206 Professor Silvanus P. Thompson [May 8, 



flat aluminium plate on its extremity. From this flat plate tlie 

 kathode rays shot forward against the bulging end of the tube ; and, 

 without any aluminium window rays which were capable of exciting 



fluorescence, found their way 

 t -J "N through the glass walls. Lenard 



I , ^. .^-.w..-. ^ had so boxed up his tube with 



I J brass cap and metal case, that if 



]1| anything in the way of rays 



cJJ struggled through the glass walls 



I of his tube he might not notice it. 



UJS Possibly he never looked for it. 



Fig. 8. Tf Roentgen made the fortunate ob- 



servation that when his tube was 

 closely covered with opaque black card it still could cause fluores- 

 cence on a screea covered with platino-cyanide of barium on which 

 shadows were cast. From seeing the shadows thus to securing their 

 imprint permanently on a photographic plate was but a small step, 

 and the discovery that they could pass freely through a sheet of the 

 metal aluminium was a natural result of an inquiry as to the trans- 

 parency of different materials. Aluminium is to these rays much 

 more transparent than ordinary glass. No lens can focus them, nor 

 mirror reflect them ; and, unlike the kathode rays within the tube, 

 they are not deflected by the magnet, 



The criterion which we have at present as to whether any rays 

 from any other source are or are not the same as the X-rays is that 

 they shall bo able to fulfil the following four-fold test : — They 

 must be capable of exciting luminescence ; they must be capable of 

 impressing an image on a photographic i)late ; they must be capable 

 of passing through aluminium ; and they must be incapable of being 

 deflected by a magnet. In addition they must — so far as present 

 evidence goes — be incapable of being either refracted or polarised. 

 Any rays that will fulfil these tests must for the present be considered 

 identical with X-rays. 



Now it has been suggested that the X-rays are the same as ultra- 

 violet light. This is certainly not so, for ultra-violet light, as known to 

 us by the researches of Stokes, Tyndall, Becquerel and Cornu, will 

 not go through aluminium and is not deflected by a magnet, though it 

 will excite luminescence and take photographs. Furthermore ultra- 

 violet light can be refracted and polarised. 



It has also been suggested that the X-rays are merely invisible 

 heat-rajs. But this is certainly untrue also, because although Abney 

 has succeeded in taking photographs by heat rays, they will not go 

 through aluminium, are not deflected by the magnet, and instead of 

 exciting phosphorescence they destroy it, as Goethe found out nearly 

 a hundred years ago. 



Neither are they Hertzian waves of longer period than the heat 

 waves. 



So far as is at present known there is no other way of producing 



