332 L. Page — A Century's Progress in Physics. 



water causes the fringes to shift, and from the amount of 

 this shift the velocity imparted to the light by the motion 

 of the stream is computed. The divergence between the 

 experimental value of this quantity and that calculated 

 from FresnePs coefficient of entrainment was found by 

 Michelson and Morley to be less than one percent, which 

 was about their experimental error. Thus FresnePs 

 expression for the velocity of light in a moving medium is 

 entirely confirmed by experiment. The derivation of it 

 accepted to-day, however, is very different from his orig- 

 inal deduction. 



It has been noted that the phenomena of polarization 

 led Newton to reject the wave theory of light. The only 

 type of wave known to him was the longitudinal wave, 

 in which the vibrations of the particles of the medium are 

 in the same direction as that of propagation of the wave, 

 and it was impossible to suppose that such a wave could 

 have different properties in different directions at right 

 angles to the line in which it is advancing. But in 1817 

 Young suggested that this inconsistency between the 

 wave theory and the facts of polarization could be 

 removed by supposing the vibrations constituting light to 

 be executed at right angles to the direction of propaga- 

 tion. Thus in ordinary light the vibrations are to be 

 conceived as taking place haphazard in all directions in 

 the plane perpendicular to the ray, while in plane polar- 

 ized light these vibrations are confined to a single 

 direction. This supposition explained so many of the 

 puzzling results of experiment, that it was accepted at 

 once and led to the complete vindication of the undula- 

 tory theory. 



Elastic Solid Theory. — Shortly afterwards Poisson 

 succeeded in solving the differential equation which 

 determines the motion of a wave through an elastic 

 medium. His solution shows that such a medium is 

 capable of transmitting two types of wave — one longi- 

 tudinal, the other transverse. If h denotes the volume 

 elasticity, r\ the rigidity and p the density of the medium, 

 the velocities of the two waves are respectively 



S- 



*±1* and a/ 1 



P P 



Now a solid has both compressibility and rigidity, 

 and transmits in general both types of wave. A 



