March 13, 1896.] 



SCIENCE. 



395 



seem to be very marked, and the interpre- 

 tation of the result obtained presents a very 

 interesting problem. 



We have also demonstrated the possibility 

 of detecting by the Rontgen process flaws or 

 blow holes in metal plates. The writer had 

 prepared for him three pieces of aluminum 

 about 5 mm. thick, within which were made 

 several hidden flaws and holes, and in one 

 of them was placed a plug of some foreign 

 substance, lead. A picture of the pieces 

 reveals exactly the positions of all the holes, 

 and a darker streak shows the position of 

 the lead plug. Even the numbers which 

 were stamped with a die are plainly visible 

 in the radiogram. 



It is now desired to call attention to a 

 very interesting incident in connection with 

 this wonderful discovery. The writer has 

 in his possession a plate showing the im- 

 pression of two coins taken on February 

 22, 1890, in the physical department of the 

 University of Pennsylvania, undoubtedly by 

 the X-rays. 



On the occasion referred to many experi- 

 ments were made, the object being to photo- 

 graph the brush discharge, from a powerful 

 induction machine, directly upon the sensi- 

 tive plate, without any camera. Incidentally 



also the impressions of coins were obtained 

 by sparking them when in contact with the 

 sensitive film. After these experiments had 

 been completed, a number of Crookes tubes 

 were brought out and operated for the 

 pleasure and amusement of Mr. W. N. Jen- 

 nings, in connection with whom the work 

 had been done. 



A few days later Mr. Jennings, who had 

 taken the plates home for development, re- 

 ported the appearance, on one of them, of 

 too very mj^sterious discs quite different in 

 character from those obtained by the spark- 

 ing process. No explanation was found at 

 the time to account for the phenomenon, and 

 the matter was forgotten till recently, when 

 the occasion was recalled and the old plate 

 was produced from a lot of so-called failures. 

 On repeating the experiment by operating a 

 Crookes tube for ten minutes, in the vicinity 

 of an enclosed photographic plate having 

 two coins on the outside of the box, it is 

 found that the coin shadows are strikingly 

 similar to the mysterious discs upon the old 

 plate. The blurred appearance of one edge 

 is a distinctive feature of a Rontgen picture. 

 A print from the original plate is shown in 

 Fig. 2. The writer and his associate wish 



Fig. 2. 



to claim no credit for the interesting acci- 

 dent, but the fact remains that without 



