20 THE 1896 ANNUAL ADDEESS. 



of the aluminium foil in air, where he could examine them at 

 pleasure, using for their detection sometimes a phosphor- 

 escent or fluorescent screen, sometimes a photographic plate. 

 He found that under these conditions they were quickly 

 deflected from their original direction and dispersed, so that 

 they could not be traced far, just like rays of light in a 

 turbid medium, such as water to which a little milk has been 

 added ; whereas in a subsequent series of experiments, to 

 which reference has already been made, in which tlie 

 cathodic rays were received into a second tube, the disper- 

 sion became less and less as the exhaustion proceeded, until 

 at the highest attainable approach to a perfect vacuum the 

 dispersion almost disappeared, and the rays were traced 

 straight onwards for a metre and more, and that, without 

 being enlarged by difii'action, as would be the case with 

 rays of light. 



Lenard mentioned incidentally tliat these cathodic rays, 

 as he supposed they were, were able to pass through the 

 hand even. He missed the discovery of the " X " rays because 

 he had, I may say, the cathodic rays too much in his head, 

 and attributed the whole effect on either side of the wall to 

 the cathodic rays. Really the effect is due in part to the 

 cathodic rays, and in part to the Rontgen rays, the existence 

 of which he was not aware of. They cannot be distinguished 

 merely by their effect on a fluorescent screen or on a photo- 

 graphic plate, since both these recipients are affected by the 

 rays of both kinds. 



Such was the state of things when Rontgen made his 

 remarkable discovery. According to an account which I saw 

 in one of the newspapers (we cannot vouch for the truth 

 of everything we see in the newspapers), the discovery was 

 made in the first instance accidentally. I cannot give you 

 more authentic information than that, but he had been 

 working with a Crookes tube and he observed that a photo- 

 graphic plate, enclosed in the usual case in which these plates 

 are enclosed when you want to protect them from light, 

 showed on development certain markings on it; so he put 

 the whole apparatus as it had been, with a photographic 

 plate in its case in the same position as before, and the 

 thing was repeated. That is according to the account in 

 the newspapers. A very remarkable discovery was the 

 result. He found that rays were capable of coming out 

 of some part of a Crookes tube which had the remarkable 

 property of passing through substances that are opaque to 



