Probably the mass necessary to demonstrate any action of the gravitational kind would be impractically large, unless the earth itself could be used. Now, by staking out mirrors at the corners of a field, it is arithmetically quite possible to arrange for a perceptible shift of the bands due to the rotation of the earth, if it carries ether round with it; but it does not seem possible to experimentally observe that shift, unless some method could be devised of making the observer and his apparatus independent of the rotation.
It is to be observed, that since a motion of the disks relatively to the observer and the light causes no effect, the ether being stationary, it follows that a motion of the light and observer would produce an effect, since they would be moving relatively to the ether. Hence if, instead of spinning only the disks, the whole apparatus, lantern, optical frame, telescope, observer and all were mounted on a turntable and caused to rotate, a reversible shift of the bands should be seen. It would not matter in the least whether the disks were revolving or not, and they might just as well be absent. The effect would be of an aberrational kind, the opposite light beams being accelerated and retarded by the motion appropriately. In an actual experiment of this kind, centrifugal force would give some trouble by introducing strains, and rapid rotation would be uncomfortable for the observer; but really rapid rotation should be unnecessary to show the effect. My present optical apparatus mounted on a turn-table revolving 4 times a minute should show something, viz.: th band shift each way. A certain amount of discomfort during the accelerative stages of any speed could hardly be avoided, and even during steady motion there would be some inconvenience; for instance, at 30 revolutions a minute the observer’s weight, at a metre and a half from the centre, would be half as much again, and would be inclined at 45° to the vertical. This, however, might be tolerated.
If the ether is stationary near the earth, that is, if it be neither carried round nor along by that body, then a single interference square, 1 kilometre in the side, would show a shift of rather more than one band width, due to the earth’s rotation in these latitudes; see p. 772, 'Phil. Trans.' 1893. But as the effect depends on the area of the square, a size of frame capable of mechanical inversion is altogether too small; there may, however, be some indirect ingenious way of virtually accomplishing a reversal of rotation — something for instance based on an interchange of source and eye – and if so, it would constitute the easiest plan of examining into the question of terrestrial ether drift.
If matter conceivably drags the ether with it in proportion to its mass, an ordinary lump of matter can hardly be expected to cope with the heart and to shift it in opposition to that body; nevertheless, since nothing is known on the subject one way or the other, it was thought well to give a more massive body a chance, by rotating a solid piece of iron about three-quarters of a ton in weight, and with a much narrower groove or channel cut in it for the passage of the light. It was easy to arrange at the same time for the magnetisation of this piece of iron when
