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. It is even more strange that it may now be said of energy that there are signs of atomicity. It may seem absurd to think that the energy which is transformed in any operation is transformed in multiples of a universal unit or units, so that the operation cannot be arrested at any desired stage but only at definite intervals. Indeed we have no right to assert that this is always true. But undoubtedly there are cases in which the atomicity of energy is clear enough, as for example in the interchange of energy between electrons in motion and radiation. It is remarkable that when radiation sets an electron in motion, the electron acquires a perfectly definite speed depending only on the wave-length of the radiation and not on its intensity, and has apparently absorbed from the radiation a definite unit of energy. Radiation of a particular wave-length cannot spend its energy in this way except in multiples of a certain unit, because each of the electrons which it sets in motion has the same initial energy, which it must have got from the radiation. In other words, energy of radiation of the particular wave-length can only be transformed into energy of movement of electrons in multiples of a certain 'quantum' peculiar to that wave-length. The intensity of the radiation, that is to say, the amount of energy moving along the beam, can only affect the number of electrons set in motion and not the speed of any one of them. During the last few years a very extraordinary theory has been developed on the basis of these and similar facts. I doubt if it would be more profitable to give further instances at present, but I have mentioned it because it seems to show looming on the horizon of our knowledge another tendency of Nature to make use of the atomic principle. I will only add that the whole position of physics is indeed at this time of extraordinary interest, and at any moment there may be some great discovery or illuminating thought which will explain the present startling difficulties and open up new worlds of thought. FOR REFERENCE Bragg, _Rays and Crystals_ (Ball & Sons). FOOTNOTES: