176 THE LIMITATIONS OF SCIENCE 



quantities appear in the mathematical expression in a 

 symmetrical manner ; so also are their momenta or their 

 masses multiplied by their velocities. But this is not 

 the case for energy since it involves the square of the 

 velocity. The squared velocity remains positive al- 

 though we change direction and the composition of two 

 energies is always an addition. For true relativity, a 

 change in direction must be accompanied by a change 

 in sign. The extension of the idea of relativity to in- 

 volve cases of radiant energy is partly responsible 

 for the conclusion that mass is a function of energy. 



Professor Einstein's second postulate has been 

 widely discussed and many attempts have been made to 

 interpret it. At first sight it seems absurd to say that 

 the mechanical motion of a body emitting light 

 does not affect the velocity of the light emitted. But 

 the fact remains that all our experiments, and they 

 have been most accurate and searching, fail to find 

 any difference in the velocity of light whether the 

 sources and the recipient are at rest or in motion. As 

 we have seen, Professor Einstein, with admirable 

 directness, goes straight to the point: he accepts the 

 reliability of these experiments; assumes the constancy 

 of V as a postulate, determines its effect on the space 

 dimensions of bodies and on time, and finds that we 

 must change our previous ideas of these radically. Let 

 us, when discussing V, consider it under two heads : 

 first, when light passes through transparent material 





