458 Messrs. Threlfall and Pollock on some 



could not have escaped our notice. In some experiments the 

 Rontgen tube was placed so that the line of kathode discharge 

 made an angle of about 30° with the path BO, and in others 

 made an angle of 90° with the same line. No disturbance of 

 the bands could be detected when the coil was started or 

 while it was working, which shows at once that within the 

 limits of accuracy imposed by the experimental conditions, 

 the Rontgen radiation is unaccompanied by aether streams. 

 This conclusion refers, of course, only to air, and it therefore 

 appeared desirable to make an additional experiment, replacing 

 the air by a substance of greater density. Pure benzene 

 was selected as a suitable liquid, and a glass tank 16'4 cm. 

 long, and several centimetres wide and deep, was placed in 

 the path BC. The radiating tube, which was placed above 

 the free surface of the benzene, was arranged to radiate on to 

 the path in all directions, and in some cases was actually 

 immersed in the benzene so that its active surface was in the 

 field of view of the telescope. 



No disturbance of the bands commencing when the coil 

 started, and stopping when it stopped, was ever detected, 

 though a great many excellent observations were made. Of 

 course, effects due to the heating and electrification of the 

 glass can be easily distinguished from those we are in search 

 of in view of their persistence after the coil is put out of 

 action. The tube employed gave quite visible fluorescence by 

 barium platinocyanide at a metre's distance and through 

 aluminium *7 mm. thick. We can get an idea of the order 

 of the minimum aether velocity which could be detected 

 by this means. The path BC being some 25 cm. long, 

 we will suppose that only 10 cm. of it are influenced by 

 the tube and that the radiation is along the path. The sen- 

 sitiveness of the method was the same whether we employed 

 air or benzene. 



A shift of the bands amounting to a fifth of the distance 

 from band to band would be produced by an aether velocity 

 sufficient to change the time of passage of light over the path 

 by one tenth of a period. As sodium light was employed, 

 we may call this 



5*9/3 x 10" 16 seconds. 



But the time required for light to travel over ten centimetres 

 in air is about 



1/3 x 10 ~ 9 seconds ; 



or the velocity is not changed by the radiation by more 

 than six parts in ten million, say. This is about 177 metres 



