EXPERIMENTS ON ETHER-DRIFT 3 r 



physical quantities involved in any particular experiment as to 

 produce, on the whole, no observable effect, and that, therefore, 

 every conceivable attempt to measure ether-drift is predestined 

 to failure. But it is to be remembered that it is unnecessary 

 to assume any change other than that of length, in order to 

 account completely for the two negative results previously re- 

 ferred to. There is, therefore, some justification for the attempts 

 which have been made to discover whether the supposed altera- 

 tion of dimensions does occur or not ; although, in the light of 

 their results, one is almost driven to the conclusion that its 

 detection will really always be impossible. The experiments 

 are, however, worth recording, if for no other reason, on account 

 of their extreme delicacy. The quantity to be measured is again 

 extremely small, being only a change of length of one part in 



two hundred million, it being the fraction \ (y) of the whole; 



for example, if we consider a particular diameter of the earth 

 perpendicular to the direction of motion through the ether, it is 

 supposed to be reduced by a few centimetres only when it 

 becomes parallel to the ether-drift. 



No direct attempt at measuring the change can succeed, 

 because it is necessary that equal lengths of all materials suffer 

 equal changes in length. The Michelson-Morley experiment 

 was carried out using both wooden and stone frames ; and the 

 material used as the dielectric in the Trouton-Noble condenser 

 was mica, while the plates were of tinfoil. It is obviously the 

 natural conclusion, in view of the supposed relative alteration in 

 size being equal for four such different substances, that all solid 

 bodies, at any rate, are affected similarly. In the experiments 

 to be described, quantities were measured which might reason- 

 ably be expected to vary with alteration in length. 



The first of these investigations — that in which Lord 

 Rayleigh x determined that the motion of the earth through the 

 ether produces no sensible double refraction — was based on the 

 following considerations : It is well known that if a piece of 

 unstrained glass be placed between two crossed nicols, no light 

 is thereby caused to pass through the analysing nicol; the 

 application of stress to the glass, besides deforming it, causes it 

 to become doubly refracting, thus producing a partial revival of 

 the light through the analyser. Is it not, therefore, to be 



1 Lord Rayleigh, Phil, Mag. 6th Series, vol. iv. 1902. 



