136 T. MANSELL THE BONTGEN OR "x" RAYS. 



difference between " Crookes tubes" and " Geissler tubes"; and 

 I therefore show some of both. Crookes' and Geissler's tubes, so 

 that it may be understood more exactly what happens when these 

 photographs are taken. 



The chief difference between Geissler tubes and Crookes tubes is 

 this : In a Geissler tube the exhaustion is very much less than in 

 a Crookes tube, the light which we see in the Geissler tube being 

 due to the luminescence of the residual gas. 



The one now shown is a Geissler tube with a fancy pattern of 

 glass tubing inside, and a small amount of air ; when the current 

 is turned on, it will be seen that this will light up in different 

 colours, due to the different qualities of glass which have been 

 used in the' making of the tube. 



In a Geissler tube with different fluid substances surrounding 

 the inner tube through which the current is passed, although by 

 the ordinary electric light these substances appear to be of the 

 same colour, when the current is passing they will fluoresce with 

 different colours. If the Geissler tube be further exhausted. 

 a different phenomenon will be obtained — that of Crookes. If 

 the current be turned on through a Crookes tube, it will be 

 observed that the tube is not so strongly illuminated as was the 

 case in the Geissler tube, but owing to radiation which takes place 

 from the large terminal, the flat piece of mica inside the tube 

 coated with calcium-sulphide will become intensely luminescent. 



The discovery of the "X" rays by Professor Eontgen was 

 due, as is so often the case, in some measure to an accident. He 

 was experimenting with a Crookes tube wrapped in an opaque 

 material, and on his bench he had laid a piece of sensitive paper, 

 on which he found, after a powei-ful current had passed through 

 the tube, that a black mark had been formed. 



Eetracing our steps we have, first, a tube in which the gas 

 is luminescent ; then one in which the gas loses most of its 

 luminosity ; and lastly, one containing a solid substance upon 

 which the cathode rays from the negative electrode play, rendering 

 it luminescent. 



The cathode rays, as shown by Lord Kelvin in his works, are 

 due to the residual gas particles contained in the tube being 

 electrified. The real work done is not by these cathode rays inside 

 the tube, but by the invisible or "X" rays outside the tube, and 

 when we come to Riintgen Photography we have to some extent 

 ceased to have to do with the cathode rays. 



"We are unable at the present time to decide what these rays 

 on the outside of the tube are. Is it that the cathode rays, after 

 being filtered through the glass, become converted into something 

 else, or is it that the cathode rays, beating on one side of the 

 glass, set up some form of vibration on the other side of the 

 glass ? Some say that they are very long waves, corresponding to 

 infra-red light; others say that they are short, corresponding to 

 ultra-violet light. My conclusion is that they are short waves, 

 and correspond to ultra-violet light. 



