200 POLARIZATION OF LIGHT. [SECT. xxi. 



SECTION XXI. 



Polarization of Light Defined Polarization by Refraction Properties of the 

 Tourmaline Double Refraction All Doubly Refracted Light is Polarized 

 Properties of Iceland Spar Tourmaline absorbs one of the two Refracted 

 Rays Undulations of Natural Light Undulations of Polarized Light 

 The Optic Axes of Crystals M. Fresnel's Discoveries on the Rays passing 

 along the Optic Axis Polarization by Reflection. 



IN giving a sketch of the constitution of light, it is impossi- 

 ble to omit the extraordinary property of its polarization, 

 " the phenomena of which," Sir John Herschel says, " are so 

 singular and various, that to one who has only studied the 

 common branches of physical optics it is like entering into 

 a new world, so splendid as to render it one of the most 

 delightful branches of experimental inquiry, and so fertile 

 in the views it lays open of the constitution of natural bodies, 

 and the minuter mechanism of the universe, as to place it 

 in the very first rank of the physico-mathematical sciences, 

 which it maintains by the rigorous application of geometrical 

 reasoning its nature admits and requires." 



Light is said to be polarized, which, by being once reflected 

 or refracted, is rendered incapable of being again reflected or 

 refracted at certain angles. In general, when a ray of light 

 is reflected from a pane of plate-glass, or any other substance, 

 it may be reflected a second time from another surface, and 

 it will also pass freely through transparent bodies. But, if a 

 ray of light be reflected from a pane of plate-glass at an angle 

 of 57, it is rendered totally incapable of reflection at the 

 surface of another pane of glass in certain definite positions, 

 but it will be completely reflected by the second pane in other 

 positions. It likewise loses the property of penetrating 

 transparent bodies in particular positions, whilst it is freely 



