simultaneously for different colours during the eclipse of a fixed star
by its dark neighbour. By means of similar considerations based on
observations of double stars, the Dutch astronomer De Sitter was also
able to show that the velocity of propagation of light cannot depend on
the velocity of motion of the body emitting the light. The assumption
that this velocity of propagation is dependent on the direction in
space is in itself improbable.
In short, let us assume that the simple law of the constancy of the
velocity of light _c_ (in vacuum) is justifiably believed by the child
at school. Who would imagine that this simple law has plunged the
conscientiously thoughtful physicist into the greatest intellectual
difficulties? Let us consider how these difficulties arise.
Of course we must refer the process of the propagation of light (and
indeed every other process) to a rigid reference-body (co-ordinate
system). As such a system let us again choose our embankment. We shall
imagine the air above it to have been removed. If a ray of light be
sent along the embankment, we see from the above that the tip of the
ray will be transmitted with the velocity _c_ relative to the
embankment. Now let us suppose that our railway carriage is again
travelling along the railway lines with the velocity _v_, and that its
direction is the same as that of the ray of light, but its velocity of
course much less. Let us inquire about the velocity of propagation of
the ray of light relative to the carriage. It is obvious that we can
here apply the consideration of the previous section, since the ray of
light plays the part of the man walking along relatively to the
carriage. 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,
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