1896.] 



on Electric Shadoivs and Luminescence. 



215 



which had lost its thermo-luminescent property by prolonged heating 

 was partially though not completely revivified. 



I referred earlier to the rays recently discovered by M. H. 

 Becquerel. In February last M. Becquerel, and independently I 

 myself,* made the observation that uranium salts emit some rays 

 which very closely resemble the X-rays, since they will pass through 

 aluminium and produce photographic action. It remains to be seen 

 whether these rays are identical with those of Roentgen. 



Finally, let me briefly exhibit two results of my own work. 

 There is now shown (Fig. 14) the photographic shadow of two 

 half-hoop ruby rings. One of them is of real rubies, the other of 

 imitation stones. By artificial light it is difficult to distinguish one 

 from the other, but when viewed by the 

 X-rays there is no mistaking the false 

 for the true. The real rubies are highly 

 transparent, those of glass are practically 

 opaque. 



After gaining much experience in judg- 

 ing by photography of the relative trans- 

 parency of materials, I made a careful 

 research f to discover whether these rays 

 can be polarised. At first I used tour- 

 malines of various thicknesses and colours. 

 More recently I have tried a number of 

 other dichroic substances, andalusite, sul- 

 phate of nickel, of nickel and ammonium, 

 sulphate of cobalt, and the like. The 

 method used for all was the following. 

 A slice of the crystal was broken into 

 three parts. One part was laid down, and 

 upon it were superposed the other two in 

 such a way that in one the crystallographic axis was parallel, in 

 the other perpendicular, to the crystallographic axis in the first 

 piece. If there were any polarisation the double thickness where 

 crossed in structure would be more opaque than the double thick- 

 ness where the structure was parallel. Not the slightest trace of 

 polarisation could I observe in any case. Of numerous other ob- 

 servers who have sought to find polarisation, none has yet produced 

 a single uncontestable case of polarisation. 



At the present moment interest centres around the use of 

 luminescent screens for observing the Eoentgen shadows, and in this 

 direction some advances have been claimed of late. It should, 

 however, not be forgotten that Eoentgen's original discovery was made 

 with a screen covered with platino-cyanide of barium. Here is a 

 piece of card covered with patches of several different kinds of lumi- 



FiG. 14. 



* See ' Philosophical Magazine ' ; July 1896. 

 t Ih. August 1896. 



