other experiments, as for ex- 

 ample, those upon the beautiful colours caused by polariza- 

 tion, were carried much farther by the eminent French 

 chemist, M. Biot (born 1774, died 1862), and by Sir David 

 Brewster (born 1784, died 1868), but they are too long and 

 difficult to be explained here. As I said at the beginning 

 of this chapter, the ' Theory of Light ' requires a special 

 study, and if you have understood something of the move- 

 ment of the supposed ether waves how they can interfere 

 with each other and produce light or darkness, how they 

 produce coloured rings in the soap-bubble, and how their 

 vibrations are altered in passing through a crystal or in re- 

 flection at certain angles you have learnt as much as can be 

 easily grasped of the discoveries of Young and Fresnel. 



Chief Works consulted. Young's ' Lectures on Natural Philosophy,' 

 1845; Peacock's 'Life and Works of Young;' Arago's ' Eloge of 

 Fresnel ;' Herschel's 'Lectures on Familiar Subjects;' Tyndall, 'On 

 I jght ; ' Spottiswoode's ' Polarization of Light ; ' Whewell's ' Inductive 

 Sciences ; ' ' Encyclopaedia Britannica ' Sixth Dissertation. 



