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 just before stopping, or sometimes a second or two after stopping. When it was over, the bands were absolutely in the old place; its effect had been to wave them about slowly and slightly.

In all these experiments the brass collar coupling, with the copper wire force-transmitters, was used to connect the two axles, instead of the old friction clutch, which was insufficient and not so dependable.

Possible Time Effect.

On June 6 we kept the disks spinning for three hours at 1900 revolutions, to see if any shift developed with time. The result on this particular occasion was an apparent shift followed by a blur and invisibility of the fringes. They did not recover on stopping, but could be brought into visibility by moving some of the screws. This was evidently a bad experiment, and the apparatus was overhauled and steadied up.

Took another spin next day, at a speed of 2400 revolutions, for three hours, and not the slightest shift developed itself in this time.

By June 23, the step bearing at the bottom of the axle shaft was replaced by a new one, and other mechanical conditions were improved. The machine now ran up to 2400 without a tremor, and a current of 30 to 35 amperes was sufficient to do the driving. An observation was made with the drum in, with glazed windows, no top or bottom (as before), the light going three times round, and the speed being kept at 2100 for two hours. At the first instant there was a shift of $$\frac{1}{20}$$th band, but it did not increase, nor did it recover on stopping. Went on with a reverse spin, also at 2100, under the same conditions, and saw not a trace of shift at going or stopping, or during long spin — only the usual flicker as the speed got very slow. The bands were distinct all the time. A good experiment. We conclude that time has nothing to do with the matter.

Attempt to Observe the Air Efect.

From June to November, 1893, continual attempts were made (except during a month's vacation), by careful and repeated setting of the micrometer wires on the bands, by taking the average of a set of readings at each speed and plotting them, to get some dependable record of the true air effect, free from disturbing causes. This labour was undertaken not only because it was thought of interest to observe this hitherto unobserved small quantity, but also because its detection would emphasise the truly negative character of the ether effect.

Taking $$\mu$$ for air as 1.00029, the $$k=1-\tfrac{1}{\mu^{2}}=.00058\theta$$, and the fraction of a band shift observed being $$x$$ at an angular speed $$\omega$$, and with $$n$$ light-journeys round the optical square of side $$a$$, we should have $$kn\omega a^{2}=4\times10^{5}x$$ (see p. 772, loc. cit.); wherefore, writing $$\omega=2\pi N$$, and considering that the observable $$x$$ is $$\frac{1}{100}$$th of a