X-rays and localisation / by James Mackenzie Davidson.

  • Davidson, James MacKenzie.
Date:
[1902]
Catalogue details

Licence: In copyright

Credit: X-rays and localisation / by James Mackenzie Davidson. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    if a tube is broken and air is allowed to rush in through the opening, there is about a million times more air in it at the ordinary pressure than was the case before the opening was made. When we pass a discharge through such a tube from the negative or cathode side, a stream of particles starts, and anything upon which they impinge is made to glow. This cathode ray has the capacity of producing great heat ; it will make platinum red hot, and even melt it; it will cause substances to fluoresce and phosphoresce ; it will only travel in straight lines, it will not turn round corners, and it is deflected by a magnet; it will cast shadows of any solid substances placed in its path. All these observations had been made by Crookes, and necessarily created much interest. Dr. Philip Lenard, in 1894, endeavoured to bring the cathode rays outside the tube into the open, and as glass was opaque to them, he endeavoured to make an opening for them by introducing a small window of thin aluminium foil opposite the cathode. He succeeded, for the rays which passed through his aluminium window produced fluorescence. They affected a photo- graphic plate. He found that they would pass through thin sheets of aluminium, and even of copper; he also found them to affect a photographic plate through metal. Professor Kontgen of Wiirtzburg took up the subject where Lenard left it. When I had the pleasure of an interview with Professor Eontgen, shortly after his discovery, I asked him what intention he had in his mind when he commenced the research. He said that he was looking for any invisible rays that might be pro- duced from a Crookes' tube. The next question I asked him was, why had he used a screen of Barium platino-cyanide. He replied that he had used this substance because, by its brilliant fluorescence it was capable of revealing the invisible rays of the spectrum. He then told me that he had covered the Crookes' tube with black paper, so that when it was excited by the electric current no visible light was seen, but he suddenly observed that a piece of cardboard, which had been covered with crystals of Barium platino-cyanide, glowed brilliantly. I asked him what he thought when he saw this. His reply was, I did not think ; I experimented. He showed me the original screen with which this great discovery was made, and I suggested that such an historical screen should be carefully protected in a glass case, and I trust that my suggestion has been carried out. He speedily found that various substances interposed between a Crookes' tube and a fluorescent screen allowed the rays to pass through in varying degrees, and doubtless he was immensely surprised and interested to observe that when he interposed his hand, that bis bones inside his flesh were revealed on the screen. One of the earliest experiments which he tried was to take a photograph througli a deal door between two of his laboratories. He placed the tube on one side of the door and aflixed to it a strip of platinum, and at a corresponding part on the other side of the door he placed a photo-