Professor Lenard's fundamental idea was to study the cathode rays
under conditions different from those in which they are produced.
These rays are born in a very rarefied space, under conditions
perfectly determined by Sir W. Crookes; but it was a question whether,
when once produced, they would be capable of propagating themselves in
other media, such as a gas at ordinary pressure, or even in an
absolute vacuum. Experiment alone could answer this question, but
there were difficulties in the way of this which seemed almost
insurmountable. The rays are stopped by glass even of slight
thickness, and how then could the almost vacuous space in which they
have to come into existence be separated from the space, absolutely
vacuous or filled with gas, into which it was desired to bring them?
The artifice used was suggested to Professor Lenard by an experiment
of Hertz. The great physicist had, in fact, shortly before his
premature death, taken up this important question of the cathode rays,
and his genius left there, as elsewhere, its powerful impress.
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