This selective action of the atom on radiation is put in evidence in a variety of ways; it is perhaps most simply shewn in the spectra of the sun and stars. Dark lines similar to those which Fraunhofer observed in the solar spectrum are observed in the spectra of practically all stars (see Plate VIII, p. 51), and we can now understand why this must be. Light of every possible wave-length streams out from the hot interior of a star, and bombards the atoms which form its atmosphere. Each atom drinks up that radiation which is of precisely the right wave-length for it, but has no interaction of any kind with the rest, so that the radiation which is finally emitted from the star is deficient in just the particular wave-lengths which suit the atoms. Thus the star shews an _absorption spectrum_ of fine lines. The positions of these lines in the spectrum shew what types of radiation the stellar atoms have swallowed, and so enable us to identify the atoms from our laboratory knowledge of the tastes of different kinds of atoms for radiation. But what ultimately decides which types of radiation an atom will swallow, and which it will reject? Planck had already supposed that radiation of each wave-length has associated with it a certain amount of energy, called the “quantum,” which depends on the wave-length and on nothing else. The quantum is supposed to be proportional to the “frequency” (p. 115), or number of vibrations of the radiation per second[12], and so is _inversely_ proportional to the wave-length of the radiation—the shorter the wave-length, the greater the energy of the quantum, and conversely. Red light has feeble quanta, violet light has energetic quanta, and so on. [12] To be precise, if _v_ is the frequency of the radiation, its quantum of energy is _h__v_, where _h_ is a universal constant of nature, known as Planck’s constant. This constant is of the physical nature of energy multiplied by time; its numerical value is: 6·55 × 10⁻²⁷ ergs × seconds. Einstein now supposes that radiation of a given type can effect an atomic or molecular change, only if the energy needed for the change is precisely equal to that of a single quantum of the radiation. This is commonly known as Einstein’s law; it determines the precise type of radiation needed to work any atomic or molecular penny-in-the-slot mechanism[13]. [13] In the form of an equation: _E_₁ - _E_₂ = _h_ν, where _E_₁, _E_₂ are the energies of the material system before and after the change, ν is the frequency of the radiation, and _h_ is Planck’s constant already specified.