direction of motion through a moving material medium 

 of index of refraction n an absolute velocity greater than 

 that when the medium is at rest by an amount 



which is only a fraction of the velocity v which would 

 have to be added if convected matter carried along all 

 the ether which resides within it. This expression was 

 tested directly, first by Fizeau in 1851, and later by 

 Michelson and Morley (31, 377, 1886) in this country. 

 The experiment consists in bifurcating a beam of light, 

 passing one half in one direction and the other in the 

 opposite direction through a stream of running water. 

 On reuniting the two rays the usual interference fringes 

 are produced. Reversing the direction of motion of the 

 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 Fresnel's coefficient of entrainment was found by 

 Michelson and Morley to be less than one percent, which 

 'was about their experimental error. Thus Fresnel's 

 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- 



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