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(1) The time-interval (time) between two events is independent of

the condition of motion of the body of reference.

(2) The space-interval (distance) between two points of a rigid

body is independent of the condition of motion of the body of

reference.

If we drop these hypotheses, then the dilemma of Section VII

disappears, because the theorem of the addition of velocities

derived in Section VI becomes invalid. The possibility presents

itself that the law of the propagation of light in vacuo may be

compatible with the principle of relativity, and the question

arises: How have we to modify the considerations of Section VI in

order to remove the apparent disagreement between these two

fundamental results of experience? This question leads to a

general one. In the discussion of Section VI we have to do with

places and times relative both to the train and to the

embankment. How are we to find the place and time of an event in

relation to the train, when we know the place and time of the

event with respect to the railway embankment? Is there a

thinkable answer to this question of such a nature that the law

of transmission of light in vacuo does not contradict the

principle of relativity?