V. LIGHT. 



IN entering on the subject of LIGHT, it will be well to de- 

 scribe briefly, and in a manner as far as may be inde- 

 pendent of theory, the effects to which the term polarisation 

 has been applied. 



When light is reflected from the surface of water, glass, 

 or many other media, it undergoes a change which disables it 

 from being again similarly reflected in a direction at right 

 angles to that at which it has been originally reflected. 

 Light so affected is said to be polarised ; it will always be 

 capable of being reflected in planes parallel to the plane in 

 which it has been first reflected, but incapable of being re- 

 flected in planes at right angles to that plane. At planes 

 having a direction intermediate between the original plane of 

 reflection, and a plane at right angles to it, the light will be 

 capable of being partially reflected, and more or less so ac- 

 cording as the direction of the second plane of reflection is 

 more or less coincident with the original plane. Light, again, 

 when passed through a crystal of Iceland spar, is what is 

 termed doubly refracted, i. e. split into two divisions or beams, 

 each having half the luminosity of the original incident light ; 

 each of these beams is polarised in planes at right angles to 

 each other ; and if they be intercepted by the mineral tour- 

 maline, one of them is absorbed, so that only one polarised 

 beam emerges. Similar effects may be produced by certain 



