THE ANNOUNCEMENT OF THE DISCOVERY OF X-RAYS. Röntgen's experiments with cathode ray tubes showed that some rays were causing fluorescence some distance away from the sealed tube, although the room was in darkness. Further trials with different materials showed that the rays could penetrate some substances but not others. "Röntgen then placed his own hand transverse to the rays and saw the macabre pattern of the bones in his hand set in a fainter outline of the flesh. To capture this spectral sight, Röntgen replaced the screen with a photographic plate to record a sight never before seen by man. It became clear to him that this was a new form of light, invisible to the eye and which had never been observed or recorded" (Bern Dibner, The New Rays of Professor Röntgen
, 1963, p.18). He called them x-rays because they were unknown (they are now generally called Röntgen rays in languages other than English).
Röntgen's secret experiments in November and December 1895 were first announced in the first paper, written in December 1895, and published simultaneously in the journal (as here) and in offprints (dated "Ende 1895" and in printed wrappers). The second part of the paper was published in the same journal in March 1896, containing confirmation of some earlier suppositions due to further experimentation, and some refinements to the equipment used. It also contains the famous photograph of the hand of Röntgen's colleague, Professor von Kölliker, which was made during Röntgen's only public demonstration of his discovery, in late January 1896 (this photograph was not reproduced for the offprint).