the Michelson Interference Experiment. 151 



hj the assumption of a stationary aether, a property which 

 has been attributed as a necessary one to the aether by the 

 majority of physicists. But to explain the experiment of 

 Michelson it was necessary to introduce new hypotheses, 

 and one of these we will here take into consideration. We 

 shail first show the general arrangements of the apparatus 

 in Michelson's experiment. 



From the source L (fig. 1) light-rays are emitted to the 

 glass S slightly silvered on one side, so as to reflect a por- 

 tion of the light to the mirror S x and to allow the rest to go 

 through to the mirror S 2 . From Si as well as from S 2 the 

 light is once more reflected and the rays meet at last in the 

 telescope K, and produce by their interference a system of 

 bright and dark fringes. The fundamental idea of the 

 experiment is that, if the cether remains at rest, a translation 

 given to the apparatus must of necessity produce a change 

 in the differences of phase. The whole arrangement was 

 mounted upon a slab of stone floating on mercury. The 

 initial situation was : SS 2 coinciding with the direction 

 of the earth's translation, SA and SSi perpendicular to it 

 and to a vertical axis. In addition was SS 1 = SS 2 = Z. 



v 2 

 Neglecting terms of higher order than — 2 , one can show that 



c 



the light would take the time 



■to go to and fro between S and S l5 and the time 



to and fro between S and S^ The motion produces a dif- 

 ference of phase between the two beams to the extent of 



i v - 



r 



The apparatus was turned 90° about the vertical axis. A 

 measurable displacement of the interference-bands should 

 then have been observed. No such displacement was, how- 

 ever, discovered however the apparatus was orientated with 

 respect to the direction of the earth's translation. 



2. The attempt at an explanation of Michelson's experi- 



