rected 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.

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 w