38 SCIENCE PROGRESS 



light in a moving bod)- is, nevertheless, the same as that which 

 a partially moving ether would be expected to produce. This 

 has been shown by the direct experiment of Fizeau 1 on the effect 

 of moving water on the velocity of light. The experiment has 

 later been repeated and confirmed by Michelson and Morley. 2 

 It consisted in causing to interfere two beams of light, one of 

 which travelled with, and the other against, running water. 

 The position of the interference bands was found to be different 

 according to whether the water was moving or not. The shift 

 was such as to indicate that the light travelling with the water 

 was accelerated, and the other beam retarded, not by the full 

 velocity of the water, but by an amount less in the proportion 

 just calculated ; and the effect observed was exactly equivalent 

 to that which would be produced by a partial drag of the ether 

 by the moving water. 



It is noteworthy, too, that a supposition of this kind is 

 necessary to account for the observation of Airy and Hoek that 

 the angle of stellar aberration is independent of the material 

 filling the telescope. On the assumption that the ether moves 

 freely through a water-filled telescope, one would expect that 

 the observed aberration would be greater than when it is filled 

 with air, because light travels more slowly in water than in air. 

 The aberration is found, however, to be the same in both cases ; 

 and this is exactly accounted for by postulating the partial drag 

 previously referred to. 



Little more remains to be said. This paper is intended 

 merely as a review of the experimental work on ether-drift. 

 Much has been written with a view to explaining this series 

 of negative results. The supposition that the ether in the 

 neighbourhood of large bodies like the earth is carried wholly 

 with them at once introduces difficulties with regard to stellar 

 aberration. Moreover, it is surely improbable that, while the 

 ether inside moving matter appears to be only partially dragged 

 with it, the ether just outside should appear to be dragged 

 forward with the full velocity. On the other hand, supposing 

 the ether near the earth to be free, explanations of the negative 

 results can be formulated by conceiving that physical quantities 

 in general are dependent upon the relative motion of the 

 observer and the ether, and that in such a manner as to make 



1 Fizeau, Ann. de Chimie el de Physique, 3rd Series, torn. xvii. 1859. 



2 Michelson and Morley, American Journal of Science (3), vol. xxxi. 1886. 



