296 THE PRINCIPLES OF SCIENCE. 



Newton has, in any of his works, displayed an under- 

 standing of the phenomena of interference inseparable from 

 the notion of waves. 



While the general principles of undulatory or harmonic 

 motion will be the same in whatever medium the motion 

 takes place, the circumstances must often be excessively 

 different. Between light travelling 186,000 miles per 

 second and sound travelling in air only about 1 100 feet in 

 the same unit of time, or almost 900,000 times as slowly, we 

 cannot expect a close outward resemblance. There are 

 great differences, too, in the character of the vibrations. 

 Gases scarcely admit of transverse vibration, so that sound 

 travelling in air is a longitudinal wave, the particles of 

 air moving backwards and forwards in the same line in 

 which the wave moves onwards. Light, on the other 

 hand, appears to consist entirely in the movement of 

 points of force transversely to the direction of propaga- 

 tion of the ray. The light-wave is partially analogous to 

 the bending of a rod or of a stretched cord agitated at one 

 end. Now this bending motion may take place in any 

 one of an infinite number of planes, and waves of which 

 the planes are perpendicular to each other cannot interfere 

 any more than two perpendicular forces can interfere. 

 Now the whole of the complicated phenomena of polar- 

 ized light arise out of this transverse character of the 

 luminous wave, and we must not expect to meet any 

 analogous phenomena in atmospheric sound-waves. It is 

 conceivable that in solids we might produce transverse 

 sound undulations, in which many of the phenomena of 

 polarization might be reproduced. But it would appear 

 that even between transverse sound and light-waves the 

 analogy holds true rather of the principles of harmonic 

 motion than the circumstances of the vibrating medium ; 

 'from experiment and theory it is inferred that the plane 

 of polarization in plane polarized light is perpendicular to 



