Bohr’s great discovery was that this same quantity _h_ is involved in the orbits of the planetary electrons in atoms, and that it limits the possible orbits in ways for which nothing in Newtonian dynamics had prepared us, and for which so far, there is nothing in relativity-dynamics to account. According to Newtonian principles, an electron ought to be able to go round the nucleus in any circle with the nucleus in the centre, or in any ellipse with the nucleus in a focus; among possible orbits, it would select one or another according to its direction and velocity. But in fact only certain out of all these orbits occur. Those that occur are among those that are possible on Newtonian principles, but are only an infinitesimal selection from among these. It will simplify the explanation if we confine ourselves, as Bohr did at first, to circular orbits; moreover we will consider only the hydrogen atom, which has one planetary electron and a nucleus consisting of one proton. To define the circular orbits that are found to be possible, we proceed as follows: multiply the mass of the electron by the circumference of its orbit, and this again by the velocity of the electron; the result will always be _h_ or 2_h_, or 3_h_, or some other small exact multiple of _h_, where _h_, as before, is “Planck’s constant”. There is thus a smallest possible orbit, in which the above product is _h_; the radius of the next orbit, in which the above produce is 2_h_, will have a length four times this minimum; the next, nine times; the next, sixteen times; and so on through the “square numbers” (_i.e._ those got by multiplying a number by itself). Apparently no other circular orbits than these are possible in the hydrogen atom. Elliptic orbits are possible, and these again introduce exact multiples of _h_: but we need not, for our purposes, concern ourselves with them. When a hydrogen atom is left to itself, if the electron is in the minimum orbit it will continue to rotate in that orbit so long as nothing from outside disturbs it; but if the electron is in any of the larger possible orbits, it may sooner or later jump suddenly to a smaller orbit, either the minimum or one of the intermediate possible orbits. So long as the electron does not change its orbit, the atom does not radiate energy, but when the electron jumps to a smaller orbit, the atom loses energy, which is radiated out in the form of a light-wave. This light-wave is always such that its energy divided by its frequency is exactly _h_. The atom may absorb energy from without, and it does so by the electron jumping to a larger orbit. It may then afterwards, when the external source of energy is removed, jump back to the smaller orbit; this is the cause of fluorescence, since, in doing so, the atom gives out energy in the form of light.