98 Prof. 0. Lodge. Aberration Problems: [Mar. 81, 



IV. " Aberration Problems : a Discussion concerning the Con- 

 nexion between Ether and Matter, and the Motion of the 

 Ether near the Earth." By Oliver Lodge, F.R.S., Pro- 

 fessor of Physics, University College, Liverpool. Received 

 March 31, 1892. 



(Abstract.) 



The paper begins by recognising the distinction between ether in 

 free space and ether as modified by transparent matter, and points 

 out that the modified ether, or at least the modification, necessarily 

 travels with the matter. The well-known hypothesis of Fresnel is 

 discussed and re-stated in modern form. 



Of its two parts, one has been verified by the experiment of Fizeau, 

 the other has not yet been verified. Its two parts are. (1) that inside 

 transparent matter the velocity of light is affected by the motion of 

 tli at matter, and (2) that immediately oatside moving matter there 

 is no such effect. The author proceeds to examine into the truth of 

 this second part, (1) by discnssing what is already known, (2) by 

 fresh experiment. 



The phenomena resulting from motion are four, viz. : — 



1. Changes in direction, observed by telescope and called aberration. 



2. Change in frequency, observed by spectroscope and called 

 Doppler effect. 



3. Change in time of journey, observed by lag of phase or shift of 

 interference bands. 



4. Change in intensity, observed by energy received by thermopile. 

 After a discussion of the effects of motion in general, which differ 



according as projectiles or waves are contemplated, the case of a fixed 

 source in a moving medium is considered ; then of a moving source 

 in a fixed medium ; then the case of medium alone moving past source 

 and receiver ; and, finally, of the receiyer only moving. 



It is found that the medium alone moving causes no change in 

 .direction, no change in frequency, no detectable lag of phase, and 

 probably no change of intensity; and hence arises the difficulty of 

 ascertaining whether the general body of the ether is moving 

 relatively to the earth or not. 



A clear distinction has to be drawn, however, between the effect of 

 general motion of the medium as a whole and motion of parts of the 

 medium, as when dense matter is artificially moved. The latter kind 

 of motion may produce many effects which the former cannot. 

 A summary of this part of the discussion is as follows : — 



Source alone moving produces a real and apparent change of 

 colour ; a real but not apparent error in direction ; no lag of 



