116 Mr. A. A. Campbell Swinton. On the Luminosity 



Society, he exhibited in action a similar arrangement, but with the 

 block of lime replaced by a flat plate of thorium oxide. In this case 

 the concave electrodes were of such a curvature and were placed so far 

 apart, that the two sides of the thorium plate in each case intersected 

 the diverging cone of cathode rays. Under these conditions nearly a 

 square inch of the thoria surface on each side of the plate became 

 highly incandescent, and a very powerful light was obtained for some 

 minutes at a time, but only at a critical and highly unstable degree of 

 vacuum. 



The writer has now applied this method to the investigation of the 

 comparative luminosity of different mixtures of the rare earths. 



One form of the tube employed was constructed as shown in the above 

 figure, where C, C are two spherically concave discs of aluminium 1-125 

 inches diameter, and 6 inches radius of curvature. These electrodes 4 

 are placed about 7 inches apart, and were connected to the secondary 

 terminals of a 10-inch Ruhmkorff coil, the primary of which was 

 supplied through a variable resistance, with alternating electric current 

 at 100 volts pressure from the main. The tube was connected 

 through a drying tube, containing phosphorus pentoxide, to a pair of 

 Toepler pumps, and also to a McLeod gauge. 



The mantle M M to be experimented upon, was mounted on a 

 platinum wire frame and placed between the two electrodes, so that 

 as the electric current alternated, and each electrode became in turn 

 cathode, the mantb was subjected on alternate sides to cathode ray 

 bombardment. The curvature of the electrodes was such as to give 

 almost parallel beams of cathode rays, so that a considerable ring 



