680 



Lord Ikryleigh : Does Motion through 



board be turned through a right angle so as to point east and 

 west, both directions of vibration for light passing the tube 

 are transverse to the earth's motion, and therefore no double 

 refraction could manifest itself. The question is whether 

 turning the board from the north and south position to the 

 east and west position makes any difference. In no case is 

 any effect to be expected from a rotation through 180°, and 

 such effect as a rotation through 90° may entail must be of 

 the second order in the ratio which expresses the velocity of 

 the earth relatively to that of light. 



It should not be overlooked that according to the theory of 

 a stationary aether, we have to do not only with the motion 

 of the earth in its orbit, but also with that of the sun in 

 space. The latter is supposed to be much the smaller, and to 

 be directed towards the constellation Hercules. In the month 

 of April, when successful experiments were first made, the 

 two motions would approximately conspire. 



If the suggested double refraction, due to the earth's motion, 

 were large enough, it would suffice to set the analysing nicol 

 to extinction in one position of the board, and to observe the 

 revival of light consequent on a rotation of the latter 

 through 90°. But a more delicate method is possible and 

 necessary. Between the polarizing nicol and the liquid 

 column we introduce a strip of glass whose length is horizontal 

 and transverse to the board. This strip, being supported (at 

 two points) near the middle of its length, and being some- 

 what loaded at its ends, is in a condition of strain, and causes 

 a revival of light except in the neighbourhood of a horizontal 

 band along the " neutral axis." Above and below r this band 

 the strained condition of the glass produces just such a 

 double refraction as might be caused by the motion of the 

 liquid through the aether, so that the existence of the latter 

 would be evidenced by a displacement of the dark band 

 upwards or downwards. In order the better to observe a 

 displacement, two horizontal wires are disposed close to the 

 bent glass so as just to inclose the band, and a small opera - 

 glass focussed upon these is introduced beyond the analysing 

 nicol. The slightest motion of the band is rendered evident 

 by changes in the feeble illumination just inside the wires. 



The board is mounted upon a point so as to revolve with 

 the utmost freedom. The point is carried on the table and 

 laces upwards. The bearing is a small depression in an iron 

 strap, rigidly attached to the board, and raised sufficiently to 

 give stability. The gas-leading tubes are connected in such 

 a manner as to give rise to no forces which could appreciably 

 vary as the board turns. 



