A 60 Messrs. Threlfall and Pollock on some 



surface of the tube is brought up to the path CA, and the 

 Rontgen radiation allowed to traverse it in a variety of direc- 

 tions from parallelism to normality, and is even thrown on to 

 the mirror at C. For experiments on benzene, troughs of 

 that liquid as similar as possible are inserted in both paths, 

 the one in the path AB acting merely as a compensator. 

 This experiment is much more difficult than the one previously 

 described, and the benzene requires to be well stirred if good 

 definition is required. In the experiments in air a shift of 

 the bands by §L of the width of a band could be observed, 

 while in benzene a change of about half the distance from 

 band to band only could be seen. A widening of the bands 

 to about half the above amounts could have been detected in 

 each case. When the benzene was used the fringes were 

 unsteady, and opportunities for observation had to be waited 

 for. Of course, when the active surface of the Rontgen tube 

 dipped into the benzene so as to appear in the field of view 

 of the telescope great disturbances due to thermal and electric 

 changes became visible. These, however, did not appear 

 instantaneously on starting the coil, nor did they disappear on 

 stopping it. In no case was any real effect observed. 



The chief interest of this experiment lies in the fact that if 

 the Rontgen radiation consisted of longitudinal aether waves, 

 i. e,, waves of longitudinal aether displacement, some effect was 

 to be expected. If the waves are long compared with the path 

 AC (the path AB being screened) a widening of the fringes, 

 or in the extreme case a total disappearance of the fringes, is 

 to be anticipated. This can easily be realized by blowing an 

 organ-pipe in the neighbourhood. If the waves are short 

 compared with the dimensions AC then all will depend on 

 the azimuth of the tube, or rather on the inclination of the 

 Rontgen-ray path to the path of the light rays. This appears 

 from the fact that whole waves would produce no effect in 

 the case contemplated — everything would depend on the 

 fractional parts of the waves included in or projected on AC. 



The conclusion to be drawn from the experiment is that 

 neither in air nor benzene are the light- transmitting properties 

 of the aether interfered with. The limits of observational 

 accuracy are of the same order as in the case of Experiment II. 



Action of Rontgen 1 § Rays on a Selenium Cell, 

 It so happened that one of us was engaged early in the 

 year in experimenting with photo-resistance cells made of 

 selenium which had been laboriously purified and which was 

 probably as pure as any that has ever been obtained. Con- 

 trary to expectation, such cells showed a quite normal light- 



