of the Earth and the Luminiferous either. 



451 



the effect of the motion of the earth through the aether on the 

 path of the ray at right angles to this motion was overlooked*. 

 The discussion of this oversight and of the entire experiment 

 forms the subject of a very searching analysis by H. A. 

 Lorentzf, who finds that this effect can by no means be dis- 

 regarded. In consequence, the quantity to be measured had 

 in fact but half the value supposed, and as it was already 

 barely beyond the limits of errors of experiment, the conclu- 

 sion drawn from the result of the experiment might well be 

 questioned ; since, however, the main portion of the theory 

 remains unquestioned, it was decided to repeat the experiment 

 with such modifications as would insure a theoretical result 

 much too large to be masked by experimental errors. The 

 theory of the method may be briefly stated as follows : — 



Let sa, fig. 1, be a ray of light which is partly reflected in 

 ab, and partly transmitted in ac, being returned by the 



Fig. 1. 



/ 



d 



mirrors b and c along ba and ca. ba is partly transmitted 

 along ad, and ca is partly reflected along ad. If then the 

 paths ah and ac are equal, the two rays interfere along ad. 

 Suppose now, the aether being at rest, that the whole appa- 

 ratus moves in the direction sc, with the velocity of the earth 

 in its orbit, the directions and distances traversed by the rays 

 will be altered thus : — The ray sa is reflected along ab, fig. 2 ; 



* It may be mentioned here that tlie error was pointed oat to the 

 author of the former paper by M. A. Potier, of Paris, in the winter of 

 1881. * . 



t " De l'lnfluence du Mouvement de la Terre sur les Phen. Lum." 

 Archives Aeerkmdaises, xxi. 2 me livr. (1886). 



2H 2 



