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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.