co-ordinates of $K$ and $K'$. It thus appears that, of all Galileian
co-ordinate systems, one ($K$) corresponding to a particular state of
motion is physically unique. This result was interpreted physically by
-regarding $K$ as at rest with respect to a hypothetical æther of space.
+regarding $K$ as at rest with respect to a hypothetical æther of space.
On the other hand, all coordinate systems $K'$ moving relatively to $K$
-were to be regarded as in motion with respect to the æther. To this
+were to be regarded as in motion with respect to the æther. To this
motion of $K'$ against the {\ae}ther (``{\ae}ther-drift'' relative to $K'$) were
attributed the more complicated laws which were supposed to hold
relative to $K'$. Strictly speaking, such an {\ae}ther-drift ought also to
mirrors so arranged on a rigid body that the reflecting surfaces face
each other. A ray of light requires a perfectly definite time T to
pass from one mirror to the other and back again, if the whole system
-be at rest with respect to the æther. It is found by calculation,
+be at rest with respect to the æther. It is found by calculation,
however, that a slightly different time $T'$ is required for this
process, if the body, together with the mirrors, be moving relatively
to the {\ae}ther. And yet another point: it is shown by calculation that
clearly detectable. But the experiment gave a negative result---a
fact very perplexing to physicists. Lorentz and FitzGerald rescued the
theory from this difficulty by assuming that the motion of the body
-relative to the æther produces a contraction of the body in the
+relative to the æther produces a contraction of the body in the
direction of motion, the amount of contraction being just sufficient
to compensate for the differeace in time mentioned above. Comparison
with the discussion in Section 11 shows that also from the
method of interpretation is incomparably more satisfactory. According
to this theory there is no such thing as a ``specially favoured''
(unique) co-ordinate system to occasion the introduction of the
-æther-idea, and hence there can be no æther-drift, nor any experiment
+æther-idea, and hence there can be no æther-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
the effect of gravitation. The results of observation have been
collected together, and discussed in detail from the standpoint of the
question which has been engaging our attention here, in a paper by E.
-Freundlich entitled ``Zur Prüfung der allgemeinen
+Freundlich entitled ``Zur Prüfung der allgemeinen
Relativit\"ats-Theorie" ({\it Die Naturwissenschaften}, 1919, No. 35,
p. 520: Julius Springer, Berlin).