Early in 1896 just a few months after Roentgen had startled the
scientific world by the announcement of the discovery of the X-rays,
Professor Henri Becquerel of the Natural History Museum in Paris
announced another discovery which, if not as mysterious, was more
puzzling and still continues a puzzle to a great degree to the present
time. Studying the action of the salts of a rare and very heavy mineral
called uranium Becquerel observed that their substances give off an
invisible radiation which, like the Roentgen rays, traverse metals and
other bodies opaque to light, as well as glass and other transparent
substances. Like most of the great discoveries it was the result of
accident. Becquerel had no idea of such radiations, had never thought
of their possibility.

In the early days of the Roentgen rays there were many facts which
suggested that phosphorescence had something to do with the production
of these rays It then occurred to several French physicists that X-rays
might be produced if phosphorescent substances were exposed to sunlight.
Becquerel began to experiment with a view to testing this supposition.
He placed uranium on a photographic plate which had first been wrapped
in black paper in order to screen it from the light. After this plate
had remained in the bright sunlight for several hours it was removed
from the paper covering and developed. A slight trace of photographic
action was found at those parts of the plate directly beneath the
uranium just as Becquerel had expected. From this it appeared evident
that rays of some kind were being produced that were capable of passing
through black paper. Since the X-rays were then the only ones known
to possess the power to penetrate opaque substances it seemed as though
the problem of producing X-rays by sunlight was solved. Then came the
fortunate accident. After several plates had been prepared for exposure
to sunlight a severe storm arose and the experiments had to be abandoned