For 



of the JEther in an Electromagnetic Field. 25 



Let T = nT, where T is the period of vibration of the light 



used. There will therefore be a displacement of the fringes 



through In bands. Taking one fifth of the breadth of a 



band as the minimum displacement which the eye could have 



detected — and such a displacement could certainly have been 



observed — we have -. 



n= — , 

 10' 



2nd 1 m 



*'• rW'-JO 



Neglecting u 2 , for a first approximation we have 

 U ~20' d ' 



~ 20'~d' 



r = 3x 10 10 centim. per second, 



\= 0-00059 millim., 



d=77 centim., 

 we have 



u= 11-5 metres per second. 



So that any velocity of the aether exceeding 11*5 metres per 

 second would have been detected ; and the experiment there- 

 fore shows that if the aether does move at all, its velocity does 

 not attain to even this small magnitude. 



In our first experiment, as d=27 centim., a velocity of the 



77 

 aether of at least ^= x 11*5 metres per second would have been 



detected. 



In our third experiment we dispensed with the condensers 



and circuit round the mirrors, and interposed in the path of 



the light along the side be of the square (fig. 1) an electrolytic 



cell (breadth 45 millim., length 61 millim., height 42 millim.) 



with bottom and sides of plate glass and ends of brass. These 



brass plates formed the electrodes of the cell, so that when 



connected to storage-cells there was a uniform current through 



the electrolyte (acidulated water). The cell rested on one 



pole of an electromagnet with the other pole vertically over 



it, and was so placed that the beams of light traversed it at 



right angles to the current. We therefore had a vertical 



magnetic field, a horizontal electric current, and the beams of 



light at right angles to both. The interference-fringes were 



now obtained by a sodium- flame at the slit of the collimator, 



