Cross" in the Field of Electro-magnetic Radiation. 153 



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, 6, c, or d), the arms of the cross will not fall on the 

 receiver, and there should be a response in the receiver. 



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 



FIG. 1. The paper disc. AB, CD are the vertical and horizontal diameters. 



perpendicular to the axis. There was now no action on the receiver; 

 but as soon as the disc was tilted, however slightly, an action was 

 immediately produced on the receiver. 



The disc was now mounted on a stand, between the two screens. 

 By means of sliding arrangements the disc could be raised or 

 lowered, or moved laterally. 



In the next experiment, the centre of the disc was first adjusted 

 on the axis, and the disc moved vertically up and down. No effect 

 was produced when this was being done. 



The centre of the axis was again adjusted on the axis, and the 

 disc moved laterally on the horizontal slide. In this rase, too, there 

 was no action. 



By adjusting the vertical sliding rod the centre of the disc was 

 next placed vertically above or below the axis. The disc was then 

 moved laterally either to the right or to the left. In this way the 



