1896.] Electric Shadows and Luminescence. 191 



WEEKLY EVENING MEETING, 

 Friday, May 8, 1896. 



LuDWiG MoND, Esq. Ph.D. F.K.S. F.C.S. Manager and 

 Vice-President, in the Chair. 



Professor Silvanus P. Thompson, D.Sc. F.R.S. M.B.I. 

 Electric Shadows and Luminescence. 



The early days of the year 1896 were marked by the announcement 

 telegraphed from Vienna to the effect that Professor Roentgen, a 

 man whose name though little known outside the world of science 

 was well known and highly esteemed by those who were initiates in 

 physics, had discovered the existence of rays of a new and extra- 

 ordinary kind. Taking a Crookes tube, excited of coui'se by a 

 proper electric spark, and covering it up within a case of black 

 cardboard, he found it to produce in the surrounding space some 

 entirely unexpected effects. Black cardboard is impervious not only 

 to ordinary light and to radiant heat, but also to all those other 

 known kinds of invisible light beyond the violet end of tlie spectrum, 

 known as actinic waves, which are such active agents in the produc- 

 tion both of fluorescence and of photographic actions. Yet the 

 invisible emanations of the Crookes tube, which passed freely 

 through the opaque cardboard, were found by Roentgen to be capable 

 of revealing their presence in two ways. In the first place he had 

 seen them to project shadows upon a luminescent screen of paper 

 coated with the highly fluorescent substance called platino-cyanide 

 of barium, and in the second place he had been able to photograph 

 these shadows by letting them fall upon an ordinary photographic 

 plate. The discovery was singular. It revealed the existence of a 

 remarkable and hitherto unexpected species of radiation. It added 

 another to the many puzzling phenomena attendant upon the dis- 

 charge of electricity in vacuo. It proved that something which in the 

 ordinary sense in which those terms are used is neither light nor 

 electricity was generated in the Crookes tube, and passed from it 

 through substances opaque alike to both. 



But that which took the imagination of the multitude by storm, 

 and aroused an interest the intensity the like of which has not been 

 known to be aroused by any other scientific discovery in our times, 

 was not the fact that Professor Roentgen had seen luminescent 

 shadows from a Crookes tube, or had obtained a photograph of those 



