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According to this theory there is no such thing as a "specially

favoured" (unique) co-ordinate system to occasion the

introduction of the aether-idea, and hence there can be no

aether-drift, nor any experiment with which to demonstrate it.

Here the contraction of moving bodies follows from the two

fundamental principles of the theory, without the introduction of

particular hypotheses; and as the prime factor involved in this

contraction we find, not the motion in itself, to which we cannot

attach any meaning, but the motion with respect to the body of

reference chosen in the particular case in point. Thus for a

co-ordinate system moving with the earth the mirror system of

Michelson and Morley is not shortened, but it is shortened for a

co-ordinate system which is at rest relatively to the sun.

XVII.

MINKOWSKI'S FOUR-DIMENSIONAL SPACE