Before going farther we shall recall what has previously been said about the quantum numbers. In the undisturbed hydrogen atom, the stationary orbits can be numbered with the principal quantum numbers 1, 2, 3 ... _n_. But to each principal quantum number there corresponds not one but several states, each with its auxiliary quantum number 1, 2, 3 ... _k_, _k_ at the most being equal to the principal quantum number. In a similar way, the stationary orbits of the electrons in an atom containing several electrons can be indicated by two quantum numbers, the 3₂ orbit, for instance, being that with principal quantum number 3 and auxiliary quantum number 2. But while in the hydrogen atom the principal quantum number _n_, in the stationary orbits which are slowly rotating ellipses, is very simply connected with the length of the major axis of the ellipse, and _k_: _n_ is the ratio between the minor and major axes, still in other atoms with complex systems of electrons the significance of the principal quantum number is not so simple and the orbit of an electron consists of a sequence of loops of more complicated form (cf. Fig. 29). We must satisfy ourselves with the statement that a definition of their significance can be given, but only by mathematical-physical considerations which we cannot enter into here. It may, however, be stated that, if we restrict ourselves to a definite atom, the rule will hold that, among a series of orbits with the same auxiliary quantum number but different principal quantum numbers, that orbit in which the electron attains a greater distance from the nucleus has the higher number. Another rule which holds is, that an orbit with a small auxiliary quantum number in comparison with its principal quantum number (as 4₁ for instance, cf. Fig. 29), will consist of very oblong loops with a very great difference between the greatest and least distances of the electron from the nucleus, while the orbit will be a circle when the two quantum numbers are the same as for 1₁, 2₂, 3₃. Although each orbit has two quantum numbers, we often speak simply of the 1-, 2-, 3- ... _n_-quantum orbits, meaning here the orbits with the principal quantum numbers 1, 2, 3 ... _n_.