1910.] TROWBRIDGE— THE ETHER DRIFT. 53 



The reason why we are left with these two possibiHties is that 

 the increased velocity of propagation of light in the direction of 

 the moving matter which Fizeau clearly demonstrated by direct 

 experiment may be due to the ether being dragged by the moving 

 matter or again it may be that the ether is at rest while the matter 

 moving through it in some way affects the speed of propagation 

 of light. 



It has been found impossible to develop a theory based on the 

 first of these possibilities as an hypothesis without introducing very 

 arbitrary assumptions as to the relation existing between the ether 

 and moving matter but if the second possibility be adopted as an 

 hypothesis, viz : the ether at rest in space ; it has been found pos- 

 sible, by the Dutch physicist, Lorentz, to develop a theory without 

 further arbitrary assumptions which is in accord with results of a 

 considerable number of optical experiments. 



If the ether be at rest it will be in motion relative to the earth 

 as it moves in its orbit and one might expect to find a different 

 velocity of propagation of light according as it is measured with 

 or against the supposed drift of the ether. The effect to be 

 expected will depend on the ratio of the velocity of the moving 

 earth to the velocity of light — this ratio in round numbers is one 

 ten-thousandth. There are many ways of detecting a variation in 

 the velocity of light by this amount and a number of physicists 

 have attempted to detect such a charge according as the velocity 

 was measured along or across the direction of the supposed ether 

 drift. All these experiments have given negative results and it is 

 in favor of the theory of Lorentz that it explains the absence of 

 this so-called first order effect when the light source and the 

 observer are in motion together, but even on the Lorentz theory 

 there should be an effect observable under these conditions which 

 is proportional to the square of the ratio of the earth's velocity to 

 the velocity of light — that is, proportional to one one-hundred 

 millionth. This is called a second order effect. 



Messrs. Michelson and Morley devised an experimental arrange- 

 ment of sufficient sensitiveness to detect a second order effect due 

 to relative motion of the earth and the ether, and they concluded 

 that the earth must drag the ether along with it in its motion. 



