SECT. XXI. POLARIZATION OF LIGHT. 179 



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 impossible to 

 omit the extraordinary property of its polarization, "the pheno- 

 mena 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 experi- 

 mental 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 1 by the rigorous appli- 

 cation 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 re- 

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

 reflected from a pane of platen-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 transmitted by them in 

 others. Light, so modified as to be incapable of reflection and 

 transmission in certain directions, is said to be polarized. 



Light may be polarized by reflection from any polished surface, 

 and the same property is also imparted by refraction. It is pro- 

 posed to explain these methods of polarizing light, to give a short 



