The smallest orbits which the electron can describe in the hydrogen atom are shewn in fig. 10. The smallest orbit of all, of diameter 1, is marked 1₁; beyond this come two orbits of diameter 4 marked 2₁ 2₂; then three orbits of diameter 9 marked 3₁, 3₂, 3₃; and four orbits of diameter 16 marked 4₁, 4₂, 4₃, 4₄. The diagram stops here for want of space, but the available orbits go on indefinitely. Even under laboratory conditions, electrons may move in orbits of a hundred times the diameter of that marked 1₁. Under the more rarefied conditions of stellar atmospheres the hydrogen atom may swell out to even greater dimensions, and stellar spectra provide evidence of orbits having over a thousand times the dimensions of the 1₁ orbit. Such an orbit would be represented in fig. 10 by a circle four yards in diameter. [Illustration: Fig. 10. The arrangement of electron orbits in the hydrogen atom (Bohr’s model).] All orbits, whether elliptic or circular, which have the same diameter, have also the same energy, but the energy changes when an electron crosses over from any orbit to another of a different diameter. Thus, to a certain limited extent, the atom constitutes a reservoir of energy. Its changes of energy are easily calculated; for example, the two orbits of smallest diameters in the hydrogen atom differ in energy by 16 × 10⁻¹² ergs. If we pour radiation of the appropriate wave-length on to an atom in which the electron is describing the smallest orbit of all, it crosses over to the next orbit, absorbing 16 × 10⁻¹² ergs of energy in the process, and so becoming temporarily a reservoir of energy holding 16 × 10⁻¹² ergs. If the atom is in any way disturbed from outside, it may of course discharge the energy at any time, or it may absorb still more energy and so increase its store. If we know all the orbits which are possible for an atom of any type, it is easy to calculate the changes of energy involved in the various transitions between them. As each transition absorbs or releases exactly one quantum of energy, we can immediately deduce the frequencies of the light emitted or absorbed in these transitions. In brief, given the arrangement of atomic orbits, we can calculate the spectrum of the atom. In practice the problem of course takes the converse form: given the spectrum, to find the structure of the atom which emits it. Bohr’s model of the hydrogen atom is a good model at least to this extent—that the spectrum it would emit reproduces the hydrogen spectrum almost exactly. Yet the agreement is not quite perfect, and it is now generally accepted that Bohr’s scheme of orbits is inadequate to account for actual spectra. We continue to discuss Bohr’s scheme, not because the atom is actually built that way, but because it provides a good enough working model for our present purpose.