100 



Prof. 0. Lodge. Aberration Problems: [Mar. 31,. 



Two others, however, do appear to discriminate ; viz., an old and 

 difficult polarisation experiment of Fizeau,* which has not been 

 repeated since, and the recent famous experiment of Michelson with 

 rays made to interfere after traversing and retraversing paths at right 

 angles. 



The conclusions deducible from these two experiments are antago- 

 nistic. Fizeau's appears to uphold absolute rest of ether; Michelson's 

 upholds relative rest, i.e., drag by the earth. 



The author now attempts a direct experiment as to the effect of 

 moving matter on the velocity of light in its neighbourhood ; assum- 

 ing that a positive or negative result with regard to the effect of 

 motion on the velocity of light will be accepted as equivalent to a 

 positive or negative result, with respect to the motion of the ether. 



He gives a detailed account of the experiment, the result of which 

 is to show that such a mass as a pair of circular saws clamped 

 together does not whirl the ether between the plates to any appreci- 

 able amount, not so much, for instance, as a l/500th part of their 

 speed. He surmises, therefore, that the ether is not appreciably 

 viscous. But, nevertheless, it may perhaps be argued that enormous 

 masses may act upon it gravitationally, straining it so as perhaps to 

 produce the same sort of effect as if they dragged it with them. He 

 proposes to try the effect of a larger mass. Also to see if, when subject 

 to a strong magnetic field, ether can be dragged by matter. 



The aberrational effect of slabs of moving transparent matter is 

 considered, also the effect of a different refractive medium. 



Motion of medium, though incompetent to produce any aberrational 

 or Doppler effect, is shown to be able to slightly modify them if 

 otherwise produced. 



The Doppler effect is then entered into. The question is dis- 

 cussed as to what the deviation produced by a prism or a grating 

 really depends on : whether on frequency or wave-length. It is 

 shown that whereas the effect of a grating must be independent of 

 its motion and depend on wave-length alone, yet that the effect 

 observed with a moving grating by a moving observer depends on 

 frequency, because the motion of the observer superposes an aberra- 

 tional effect on the true effect of the grating. This suggests a means 

 of discriminating motion of source from motion of observer ; in other 

 words, of detecting absolute motion through ether ; but the smallness 

 of the difference is not hopeful. 



Michelson's experiment is then discussed in detail, as a case of 

 normal reflexion from a moving mirror or from a mirror in a drifting 

 medium. No error in its theory is discovered. 



The subjects of change of phase, of energy, of reflexion in a moving 



* 'Ann. de Chim. et de Phys.,' 1859. 

 f 'Phil. Mag.,' 1887. 



