192 Professor Silvanus P. Thompson [May 8, 



shadows ; it was the entirely subsidiary and comparatively unim- 

 portant point that to these mysterious radiations flesh is more 

 transparent than bone. 



Let me begin by showing you as a first experiment that same fact 

 which Eoeutgen announced of the production of luminescent shadows 

 by these invisible rays. Before you there stands a Crookes tube, 

 of the most modern kind,* for this particular purpose. We have 

 here an induction coil t capable of giving 6 -inch sparks, with which 

 we can send electric discharges through the tube, illuminating it with 

 its characteristic golden-green glow. I now cover over the tube and 

 exclude all ordinary light, not with a box of black cardboard but 

 with a black velvet cloth. And now in the darkness I am able 

 to show you on a sheet of paper covered with the highly fluor- 

 escent platino-cyanide of barium — the well known substance which 

 Eoentgen himself was using — the shadows of objects placed behind. 

 See how this sheet shines in the light of the tube transmuting the 

 invisible radiations into visible light. I hold my purse behind the 

 screen — you see the shadow of the metal clasp, and of the metal 

 contents (two coins and a ring), but you see not the shadow of the 

 leather purse itself, for leather is transparent to these rays while metal 

 is opaque. I hold my hand behind and you see — or at least those of 

 you who are within a few yards of me — the shadow of my hand, or 

 rather of the bones of my hand, surrounded by a fainter shadow of 

 the almost transparent flesh. 



Now the second fact that Eoentgen announced was that these 

 same rays which escape through the opaque covering and excite 

 fluorescence are also caj^able of taking photographic impressions of 

 the shadow^s. There is nothing w^hatever new about this part of the 

 subject : it is the old pliotograj)hy ; there is no " new photograi)hy." 

 Here is a common camera back, and here inside it is a photographic 

 dry-plate — quite a common dry-j^late, such as has been known for ten 

 years. This plate is covered with a black card, so that it may not 

 become fogged by the light of the room when I draw the slide. All I 

 have to do is to lay it upon the table below the Crookes tube so as to 

 cast the shadow upon it, and after due exposure develop the plate 

 in the ordinary well-understood way. Now it may be interesting 

 to see the proof of the fact that bone is less transparent than flesh. 

 So, with your permission, I will ask my little daughter to have her 

 hand photographed. (Experiment made.) 



At the time of Eoentgen's announcement, the exposure required 

 with the Crookes tubes that were then in existence was from twenty 

 minutes to, I think, two or three hours. Very shortly improvements 

 were made ; and with these modern tubes one minute is quite suffi- 



* A Crookes " focus " tube (Jackson pattern), constructed by Messrs. New- 

 ton & Co., of Fleet Street, London. 



t An Apps coil capable of giving sparks 25 centimetres in length, but on 

 this occasion excited v?ith only 5 cells, giving sparks about 6 inches iu length. 



