Section II. THE DISCOVERY OF X-RAYS
1-11. WILHELM ROENTGEN (1895)
a. Reproducing Lenard's Experiments. Between the years 1894 and 1895,
Wilhelm Conrad Roentgen attempted to reproduce his predecessor, Philippe Lenard's,
experiments. Concerned that outside light could have possibly affected Lenard's data in
some fashion, Roentgen waited until nightfall, even enclosing his Crookes Partial
Vacuum Tube in cardboard to keep its light from affecting his findings.
b. The Cardboard Screen with Barium Platinocyanide. Now, it so happened
that on a distant table, Roentgen had a small cardboard screen coated with barium
platinocyanide crystals. This screen had nothing whatsoever to do with the experiment,
as Roentgen had originally conceived it. But, it was an unforeseen and chance reaction
of the barium platinocyanide to Roentgen's experiment that was to take on enormous
importance for the discovery of x-rays.
c. Crookes Tube Turned On, Barium Platinocyanide Screen
Phosphorescent. With his laboratory in darkness, Roentgen turned on the apparatus.
From his peripheral (side vision), he noticed a change in the screen covered with
barium platinocyanide, the one that was located on a distant table. The screen had
begun to fluoresce (exhibit phosphorescence). He noted, too, that when his apparatus
was turned off, there was no effect. With the apparatus on, the screen glowed. Moving
the screen closer caused the fluorescence to intensify or brighten. Realizing that this
invisible source had already penetrated the wrapping of his tube, he experimented with
other materials and discovered that books with many pages had little effect. Very dense
objects almost totally blocked the invisible beam. In the course of trying many different
objects, he noticed that that the image of his hand, as projected onto the screen made
the flesh virtually invisible, while the bones were very apparent. It was immediately
obvious to Roentgen that this could be a useful tool for medical observation. Thus, it
was on November 8, 1895, when Roentgen noticed a faint luminescence from the
cardboard screen coated with barium platinocyanide crystals that x-rays were
discovered. Roentgen felt sure that others must have already observed what he had
just seen. So, he spent many hours reviewing publications of the day to learn more
about what he had just witnessed. By daybreak it became clear to him that he seemed
to have been the first to observe this phenomenon.
d. Substituting Photographic Film for the Screen. In his next round of
experiments, Roentgen substituted photographic film for the screen. To prove to his
contemporaries the value of his discovery, Roentgen persuaded his wife, Berta, to place
her hand atop the fluorescing screen on which he had placed a piece of the light-
sensitive photographic film. With the Crookes Tube above her hand, he again turned on
the apparatus. Historians estimate that with the limited power of his equipment, this first
radiograph may have required as long as fifteen minutes of exposure.