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 or paddles, would have a chance of success, unless there existed a trace of something akin to viscosity by which the medium could be got hold of, and as the previous arrangement of apparatus seemed as well calculated as any other to detect the existence of a trace of viscosity, whereby ether in the immediate neighbourhood of moving matter should sooner or later be more or less carried along by it, no fundamental change in the mode of experiment seemed necessary; only improvement in details, and some modifications, in order to secure a closer and a wider generalisation.

Hitherto the experiments had been conducted with a pair of hard steel disks like circular saws, clamped together on a vertical axis, at a distance apart of one inch.

These disks had been spun, at a speed not exceeding 1250 revolutions a minute in the most accurate experiments, and the effect of the motion on a bifurcated beam of light, whose two halves travelled in opposite directions several times round in the space between the disks, was observed. One half of the light travelled in the same sense as the motion, while the other half travelled in the opposite sense; the two half beams were made to interfere in the field of view of a micrometer eye-piece, and a shift of the central band of the system by so much as the hundredth part of the width of a band could be observed. In making the above careful estimate of the result, however, the safe course was taken of assuming that $$\tfrac{1}{20}$$th of a band shift was the minimum certainly detectable.

There were some modifications still to be made before accepting a definitely negative result of experiment.

1st : to steady the motion, so that quantitative readings could be taken without tremor at a much higher speed of rotation.

2nd : to continue the motion for some considerable time, and to narrow the light channel or watch the effect close to a disk.

3rd : to increase the mass of the revolving matter.

4th : to magnetise the revolving material.

5th : to electrify it.

The connexion looked for between ether and matter being something of the nature of viscosity, the space between the disks may be considered rather wide; though it is difficult to suppose that any motion generated at the surface of the disks in a substance possessing any of the properties of an ordinary fluid, should not spread into the nearly enclosed space between them. It may, however, be conceivably argued that this diffusion of motion might take considerable time, and hence the modification labelled No. 2 above was called for. The modification No. 3 is to meet the argument that, even though a viscous connexion between ether and matter were disproved, it did not follow that there was not another mode of connexion competent to transmit motion from one to the other, viz.: the unknown kind of connexion which is concerned in gravitation; and to display any effect on this, a large mass must be used.