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In view of this dilemma there appears to be nothing else for it

than to abandon either the principle of relativity or the simple

law of the propagation of light in vacuo. Those of you who have

carefully followed the preceding discussion are almost sure to

expect that we should retain the principle of relativity, which

appeals so convincingly to the intellect because it is so natural

and simple. The law of the propagation of light in vacuo would

then have to be replaced by a more complicated law conformable to

the principle of relativity. The development of theoretical

physics shows, however, that we cannot pursue this course. The

epoch-making theoretical investigations of H. A. Lorentz on the

electrodynamical and optical phenomena connected with moving

bodies show that experience in this domain leads conclusively to

a theory of electromagnetic phenomena, of which the law of the

constancy of the velocity of light in vacuo is a necessary

consequence.