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We shall see later that this result, which expresses the theorem

of the addition of velocities employed in classical mechanics,

cannot be maintained; in other words, the law that we have just

written down does not hold in reality. For the time being,

however, we shall assume its correctness.

VII.

THE APPARENT INCOMPATIBILITY OF THE LAW OF PROPAGATION OF LIGHT

WITH THE PRINCIPLE OF RELATIVITY

There is hardly a simpler law in physics than that according to

which light is propagated in empty space. Every child at school

knows, or believes he knows, that this propagation takes place in

straight lines with a velocity c = 300,000 km./sec. At all events

we know with great exactness that this velocity is the same for

all colours, because if this were not the case, the minimum of

emission would not be observed 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.