56 The History and Properties of the Rontgen Rays. 



beautifully striated, the striations having an irregular motion 

 backwards and forwards along the tube. When the pump was 

 worked so that about i "10000 of the air was left, the dark space 

 round the negative electrode expanded and filled the whole tube. 

 The light only came from the surface of the glass. The dis- 

 charge now seemed to come off entirely from the cathode. It 

 moved in straight lines, and did not curve back to the anode. 

 When it struck the glass the glass gave a brilliant phosphor- 

 escence ; if it fell upon platinum, the platinum became hot. A 

 radiometer in its path rotates and phosphoresces beautifully. 

 These cathode rays cast a shadow of a solid object placed in their 

 path, and, lastly, they were deflected by a magnet like a flexible 

 wire conveying a negative current. Those were the discoveries 

 of Hettorf, Goldstein, and Crookes, and were made fourteen or 

 fifteen years ago. Crookes's explanation of these phenomena 

 was — "The molecules of the gas remaining in the tube are 

 drawn towards the cathode, get charged, are so violently repelled 

 that their shock against the glass caused it to phosphoresce and 

 the radiometer to rotate." In 1892 Hertz showed that, while 

 many substances transparent to ordinary light were opaque to 

 cathode rays, thin sheets of metal were very transparent to 

 cathode rays. Hitherto those rays had only been examined in 

 a vacuum. Lenard now pierced a hole in the tube opposite the 

 cathode, and closed this hole with a thin leaf of aluminium. 

 The cathode rays now passed out of the tube, and he found that 

 they caused fluorescence on a prepared screen. They discharged 

 a gold leaf electroscope and blackened a photographic plate, and 

 were mostly deflected by a magnet. Lenard used a tube with 

 a very small window, and only worked a few seconds at each 

 experiment, and so narrowly missed anticipating Rontgen. 

 Rontgen'sdiscovery was simply that when the cathode rays struck 

 a solid substance the point struck became an origin of an entirely 

 new set of rays. The tubes for the production ofRontgen rays were 

 mostly modifications of the so called focus tube invented by Mr. 

 Jackson, of London. The tube he. (Mr. Finnegan) was using 

 had a spherical bulb. At one side there was a cap of aluminium 



