MECHANICAL CONNEXION BETWEEN ETHER AND MATTER. 
157 
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 woi’king them¬ 
selves 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 tiiat 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 difflculty 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 sub¬ 
sequently 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 i^-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 (p," — 1) should be observable ; but if its amount be 
reckoned it will be found to be less than -j^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, amd could only be seen 
