xii Transactions of the South African Philosophical Society. 



the exhaustion proceeded the light from the anode retreated, so that 

 the dark space nearly filled the whole tube. The cathode rays 

 could only be rendered visible by impinging on some fluorescent 

 substance ; the glass walls of the tube gave with English glass a 

 bluish and with German glass a greenish phosphorescence. The 

 cathode rays proceeded in straight lines from the cathode, and were 

 deflected by a magnet. Leonard modified the Crookes' tube by 

 inserting a small aluminium window opposite the cathode. The 

 cathode rays were found to pass through the window and render the 

 outside air feebly luminous. He also showed that these cathode 

 rays were photographically active, and certain substances in thin 

 layers were transparent to them. t 



Eontgen, while experimenting with a Crookes' tube, as modified 

 by Leonard, noticed that a strip of paper painted with barium 

 platino-cyanide and glued on a block of wood became phos- 

 phorescent when the paper was turned towards the tube. He 

 turned the wooden side towards the tube and the phosphorescence 

 still appeared. This suggested to him to try the transparency of 

 various other substances. A book of one thousand pages produced 

 only a perceptible weakening ; metals in the form of foil were trans- 

 parent, but as the thickness of the metal increased the transparency 

 diminished, so that shadows were cast on the screen. Thus with a 

 wooden box containing metal weights the wood was transparent 

 while the metal weights cast distinct shadows on the screen. When 

 the hand was interposed it was found that distinct shadows of the 

 bones were visible, surrounded by much fainter outlines of the 

 surrounding tissues. He obtained similar results when the rays 

 were allowed to impinge on a photographic plate completely enclosed 

 in a dark slide, and the photographs were now familiar to every one. 

 Eontgen proved that these rays are distinct from the cathode rays, 

 and from the fact that they appeared to be incapable of regular 

 reflection and refraction through a prism, he threw out the sugges- 

 tion that they might be due to longitudinal vibrations. He concluded 

 that they originated from the points where the cathode rays impinge 

 on the surface of the glass and produce fluorescence. It had since 

 been shown that the rays could be produced without the aid of 

 electricity by merely allowing sunlight to fall on some fluorescent 

 substance. Edison in a telegram to Lord Kelvin stated that out of 

 1,800 substances examined he found that calcium tungstate when 

 properly crystallised gave the finest fluorescence with the Eontgen 

 rays, far exceeding that produced by platino-cyanide. Professor 

 Hahn had kindly prepared some of the salt, and experiments were 

 being made by Mr. Trotter and Professor Holm at the South 



