DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
773 
Taking 30-ton steel as the material, this gives as the smallest observable value 
of kjx, 
4 X 10^ / 7 
na A' 9 X 10® 
11 centims. 
na 
It is fairly easy to make sure that x is not greater than apparatus 
a is 2 feet or 60 centims., and the light may go three times round with the cover- 
glasses on; hence the limiting determination of ether-drag that can be made with it 
thus arranged is i ' /oo th, unless the setting of the micrometer wires can be relied on 
more closely than to the y^th of a band, or unless the steel will safely stand more 
than 30 tons to the square inch. 
(Evidently the larger the square the better, and a large enough square might show 
even the earth’s rotation effect, only it is difficult to see how to imitate the effect of 
stopping and reversing the rotation, at least with the unwieldy size of frame 
necessary.) 
Testing for Cause of Slight Irreversible Shift. 
44. The following experiments tend to show (and do distinctly show in my opinion) 
that such shift as is observed is independent of the width of the bands, and therefore 
is an absolute shift caused by shake or strain ; very likely by a strain caused liy a 
shake, for its effect often dies out slowly. 
Spins on 18th March. Light going three times round. 
First spin, in direction to help transmitted beam. 
Micrometer wires set and moved as before (fig. 12), but only the results of the 
readings quoted. 
Width of yellow band < 
'"at rest. 
while revolving 1200 times a minute 
at rest again. 
114 divisions. 
159 
184 
5) 
Shift of middle band . 
J shift to the light . 
\ return on stopping 
6 
5 
Found that the front mirror was too near the drum, probably touching it, hence the 
continuous widening of the band. Moved it back a bit and tightened up the nuts 
of the semi-transparent plate. 
Started again with much narrower bands, and continued spinning and stopping 
alternately without intermission, all in the same direction. 
It may be worth while to quote the actual readings of this set. The wires were set 
at each stoppage, and likewise during each spin while the speed was kept at 1260 
revolutions per minute. 
