called the discovery of
radiophony in 1880 by Graham Bell, which was foreshadowed in 1875 by
C.A. Brown. A luminous ray falling on a selenium cell produces a
variation of electric resistance, thanks to which a sound signal can
be transmitted by light. That delicate instrument the radiophone,
constructed on this principle, has wide analogies with the apparatus
of to-day.

Sec. 6

Starting from the experiments of Hertz, the history of wireless
telegraphy almost merges into that of the researches on electrical
waves. All the progress realised in the manner of producing and
receiving these waves necessarily helped to give rise to the
application already indicated. The experiments of Hertz, after being
checked in every laboratory, and having entered into the strong domain
of our most certain knowledge, were about to yield the expected fruit.

Experimenters like Sir Oliver Lodge in England, Righi in Italy,
Sarrazin and de la Rive in Switzerland, Blondlot in France, Lecher in
Germany, Bose in India, Lebedeff in Russia, and theorists like M.H.
Poincare and Professor Bjerknes, who devised ingenious arrangements or
elucidated certain points left dark, are among the artisans of the
work which followed its natural evolution.

It was Professor R. Threlfall who seems to have been the first to
clearly propose, in 1890, the application of the Hertzian waves to
telegraphy, but it was certainly Sir W. Crookes who, in a very
remarkable article in the _Fortnightly Review_ of February 1892,
pointed out very clearly the road to be followed. He even showed in
what conditions the Morse receiver might be applied to the new system
of telegraphy.

About the same period an American physicist, well known by his
celebrated experiments on high frequency currents--experiments, too,
which are not unconnected with those on electric oscillations,--M.
Tesla, demonstrated that these oscillations could be transmitted to
more considerable distances by making use of two vertical antennae,
terminated by large conductors.

A little later, Sir Oliver Lodge succeeded, by the aid of the coherer,
in detecting waves at relatively long distances, and Mr Rutherford
obtained similar results with a magnetic indicator of his own
invention.

An important question of meteorology, the study of atmospheric
discharges, at this date led a few scholars, and more particularly the
Russian, M. Popoff, to set up apparatus very analogous to the
receiving apparatus of th