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The velocity W of the man relative to the embankment is here

replaced by the velocity of light relative to the embankment. w

is the required velocity of light with respect to the carriage,

and we have

w = c - v.

The velocity of propagation ot a ray of light relative to the

carriage thus comes out smaller than c.

But this result comes into conflict with the principle of

relativity set forth in Section V. For, like every other general

law of nature, the law of the transmission of light in vacuo [in

vacuum] must, according to the principle of relativity, be the

same for the railway carriage as reference-body as when the rails

are the body of reference. But, from our above consideration,

this would appear to be impossible. If every ray of light is

propagated relative to the embankment with the velocity c, then

for this reason it would appear that another law of propagation

of light must necessarily hold with respect to the carriage-a

result contradictory to the principle of relativity.