PRESIDENT S ADDRESS — SECTION A. 2/ 



theory obviously fails to explain it, for v is altered while « and u 

 are unchanged.^ 



Experimental evidence was obtained by Fizeau of the 

 effect of a moving material medium (water) upon the velocity of 

 light propagated in it. The light from a stationary source travelled 

 with and against the stream of water, having a velocity m and 

 refractive index //. The velocity of the light in the water was 

 increased (compared to its velocity in free space) by about half 

 the velocity of the water. This increase is expressed by 



t<(l— ly) 



The possibility of an explanation of Airy's experiment becomes 

 apparent, though the velocity of the light down the telescope tube is 

 decreased by the water, the angle of aberration is unchanged, as the 

 light disturbance drifts with the water and the earth, and to a first 

 order of approximation the observed effects can be calculated by 

 Huyghens' principle and Fresnel's coefficient. 



Lodge's Experiment. — Now, if material media change the 

 velocit}' of light in this manner, it is to be expected that the ether 

 is entrained by moving matter. The experimental evidence is 

 against that view, nor, as we will see, is there any satisfactory 

 theory, which postulates such an entrainment, of the phenomena 

 we are considering. 



Lodge directly investigated the question by sending two beams 

 of light in the equatorial crevasse in a sphere of iron capable of 

 rotation, one beam being in the direction of rotation, the other 

 against it. The interference bands formed on combining the beams 

 were unaffected by the rotation of the iron. The ether, we conclude, 

 was not entrained by the moving iron. 



Fresnel Theory. — In 1818 Fresnel in a letter to Arago formulated 

 a theory, which anticipated the increase of velocity by « (l-l/^u^), 

 which takes place in Fitzeau's experiment (see above). This 

 coefficient (1 — 1//*^) has been deduced b}- Lorentz for the propa- 

 gation of light in material media containing electrons^, and assuming 

 a stationary ether. 



Thus the aberration of light in Airy's experiment, the positive 

 effect in Fizeau 's experiment are consistent with a stationary 

 ether and electro-magnetic theory ; further, Lodge's experiment 

 affords direct evidence that moving matter does not disturb the 

 ether. 



Aberration Theories. ^Stokes' Theory.- — There are two im- 

 portant theories to explain the aberration of light from stars. 

 Stokes^ assumed that the earth set the ether in its immediate 



1. The constant of aberration is slightly greater than can be reconciled with the 

 velocity of light, and solar parallax, which determines u. Plummer, Monthly Notices 

 R.A.S., April, 1909. 



2. Lorentz : "The Theory of Electrons," p. 182. There is a small term depending 

 the dispersion of the medium in addition to the above. 



3. Phil. Mag., 27, 9, 1845. 



