of the Earth and the Luminiferous JEther. 453 



The whole time of going and coming is 



and the distance travelled in this time is 

 V 2 / v 2 \ 



neglecting terms of the fourth order. The length of the 

 other path is evidently 2Da/ 1 + =^, or to the same degree 



of accuracy, 2D ( 1 + ~™ )• The difference is therefore 



v 2 

 D^g. If now the whole apparatus be turned through 90°, 



the difference will be in the opposite direction, hence the 



v 2 

 displacement of the interference-fringes should be 2D™ 



Considering only the velocity of the earth in its orbit, this 

 would be 2D x 10 _8 . If, as was the case in the first experi- 

 ment, D = 2 x 10 6 ' waves of yellow light, the displacement to 

 be expected would be 0*04 of the distance between the inter- 

 ference-fringes. 



In the first experiment, one of the principal difficulties 

 encountered was that of revolving the apparatus without pro- 

 ducing distortion ; and another was its extreme sensitiveness 

 to vibration. This was so great that it was impossible to see 

 the interference-fringes except at brief intervals when work- 

 ing in the city, even at two o'clock in the morning. Finallv, 

 as before remarked, the quantity to be observed, namely, a 

 displacement of something less than a twentieth of the 

 distance between the interference-fringes, may have been too 

 small to be detected when masked by experimental errors. 



The first-named difficulties were entirely overcome by 

 mounting the apparatus on a massive stone floating on 

 mercury ; and the second by increasing, by repeated re- 

 flexion, the path of the light to about ten times its former 

 value. 



The apparatus is represented in perspective in fig. 3, in plan 

 in fig. 4, and in vertical section in fig. 5. The stone a (fig. 5) 

 is about 1*5 metre square and 0*3 metre thick. It rests on 

 an annular wooden float bo, 1*5 metre outside diameter, 07 

 metre inside diameter, and 025 metre thick. The float rests 

 on mercury contained in the cast-iron trough cc, 1*5 centi- 



