Now it is found that when the speed approaches that of light, 186,000 miles or 3 x 10^{10} centimetres per second, the energy is higher than it should be if it followed the usual rule, viz. energy is equal to half the mass multiplied by the square of the velocity.  It would seem that an electron moving with the velocity of light would have infinite energy; or, to put the matter in another way, the experimenter in his laboratory can never hope to observe an electron moving so fast; it would be the end of his laboratory and of himself if ever it turned up.

Linked up with this result is the very strange fact that no one has ever been able to find any direct evidence of the existence of the ether, which is postulated in order to carry light-waves.  It has been pictured as a medium through which the heavenly bodies move, and to which their motions may be referred.  But when light is launched into the ether, its apparent velocity must depend on whether it travels with or against the drift of the ether through the laboratory where the measurement is made.  The experiment has been performed without the discovery of any such difference, although the method was amply accurate enough to detect the effect that might be expected.  It was afterwards shown that the negative result might be explained by supposing that a measure of length varied in length according to whether it was travelling with or against the ether.  But the continual failure of all such experiments has led to a remarkable hypothetical development with which the name of Einstein is firmly connected.  It is supposed that some flaw must exist in our fundamental hypotheses, and that if this were corrected we should then find that we ought to get the same value for the velocity of light however and whenever we measured it, and at the same time we should find that no measurement of the velocity of a body moving relative to the observer would ever equal the velocity of light.  The hypothesis denies the existence of an absolute standard to which motions can be referred, and insists that they must all be considered relatively to the observer.  It is called the principle of relativity.  Calculations of its consequences begin with the necessary changes in the fundamentals, such as Einstein has introduced.[70]

Time does not allow me to say more of the innumerable ways in which electrons play an essential part in all the processes in the world.  We have long believed that this is so, but the picture has never been so clear to us as it is now; and with our understanding our power is increased.  Yet once more the illumination of our understanding comes from our recognition that Nature has preferred the discrete to the continuous and that electricity is not infinitely divisible but is, like matter, and even more simply than matter, of an atomic structure.  And we have found the unit and learnt how to handle it.