DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
765 
by foot bellows : but this made very little difference to the bands, though the pressure 
was 6 centims. of water. A water gauge was also used to measure the centrifugal 
force of the blast : it was about 3 centims. of water, but was not easy to measure 
satisfactorily. Evidently the blast acted mainly on the mirrors. I contemplated a 
vacuous chamber, but shrank from some obvious difficulties, besides realizing that the 
residual air must give trouble anyhow. I decided to risk interposing transparent 
matter in the beam. 
So some plate-glass protectors were cut and framed, one in front of each of the 
three mirrors, the fourth at present being left still exposed, partly because it seemed 
less likely to be affected by the wind, and partly because of the very narrow space 
available between it and the disks. 
Seeing that each half-beam of light, in going three times round, has to go through 
each piece of plate-glass twice at every reflexion, or eighteen transmissions altogether, 
the intensity of the emergent light was feeble, and the bands were distorted by imper¬ 
fections in the glass. Still they could be got clear, though curled, and the cross wk-es 
could be set on them. On 24th and 25th of July, spins at 1380 revolutions a minute 
were taken, and no shift so great as of a band could be detected ; whereas, with 
the light only going twice round, there might have been a shift of 2^ bands if the 
ether had been carried full tilt. Hence it would appear that the ether was not 
carried round with the disks by so much as of their speed. 
The alignment of the shafts and other mechanical details were now attended to, so 
as to make possible higher speeds of rotation. Fresh arrangements for holding and 
adjusting the semi-transparent plate (the setting of which is a delicate matter) were 
made, and the micrometer was more firmly fitted by a much longer tube into the 
telescope. All sorts of steadyings were attended to. A pair of wires were laid 
across to the Walker Engineering Laboratory, where Professor Hele Shaw had a 
large Crompton dynamo; and so in December, 1891, my assistant, Mr. Davies, got 
the speed up to 2800 revolutions per minute. 
43 amperes and 75 volts on the terminals (4*3 H.P.) gave 2500 revolutions a 
minute with lid of box off. With lid on, it went up to 2800 revolutions, but now the 
air in the box got quite hot, and a new difficulty arose from oil. It crawled up in an 
imperceptible film, notwithstanding the traps arranged for it, and, flying from the 
under surface of the lower disk, bespattered the mirrors and spoilt them. 
More elaborate oil-catchers, to spray the oil off before it got to the disks, were there¬ 
fore arranged, and the disks were kept whirling many days to get rid of all traces of 
the oil that had already soaked between the steel and the wrought-iron cheeks. 
Having cured the oil difficulty, fresh and improved cover glasses for the mirrors were 
got from Mr. Hilger, one extra large and extra thin one for the front mirror included, 
and these were carefully framed and placed in position. The light was then with 
difficulty, and considerable skill on the part of Davies, got three times round through 
all the glasses, and, during Christmas, 1891, sjiins were taken, but the highest speeds 
