culiar habit of Professor Röntgen's, a habit which leads often to the finding of lost keys by shaking open the books on his desk. The professor 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 Röntgen is an enthusiastic amateur photographer, in fact out door photography is his recreation. Returning 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 suggestions 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 impervious to light, and the cathode rays were confined by the walls of the tube.
Röntgen continued his investigations and found that the rays from his tube would penetrate other objects, but in different degrees, and because 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 suspended. 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 Röntgen made his public announcement.
In answer to my question regarding the date of the incident nar-