447 



FiZEAü's expei'iiiients, (liougii iiuule Uy a metliod which is (heore- 

 lically as simple as it is perfect, left some doubts as to their accu- 

 racy, partly by reason of the remarkable conclusions as to relative 

 motion of ether and matter to which they gave rise, and these 

 doubts could oidy be removed by new experiments. 



35 years after Fizeau's first communication ') to the Académie 

 des Sciences, Michklson and Morley ^) repeated the experiment. 

 They intended to remove some dil'iiculties inherent to Fizeau's method 

 of observation and also, if possible, to measure accurately the fraction 

 to be applied to the velocity of the water. Michelson uses (he prin- 

 ciple of his interferometer and produces interference fringes of con- 

 siderable width without I'educing at the same time the intensity of 

 the light. The arrangement is further the same as that used by 

 FiZEAU but performed with the considerable means, which American 

 scientists have at their disposal for important scientific questions. 

 The internal diameter of the tubes in the experiment of Michklson 

 and MoRLEY was 28 m.m. and in a first series the totaP) lengthof 

 the tubes was 3 metres, in a second series a little more than 6 metres. 



From three series of experiments wiih wJdie //(//i^ Michelson found 

 results which if reduced to what they would be if the tube were 

 2x5 metres long and the velocity 1 metre per second, would be 

 as follows: 



"Series A =r double displacement 



1 0,1858 



2 0,J838 



3 0,1800" 



"The final weighted value of A for all the observations is A :^ 0,1840. 

 From this by substitution in the formula, we get e = 0.434 with a 

 possible error of ± 0,02". 



For light of the wavelength of the Z)-liiies we calculate 1 — 



J 



= 0,437. This agreement between theory and ol)servatioii is 



extremely satisfactory. 



A new formula for s was given by Lorentz ^) in 1895 viz.: 



1) Comptes rendus 33, 349, 1851. 



2) A. A. Michelson and E. W. Morlky, Influence of motion of the medium on 

 the velocity of light. Am. Journ. of Science (3) 31, 377, 1886. 



^) Viz. the sum of the lengths of the ways in tlie moving medium, traversed 

 by each of the interfering beams, or approximately twice tlie length of one of the 

 tubes. 



■*) H. A. LoRENTz Versucli einer Theorie der electrischen und optischen Erschei- 

 nungen in bewegten Körpern, p. 101, 1895. See also Theory of Electrons p. 290. 



