Y. 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 



