MECHANICAL CONNEXION BETWEEN ETHER AND AIATTEIl. 
1G5 
while spinning-, and with a Voss or Winishurst machine a constant succession of sparks 
could be maintained from the middle insidated disk to the earthed outer ones. These 
sparks were about halt an inch long, and were sharp and clear ; they usually occurred 
from its rounded edge, but sometimes from its flat surface, which on account of the 
bevel was slightly nearer the other plates than the edge was. It may be taken that 
the difference of potential concerned was not far short of 40,000 volts., and that the 
electric tension was about as much as common air can stand. 
The interference bands could now’ be seen bisected by the middle disk, and usually 
either the upper or the lower half was used for an observation, the positions of the 
bands close to one of the plates being specially Avatched, especially at and before each 
spark, Avhile the disks Avere revolving at 2800 a mi]iute and the light going three 
times round. 
One of the latest arrangements of tlie optical pai'ts, on the opposite walls of a room, so as to be unclis- 
tnrbed by tbe force or beat of the blast from the disks revolving in the middle of the room. 
L is the electric lamp; C, the collimator; T, the double micrometer telescope ; and S the double 
boiler-plate screen to protect the observer; M is the semi-transparent plate, and the light i.s 
indicated going three times round a rectangle, with ]ia]’t of its course between the disks. The 
whole is drawn to scale, the diameter of disks being- 3 feet, or nearly 1 metre. 
The experiments Avere chiefly done in Februat-y, 1894, and the bands Avere broad 
and clear. There Avas a trace of irreversible shift A\'hen the disks Avere spinning, but 
its amount was quite independent of the direction of rotation, and there Avas not the 
slightest difference Avhether the plates Avere electrified or not. 
The path of light in this set of experiments was the long oblong Avith tAvo of its 
sides betAveen the disks as already briefly mentioned. The mirrors Avere supported 
on opposite Avails of the room, and a diagram of the arrangement is annexed (fig. 8), 
the light being sent three or more times round the oblong. 
With this plan, alternate light paths go betAveen the disks, and accordingly the 
density-gradient-effect, if any, is uncompensated. A very narroAV beam Avas used, 
however, and the effect is demonstrably small, though it probably accounts for the 
irreversible shift observed. 
