22 THE 1896 ANNUAL ADDRESS. 



capable of regular reflection, but lie brought forward experi- 

 ments which show that in a certain sense they appear to 

 be capable of reflection. 



A photographic plate with the sensitive surface downwards 

 Avas placed in its case under a Crookes tube, and immediately 

 under the plate, and inside the case, were placed portions of 

 different kinds of metal, wdiich would be capable of reflect- 

 ing back the rays on to the sensitive surface, if they admitted 

 of reflection ; and it was found that the plate was much more 

 darkened over certain of those metals than where the metal 

 did not exist. There was very little darkening over ahi- 

 minium and a great deal of darkening comparatively over 

 platinum. This indicated that some effect was produced, 

 though the greater part of it is not one of regular reflec- 

 tion. He conceived tJie effect to be one of reflection such 

 as you might have from a turbid medium. 



There is, however, another mode of explanation which 

 seems worth considering, viz. : that the Rontgen rays, falling 

 upon the metal throw the molecules into a state of vibration, 

 which they communicate to the ether, by a sort of phos- 

 phorescence or fluorescence of X light ; so that the rays 

 which come from the molecules, though perhaps not of 

 exactly the same nature as the X rays that fell upon them, 

 still Jiave enough of the " X " quality about them, whatever 

 that is, to enable them to get through objects whicli are 

 opaque to ordinary light. 



Lord Blythswood, who has worked a great deal with the 

 Rontgen rays, has written a paper, which was communicated 

 to the Royal Society by Lord Kelvin, in which he establishes 

 a minute regular reflection of those rays from speculum 

 metal at an angle of about 45°. Two plane specula were 

 placed side by side so as to receive at that angle the X rays 

 coming from a Crookes tube, and a duly protected photo- 

 graphic plate was placed in such a position as to receive the 

 regularly reflected rays if there should be any. The 

 developed plate appeared to show a slight indication of the 

 junction between the mirrors ; and that the appearance was 

 not illusory was shown by Lord Kelvin, who made measure- 

 ments on the image and compared the results with what they 

 ought to be on the supposition of a regular reflection. The 

 indication was so faint that I could not myself perceive it (I 

 have not seen the negative, but only positive, copies), but 

 Lord Blythswood has given me some positive copies of a 

 negative which he subsequently obtained by reflection from 



