POLARISED LIGHT. 5 



supposed to have some analogy to the effects of magnetism 

 or electricity, or to other actions comprised under the general 

 term polar. And light so affected was therefore said to "be 

 polarised. The term is perhaps not altogether felicitous or 

 self-explanatory, but it is probably too late in the world's 

 history to change it. 



And now, passing from the direct instrumental examina- 

 tion of polarised light, it is necessary to say a few words on 

 the physical explanation which has been proposed of the 

 phenomena. Light, according to the wave theory, is supposed 

 to be due to the vibrations of a certain elastic medium which 

 is diffused through all space, and which is capable of under- 

 going vibrations. The vibrations, to the effect of which on 

 the retina the impression of light is attributed, are extremely 

 small in amplitude and extremely rapid in succession ; and, 

 for reasons too long to be enumerated here, may be regarded 

 as always taking place in planes perpendicular to the ray. 

 In these planes the actual motion may have any direction and 

 form compatible with the mechanical constitution of the 

 vibrating medium, or ether as it is called. But, in particular, 

 there are reasons for thinking that the paths or orbits are 

 always straight lines, or circles, or ellipses. The straight 

 lines, and the longer axes of the ellipses, may have any 

 direction in the perpendicular plane through the ray ; and 

 the direction of motion in the circles and ellipses may be 

 either direct or retrograde. 



We will first consider more in detail the case of rectilinear, 

 or straight line, vibrations. The general principles which 

 regulate this case are applicable, cseteris paribus, to the others; 

 but it is convenient to begin with the simplest form. Ima- 

 gine then a row of particles of this ether lying originally in 

 a straight line to be successively disturbed from their positions 

 of rest, and then allowed to fall back again. The successive 

 motion of the particles may be due to their mutual attractions 

 when one of them has been once disturbed ; or it may be due 

 to other forms of elasticity. But whatever be the physical 

 cause, the motion gives rise to a wave -like configuration which 

 is passed on throughout the row of particles, and produces 

 what is understood as the progressive motion of the waves. 

 Now in ordinary, or unpolarised light, the successive dis- 

 turbances or impulses communicated to any particle need not 

 be all in the same direction; and there is experimental 



