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 while spinning, and with a Voss or Wimshurst machine a constant succession of sparks could be maintained from the middle insulated disk to the earthed outer ones. These sparks were about half an inch long, and were sharp and clear: they usually occurred from its rounded edge, but sometimes from its Hat 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 need for an observation, the positions of the bands close to one of the plates being specially watched, especially at and before each spark, while the disks were revolving at 2800 a minute and the light going three times round.



The experiments were chiefly done in February, 1894, and the bands were broad and clear. There was a trace of irreversible shift when the disks were spinning, but its amount was quite independent of the direction of rotation, and there was not the slightest difference whether the plates were electrified or not.

The path of light in this set of experiments was the long oblong with two of its sides between the disks as already briefly mentioned. The mirrors were supported on opposite wells 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 between the disks, and accordingly the density-gradient-effect, if any, is uncompensated. A very narrow beam was used, however, and the effect is demonstrably small, though it probably accounts for the irreversible shift observed.