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hot, and this spoiled the fringes, so that at high speeds they were often invisible.

Without the drum the fringes remained visible, but the blast caused a shift often of as much as two bands. This shift came back on stopping, and sometimes rather more than came back, ultimately settling down as if slow strains were working themselves out. The drum was now replaced without floor or roof, and with only very narrow slits for the light to get through. The light was often got four times round. A smaller shift still remained, and there was nothing for it but to glaze the slits, and broaden the drum above and below, so that no trace of air blast could reach the frame, at the same time that there was plenty of ventilation to keep the air quite cool.

It need hardly be said that the presence of so many glass surfaces in the course of the beam increased the difficulty of getting the fringes distinct for the three-times round path, for each half of the beam had to undergo not only 11 reflexions as usual, before returning to the semi-transparent plate, but also 24 transmissions through panes of glass, i.e., 48 transmissions through a glass-air surface at 45°. The intensity of the beam is thereby greatly enfeebled, and the glass has to be of excellent optical quality and free from strain if good definition is to be got. Ultimately, by selecting from a number of glass plates supplied by Mr. Hilger, the patience of Mr. Davies overcame the difficulties, and fringes were got of sufficiently satisfactory quality with the beam three times round; a Brockie-Pell arc light imaged upon the aperture of the collimator, and kept finally steady by hand, being used as the source. It was found that a great width of beam was difficult to use, probably for a reason subsequently to be mentioned (varying air density due to centrifugal force), and a diaphragm was commonly used over the object glass of the collimator.

Under these conditions a set of observations were made, with the speed up to 2,800 a minute, first in one direction, then the other, and then the first way again.

In each case the bands remained visible at the highest speed, though at certain intermediate speeds, especially about 1000 and 1700, a slight tremor smudged them.

The shift observed now was moderately small but quite distinct, and was estimated with the micrometer at \tfrac{1}{16}th band. It repeated itself each time without regard to the direction of spin, and disappeared, though not instantly, when the disks stopped. It seemed probably due to some obscure residual effect of the blast, perhaps on the cover glasses of the drum. The shift was irreversible, and of reversible shift there was none.

At these higher speeds it would naturally be thought that the true theoretical effect due to whirling air (\mu^2 -1) should be observable ; but if its amount be reckoned it will be found to be less than \tfrac{1}{200}th band, and therefore not detectable for certain under the above conditions.

The only effect distinctly due to heat in the above experiments was a flicker of the bands at the lowest speeds, just before stopping. It was due to the gentle warmth of the motor, an air current rising towards the disk and mirrors when the blast was insufficient to drive it away. But it never did the least harm, and could only be seen