PROFESSOR RONTGEN'S DISCOVERY 555 



culiar habit of Professor Eontgems, a habit which leads often to the 

 finding of lost keys by shaking open the books on his desk. The pro- 

 fessor was working with the Crookes tube referred to above, observing 

 the beautiful yellowish-green fluorescence which characterized this 

 particular tube, when his wife came to call him to lunch. Laying the 

 tube, still glowing, on the book he obeyed her summons. 



Now Professor Eontgen is an enthusiastic amateur photographer, 

 in fact out door photograph}" is his recreation. Eeturning from lunch, 

 he took the plate holder which had lain under the book, with other 

 plate holders, and made several outdoor exposures. On developing 

 the plates a shadow picture of a key appeared on one of them. Much 

 puzzled, he showed the negative to some of his students, asking them to 

 suggest some explanation of the mysterious key. None of their sug- 

 gestions proved satisfactory, and he was up early the next morning 

 searching for a solution of the mystery. 



He determined to repeat precisely the operations of the preceding 

 day and, remembering the positions of the glowing tube, the book, and 

 the plate holder, he placed them as before, leaving them for the same 

 length of time as on the preceding day. On developing the plate, the 

 image of the key again appeared. The key was found in the book but 

 the mystery was not solved. Here was indeed a strange thing. Of 

 course it was known that the cathode rays would affect a photographic 

 plate, but here between the plate and the source of the rays were a book 

 and the hard-rubber slide of the plate holder, both of which are im- 

 pervious to light, and the cathode rays were confined by the walls of 

 the tube. 



Eontgen continued his investigations and found that the rays from 

 his tube would penetrate other objects, but in different degrees, and be- 

 cause of this difference in transparency he could obtain shadow pictures 

 of many interesting objects. 



The fluorescence of his tube suggested to him that other substances 

 than glass might be caused to fluoresce by the radiation from the tube. 

 An interesting field was opened before him. Other research was sus- 

 pended. Visitors were excluded, and with his research students the 

 work was continued. Fluorescent liquids were tested, one of the first 

 being a solution prepared from the horse-chestnut. The number of 

 fluorescent substances tested, including liquids and solids, was not less 

 than fifty. One of these was barium platino-cyanide, the fluorescence 

 in this case being caused by the newly-discovered radiation acting 

 through a black cardboard. Convinced by a long series of observations 

 of the photographic and fluorescent effects obtained from his tube that 

 he had discovered a new form of radiation, Professor Eontgen made 

 his public announcement. 



In answer to my question regarding the date of the incident nar- 



