54 TROWBRIDGE— THE ETHER DRIFT. [April 23, 



The results of this justly celebrated experiment of JMichelson 

 and Morley cannot be reconciled with the one theory which is in 

 accord with the other optical phenomena such as stellar aberration 

 and the Fizeau experiment, and which we have seen postulates an 

 ether at rest in space and so it became necessary either to abandon 

 the theory or to explain why the second order effect predicted by 

 the theory should not have been detected in the jMichelson-Morley 

 experiment. 



This explanation, due to Lorentz and Fitzgerald, was based 

 on an assumed shortening of the linear dimensions of matter result- 

 ing from its motion through the ether. It is true that effects neces- 

 sarily attendant on this hypothetical shortening have been sought in 

 vain, but nevertheless this so-called Lorentz-Fitzgerald objection 

 has tended to discredit the conclusions of Messrs. Michelson and 

 Morley. 



This, then, was the state of the question as to the relative motion 

 of matter and ether when Professor C. E. Mendenhall and I under- 

 took, in 1905, the work on which I am reporting at this time, (i) 

 A well-developed theory based on the assumption of a stagnant 

 ether which predicts a second order effect when the source of light 

 and observer are in motion with respect to the ether. (2) Failure 

 to detect any such effect by Michelson and Morley and the con- 

 clusion by them that the ether is at rest relatively to the moving 

 earth and hence not stagnant. (3) The theory rehabilitated by an 

 assumption that the linear dimension of matter is shortened by an 

 amount of the second order when it moves through the ether. (To 

 support this assumption good theoretical reasons were later adduced.) 



In further experimentation it was obviously necessary to devise 

 apparatus which should give indications which were independent of 

 any hypothetical change of dimensions such as that suggested by 

 Lorentz and Fitzgerald and be nevertheless sufficiently sensitive 

 to detect the optical second order effect due to its motion relative 

 to the ether. 



The device adopted by us consisted of a source of light placed 

 midway between two delicate electrical thermometers. Suppose the 

 line joining the three points to lie in the direction of the motion 



