[ 591 J 



LXV. A Repetition of FizeaiC s Experiment on the Change 

 produced by the Earth's Motion on the Rotation of a 

 Reflected Ray. By D. B. Brace *. 



THE following experiment was undertaken for the purpose 

 o£ re-examining the results which Fizeau f announced 

 he had obtained, nearly fifty years ago, in observing a change 

 in the azimuth of a plane-polarized ray when refracted 

 through a plate of glass, respectively along and against the 

 earth's motion through space. 



The results of my observations, made under similar con- 

 ditions, show that the effects which Fizeau obtained must 

 have been due to other causes than those whose effects he 

 supposed he was examining. This conforms to that which 

 we should expect from the results of other experimenters, 

 examining the problem by other modes of experimentation, 

 and also to the prevailing opinion that the results of Fizeau\s 

 observations were somewhat doubtful. In fact it appears 

 that he has so expressed himself in this matter %. Notwith- 

 standing these facts, reference is still made to the positive 

 results of this experiment, which are the only ones ever 

 obtained by experimental means on the problem of the 

 u aether drift." 



It would have been highly desirable if Fizeau himself 

 could have repeated his experiments long ago. Perhaps, 

 however, the evidence from other sources did not warrant 

 this ; but the undoubted care and skill devoted to this 

 experiment have left an impression that, perhaps, after all 

 the test involved surface conditions which did not enter in the 

 other experiments and which might give the positive results 

 obtained. 



Experience in several similar problems in polarization 

 recently has led me, incidentally, to believe that it would be 

 entirely possible to repeat this experiment with sufficient 

 sensibility to give conclusive results. Two principal factors 

 have made this possible : — first, a sensitive-strip plane 

 polarizer §; and, second, the use of a " crossed " system of 

 thin refracting plates. This system reduced the double 

 refraction to a minimum, and also compensated for the colour 

 dispersion that Fizeau had to contend with, thus allowing me 

 to use white light with, consequently, a higher sensibility in 

 the half-shade system. 



* Communicated by the Author. 



t Ann. de Chim. et de Phys. (3) Iviii. p. 129. 



% Lorentz, Versuch einer Theorie, Leiden, 1895. p. 2 



§ Phil. Mag. Jan. 1903, p. 161. 



