150.

Whereas the absorption of radiation by an oscillator takes place in a perfectly continuous way, so that the energy of the oscillator increases continuously and at a constant rate, for its emission we have, in accordance with [sect:147.] Sec. 147, the following law:

The oscillator emits in irregular intervals, subject to the laws of chance; it emits, however, only at a moment when its energy of vibration is just equal to an integral multiple $n$ of the elementary quantum $ϵ=h\nu$, and then it always emits its whole energy of vibration $nϵ$.

We may represent the whole process by the following figure in which the abscissæ represent the time $t$ and the ordinates the energy $U=nϵ+\rho ,(\rho <ϵ)\mathrm{\backslash Label}[eqn](251)\text{upshape (251)}$ 7 of a definite oscillator under consideration. The oblique parallel lines indicate the continuous increase of energy at a constant rate $\frac{dU}{dt}=\frac{d\rho }{dt}=a,\mathrm{\backslash Label}[eqn](252)\text{upshape (252)}$

which is, according to [eqn:(249)] (249), caused by absorption at a constant rate. Whenever this straight line intersects one of the parallels to the axis of abscissæ $U=ϵ$, $U=2ϵ,\dots$ emission may possibly take place, in which case the curve drops down to zero at that point and immediately begins to rise again.