SIE G. G. STOKES, BART., ON THE EOXTGEN RAYS, 15 



"because I have never, myself, made experiments with the 

 Rontgen rays. Nevertheless I have read a good deal about 

 them, following what others have done, more especially 

 where it connected itself with the subject of light, to which 

 I have paid a good deal of attention. So I cannot but have 

 a tolerably definite idea in my own mind as to the nature 

 of these Rontgen rays which has been a matter in dispute 

 and, I may say, is still in dispute, although I think opinions 

 are generally coming round to that which I will bring before 

 you in the end. 



Now before I go to the Rontgen rays chrect, T must touch 

 on previous work which gradually led up to them. 



For a very long time it has been known that an electric 

 discharge passes more readily through tolerably rarefied air, 

 than through air of greater density, and so with other gases. 

 If we have a longish closed tube, provided with electrodes at 

 the ends by means of platinum wires passing through the 

 glass, if the air be tolerably exhausted from it, an electric 

 'discharge passes, comparatively speaking, freely through it 

 forming a beautiful skein of light, if I may so speak, and 

 under certain circumstances that skein of light is divided 

 into strata in a very remarkable manner. These strata fill 

 the greater part of the tube from the positive electrode, or 

 ■anode, as it is called, till we get nearly, but not quite, to the 

 negative electrode, or cathode. There is a dark space 

 ■separating the end of the positive discharge which, as I said 

 under suitable conditions and sufficiently high exhaustion," 

 shows stratification, from a blue glow enveloping the negative 

 electrode or part of it. The luminosity about the cathode 

 is somewhat indefinitely bounded on the side of the strati- 

 fication. 



When, however, the exhaustion is carried still further, at 

 the same time the strata become wider apart,' and the lumin- 

 osity recedes from the cathode and expands, forming a sort 

 of glowing' halo much more sharply defined on the inside 

 than the outside ; in that respect resembling the ordinary 

 luminous halo — not the corona — occasionally seen round the 

 moon. We have here then, these two dark spaces, one 

 outside the halo, where the luminosity gradually fades oft', 

 and another dark space on the inside, where the luminosity 

 is more sharply defined, and which reaches to the negative 

 electrode. 



Now it is the phenomena in connection with this second dark 

 space that I have more particularly to bring before you. As 



