It was on the eighth of November, 1895, that Dr. William Konrad Roentgen, of Wuerzburg, made the discovery which seems likely to contribute so much to our knowledge of the mysterious processes of nature.  On that day Dr. Roentgen was working with a Crookes tube in his laboratory.  This piece of apparatus is well known to students and partly known to general readers.  It consists of a glass cylinder, elongated into tubular form, and hermetically closed at the ends.  When the tube is made, the air is exhausted as nearly as possible from it, and the ends are sealed over a vacuum as perfect as science is able to produce.  Through the two ends, bits of platinum wire are passed at the time of sealing, so that they project a little within and without.  The interior of the tube is thus a vacuum into which at the two ends platinum wires extend.  Electrical communication with outside apparatus is thus supplied.

It has long been known that on the discharge of an electrical current into this kind of vacuum peculiar and interesting phenomena are produced.  The platinum wires at the two ends are connected with the positive and negative wires or terminals of an induction coil.  When this is done, the electrical current discharged into the vacuum seems to flash out around the inner surfaces of the tube, in the form of light.  There are brilliant coruscations from one end to the other of the tube.  The tips of the platinum wire constituting the inner poles glow and seem to flame.  That pole which is connected with the positive side of the battery is called the anode, or upper pole, and that which is connected with the negative, or receptive, side of the battery, is called the cathode, or lower pole.  It was in his experimentation with this apparatus, and in particular in noticing the results at the cathode or lower end of the tube, that Professor Roentgen made his famous discovery.  It was for this reason that the name of “cathode rays” has been given to the new radiant force; but Dr. Roentgen himself called the phenomena the X, or unknown, rays.

In the experimentation referred to, Roentgen had covered the glass tube at the end with a shield of black cardboard.  This rendered the glow at the cathode pole completely invisible.  It chanced that a piece of paper treated with platino-barium cyanide for photographic uses was on a bench near by.  Notwithstanding the fact that the tube was covered with an opaque shield, so that no light could be transmitted, Professor Roentgen noticed that changes in the barium paper were taking place, as though it were exposed to the action of light!  Black lines appeared on the paper, showing that the surface was undergoing chemical change from the action of some invisible and hitherto unknown force!

This was the moment of discovery.  The philosopher began experimenting.  He repeated what had been accidentally done and was immediately convinced that a force, or, as it were, invisible rays were streaming from the cathode pole of the tube through the glass, and through a substance absolutely opaque, and that these rays were performing their work at a distance on the surface of paper that was ordinarily sensitive only to the action of light.