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It is clear that we have the problem of Section VI again before

us. The tube plays the part of the railway embankment or of the

co-ordinate system K, the liquid plays the part of the carriage

or of the co-ordinate system K′, and finally, the light plays the

part of the man walking along the carriage, or of the moving

point in the present section. If we denote the velocity of the

light relative to the tube by W, then this is given by the

equation (A) or (B), according as the Galilei transformation or

the Lorentz transformation corresponds to the facts.

Experiment[10] decides in favour of equation (B) derived from the

theory of relativity, and the agreement is, indeed, very exact.

According to recent and most excellent measurements by Zeeman,

the influence of the velocity of flow v on the propagation of

light is represented by formula (B) to within one per cent.

[10] Fizeau found

image015

where

image016

is the index of refraction of the liquid. On the other hand,

owing to the smallness of