generation and transformation of light are what they must be if light consisted of such energy quanta. In the following we will address that question.

## Stokes' Rule

Let monochromatic light be transformed by photoluminence into light of another frequency, and let it be assumed that according to the result just obtained the generating as well as the generated light consists of energy quanta of magnitude (*R*/*N*)*βν*, where *ν* is the corresponding frequency. The transformation process can then be interpreted as follows. Each generating energy quantum of frequency *ν*_{1} is absorbed and generates—at least with sufficiently small density of the generating energy quanta—by itself a light quantum of frequency *ν*_{2}; possibly other light quanta of frequency *ν*_{3}, *ν*_{4} etc. as well as other form of energy (e.g heat) can be generated simultaneously. Through which intermedia processes the final result comes about is immaterial. If the photoluminescing substance isn't a continuous source of energy it follows from the energy principle that the energy of the generated energy quanta are not larger than the generating light quanta; therefore the following relation must hold:

or

As is well known this is Stokes' rule.

Especially noteworthy is that with weak illumination the amount of generated light must, other circumstances being equal, be proportional to the amount of exciting light, since every incident energy quantum will cause one elementary process of the above indicated kind, independent of the action of other exciting energy quanta. In particular there will be no lower limit of the intensity of the exciting light below which the light would be incapable of exciting light.