and for stiII greater velocities the square-root becomes imaginary.  From this we conclude that in the theory of relativity the velocity c plays the part of a limiting velocity, which can neither be reached nor exceeded by any real body.

Of course this feature of the velocity c as a limiting velocity also clearly follows from the equations of the Lorentz transformation, for these became meaningless if we choose values of v greater than c.

If, on the contrary, we had considered a metre-rod at rest in the x-axis with respect to K, then we should have found that the length of the rod as judged from K1 would have been eq. 06 ;

this is quite in accordance with the principle of relativity which forms the basis of our considerations.

A Priori it is quite clear that we must be able to learn something about the physical behaviour of measuring-rods and clocks from the equations of transformation, for the magnitudes z, y, x, t, are nothing more nor less than the results of measurements obtainable by means of measuring-rods and clocks.  If we had based our considerations on the Galileian transformation we should not have obtained a contraction of the rod as a consequence of its motion.

Let us now consider a seconds-clock which is permanently situated at the origin (x1=0) of K1. t1=0 and t1=I are two successive ticks of this clock.  The first and fourth equations of the Lorentz transformation give for these two ticks : 

t = 0

and

eq. 07:  file eq07.gif

As judged from K, the clock is moving with the velocity v; as judged from this reference-body, the time which elapses between two strokes of the clock is not one second, but

eq. 08:  file eq08.gif

seconds, i.e. a somewhat larger time.  As a consequence of its motion the clock goes more slowly than when at rest.  Here also the velocity c plays the part of an unattainable limiting velocity.

THEOREM OF THE ADDITION OF VELOCITIES.  THE EXPERIMENT OF FIZEAU

Now in practice we can move clocks and measuring-rods only with velocities that are small compared with the velocity of light; hence we shall hardly be able to compare the results of the previous section directly with the reality.  But, on the other hand, these results must strike you as being very singular, and for that reason I shall now draw another conclusion from the theory, one which can easily be derived from the foregoing considerations, and which has been most elegantly confirmed by experiment.

In Section 6 we derived the theorem of the addition of velocities in one direction in the form which also results from the hypotheses of classical mechanics- This theorem can also be deduced readily horn the Galilei transformation (Section 11).  In place of the man walking inside the carriage, we introduce a point moving relatively to the co-ordinate system K1 in accordance with the equation