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the contracted length being proportional to the expression

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This, hypothesis, which is not justifiable by any

electrodynamical facts, supplies us then with that particular law

of motion which has been confirmed with great precision in recent

years.

[13] The general theory of relativity renders it likely that the

electrical masses of an electron are held together by

gravitational forces.

The theory of relativity leads to the same law of motion, without

requiring any special hypothesis whatsoever as to the structure

and the behaviour of the electron. We arrived at a similar

conclusion in Section XIII in connection with the experiment of

Fizeau, the result of which is foretold by the theory of

relativity without the necessity of drawing on hypotheses as to

the physical nature of the liquid.