95 



LIGHT. 



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

 describe 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 reflected 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 according 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 tourmaline, 

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

 emerges. Similar effects may be produced by certain other 

 reflections or refractions. A ray of light once polarised in a 

 certain plane continues so affected throughout its whole sub- 

 sequent course ; and at any indefinite distance from the point 

 where it originally underwent the change, the direction of the 

 plane will be the same, provided the media through which it 

 is transmitted be air, water, or certain other transparent sub- 

 stances which need not be enumerated. If, however, the 



