332 Lord Kelvin. Note on Lord BlythswoooV s Paper. [Mar. 19, 



I placed a vacuum-tube, A, behind a lead screen, BB, 18 in. x 

 12J in. thick. The screen had a 2-in. hole in it with a 2-in. pipe 

 attached ; 4 in. from the vacuum-tube was placed a speculum-metal 

 mirror, 4 in. x 2J, at an angle of 45° with the lead screen; 4 in. 

 from the mirror was a light-tight zinc box, E, with aluminium win- 

 dow, F ; inside came first the objects, G, stuck on to a black card- 

 board, H, then I, the photographic plate. 



The following objects were photographed in about 20 minutes : — 



1. Some brass clock wheels. 



2. A screw-cutting gauge. 



3. Two lead disks. 



4. The mirrors, being two pieces of speculum-metal used by me to 

 divide upon. In Photograph 4 the crack between the plate can just be 

 seen in the positive, but in the negative it is quite clear. 



I hope to confirm the above experiments immediately : the delay is 

 caused by the vacuum-tubes having all broken down. 



IV. " Note on Lord Blytkswood's Paper." By Lord Kelvin. 

 F.R.S. Received March 19, 1896. 



Rontgen, in (7) and (8) of his original paper.* described experi- 

 ments seeming to prove the X rays to be incapable of regular- 

 reflection. He pointed out that the result of his experiment in (8), 

 seeming possibly due to regular reflection, might be explained 

 otherwise. Communications to the French Academy of Sciences, by 

 Imbert and Bertin-Sans (March 2, ' Comptes Rendus,' pp. 524, 525) 

 and Battelli and Garbasso (March 9, ' Comptes Rendus,' p. 603) 

 refer to experiments proving not regular reflection, as from a polished 

 surface, but a " diffuse " reflection. This, as Sir Greorge Stokes has 

 suggested in letters which I have received from him in the last few 

 days, might either be due, as indicated by MM. Imbert and Bertin- 

 Sans, to the reflecting surface, though polished for ordinary light, 

 being rugged for light of the exceedingly short wave-length which 

 may be attributed with probability to the Rontgen X rays ; or else 

 to a sort of phosphorescence, or possibly fluorescence, with regard to 

 X light, produced in the substance of the mirror. One, and only one, 

 of the photographs described in Lord Blythswood's communica- 

 tion, seems possibly decisive in proving regular reflection from the 

 polished speculum-metal which he used. I enclose a copy of it, which 

 may be clearer than that which accompanies his paper. In this 

 I see quite clearly a straight line, with its two ends next the letters 

 A, B, which for brevity I shall call the line AB. The space for 



* Translation in ' Nature,' January 23, 1896. 



