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But there are two classes of experimental facts hitherto obtained

which can be represented in the Maxwell-Lorentz theory only by

the introduction of an auxiliary hypothesis, which in itself-i.e.

without making use of the theory of relativity-appears

extraneous.

It is known that cathode rays and the so-called β-rays emitted by

radioactive substances consist of negatively electrified

particles (electrons) of very small inertia and large velocity.

By examining the deflection of these rays under the influence of

electric and magnetic fields, we can study the law of motion of

these particles very exactly.