CH. xxi. POLARIZATION OF LIGHT. 181 



of unequal brightness, but the light of all four was never 

 greater than the light of the one ray, out of which they had 

 all come. These four rays continued apart while he turned 

 the second crystal more and more round ; till, when he had 

 turned it 90, or a quarter of a circle, the rays became two 

 again, with this remarkable peculiarity, that they had changed 

 characters ! The ray which before had been refracted in 

 the ordinary way now took the extraordinary direction, while 

 the other chose the ordinary one. 



This curious effect observed by Huyghens is now known 

 as the ' polarization of light ' by crystals. It is very difficult 

 to understand, and you must be content at present to know 

 that he discovered the fact. There is a beautiful explanation 

 of it, but we must wait for that till we consider the science 

 of the nineteenth century, for it is now much better under- 

 stood. Huyghens' ' Theory of Light ' was published in 1690, 

 under the title ' Traite de la Lumiere.' He remained in 

 Paris for some years ; but left it and returned to Holland 

 when the persecution of the Protestants began after the 

 revocation of the Edict of Nantes. He died in 1695. 



Chief Works consulted. HerschePs 'Familiar Lectures' art. 

 'Light;' Ty lor, 'On Refraction' 'Nature,' vol. ix. ; ' Edin. Phil. 

 Journal,' vols. ii. and iii. 'On Double Refraction;' Ganot's 'Phy- 

 sics;' Encyclopaedias ' Britannica,' ' Metropolitana,' and Brewster's, 



