The arrangement of the apparatus is the same as in fig. 19 of my previous paper. The polariser is vertical and the analyser horizontal. The paper disc is interposed between the screens with its plane at right angles to the direction of the ray.
The receiver is fixed on the prolongation of the line (which I shall call the axis), joining the centres of the polariser and the analyser.
On the supposition that the interposition of the disc produces a dark cross, the arms of the cross (with the particular arrangement of the polariser and the analyser) will lie in the projections of the vertical and the horizontal diameters of the disc, and will move in space with the movements of the disc. When the centre of the disc is on the axis, the intersection of the cross will be superposed on the receiver, and there should then be no action. If the disc be moved up and down, the centre remaining in the vertical line passing through the axis, the vertical arm of the cross will slide over the receiver. If the disc be moved laterally, with its centre in the horizontal line passing through the axis, the horizontal arm of the cross will slide over the receiver. In this, as in the last case, there should be no action on the receiver. But if the disc be displaced so that the centre does not lie in either the horizontal or the vertical line passing through the axis (the axis now cutting the disc at points such as a, b, c or d), the arms of the cross will not fall on the receiver, and there should be a response in the receiver (see fig. 21).
The experiments were now arranged as follows:—The disc was at first placed with its centre on the axis, the plane of the disc being perpendicular to the axis. There was now no action on the receiver; but as soon
