xxviii.] ANALOGY. 637 



its falsity. I am not aware, too, that Newton has, in any 

 of his works, displayed an understanding of the phenomena 

 of interference without which his notion of waves must 

 have been imperfect. 



While the general principles of undulatory motion will 

 be the same in whatever medium the motion takes place, 

 the circumstances may be excessively different. Between 

 light travelling 186,000 miles per second and sound 

 travelling in air only about 1,100 feet in the same 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 on- 

 wards. Light, on the other hand, appears to consist entirely 

 in the movement of points of force transversely to the direc- 

 tion of propagation 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. The complicated phenomena of polarised light 

 arise out of this transverse character of the luminous wave, 

 and we must not expect to meet analogous phenomena in 

 atmospheric sound-waves. It is conceivable that in solids 

 we might produce transverse sound undulations, in which 

 phenomena of polarisation 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 polarisation in plane polarised light is perpen- 

 dicular to instead of being coincident with the direction of 

 vibration, as it would be in the case of transverse sound 

 undulations. If so the laws of elastic forces are essentially 

 different in application to the luminiferous ether and to 

 ordinary solid bodies. 1 



1 Rankine, Philosophical Transactions (1856), voL cxlvi. p. 282. 



