28 president's address — section a. 



neighbourhood in motion, and that there was no " shp " at the 

 earth's surface. This hypothesis completely accounts for the aber- 

 ration of light, provided the motion of the ether is irrotational, 

 which is only possible if the ether is compressible.^ The degree^ 

 of compression necessary, however, to account for the observed 

 aberration is so huge that some effect on the velocity of light is 

 to be expected. Stokes' hypothesis of no slip and Lodge's experi- 

 ment are contradictory. The other theory of abberation is Fresnel's. 



The Absence of Expected Second Order Effects. — Michelson and 

 Morley's Experiment. — An immediate conclusion from the theory 

 that the ether is stationary and the earth moves through it is that 

 certain positive effects proportional to u^/v'^=^^^, or of a second 

 order of small quantities, are to be expected. Michelson and Morley 

 were the first to seek for such a phenomenon. A beam of light was 

 split ; one-half had, for a certain orientation of the interferometer, 

 a longitudinal path in the direction of the earth's motion, and back 

 from a mirror to the observer ; the other half of the beam had a 

 similar path, but it was at right angles to first and to the direction 

 of motion. The beams recombined, forming interference bands, 

 which, however, were unaltered by changing the orientation of the 

 interferometer. If the interferometer moved thi"ough a quiescent 

 ether, the longitudinal path would be longer than the lateral, as 

 the mirror is retreating from the light. The lateral path is also 

 increased, but not so much as the other. This sets up a difference 

 of path, which depends on the orientation of the interferometer. 

 But actually the interference bands were independent of orienta- 

 tion. One-tenth of the effect to be expected could have been 

 detected. This was the first of a series of experiments which failed 

 to bring to light small physical effects to be expected from the con- 

 vection of optical or electrical apparatus (with the earth) relative 

 to the ether, which we concluded above is stationary. 



Search for Double Refraction. — More than one point of view 

 would indicate the possibility that the convection of transparent 

 media through the ether would render them doubly refracting. 

 The Fitzgerald-Lorentz shrinkage might have the same effect on 

 the optical properties of the medium as a mechanical stress would 

 if it produced the shrinkage. Lord Rayleigh^ failed to find any 

 evidence of double refraction in carbon bisulphide or glass due to 

 the earth's motion. 



Electrical Effects. — -Trouton and Noble's Experiment. — The 

 convection of the charges of an electric condenser will produce a 

 magnetic field with a certain energy. It is to be expected that the 

 plane of the condenser will tend to set itself in a definite direction 

 with respect to the direction of motion. Trouton and Noble sus- 

 pended a condenser, but were unable to detect any couple due to 

 the earth's motion through space, and they could have detected 

 the couple calculated to act on the condenser. 



1. Planck. 



2. Lorentz "Theory of Electrons," p. 314. 



3. Phil. Mag., 4. 678 (1902). 



