This reciprocity, which may be considered as a direct consequence of the Bohr postulates, agrees with what has been said (cf. p. 50) about the correspondence between the lines in the line spectrum of an element and the dark absorption lines of that element—_e.g._, the Fraunhofer lines in the solar spectrum. Let us examine, as an example, the yellow sodium line, the D-line. Light with the corresponding frequency, 526 × 10¹² vibrations per second, is emitted by a sodium atom, when the loosest bound electron goes over from a stationary orbit with quantum numbers 3₂ to the orbit 3₁, which belongs to the normal state of the sodium atom. The transition in the opposite direction, 3₁ to 3₂, can take place under absorption of radiation only when in the light from some other source of light, which passes the sodium atoms, there are found rays with the frequency 526 × 10¹². Even if there is present radiation energy with some other frequency, the sodium atoms take no notice of this energy; they absorb only rays with the frequency stated, and every time an atom absorbs energy from a ray the energy taken is always an energy quantum of the magnitude _h_ν, _i.e._ about 6·54 × 10⁻²⁷ × 526 × 10¹² = 3·44 × 10⁻¹² ergs (1 erg is the unit of energy used in the determination of _h_). When there are present a large number of sodium atoms (as, for instance, in the previously mentioned common salt flame), the transition 3₁ to 3₂ can take place in some atoms, the transition 3₂ to 3₁ in others; therefore, at the same time there can be absorption and radiation of the light in question. Whether absorption or radiation at any given time has the upper hand depends upon various conditions (temperature, etc.). For the sake of simplicity we have here tacitly understood that there can be but one definite transition (from the normal state) corresponding to the assumption that the sodium spectrum had no other lines than the D-line. In reality this is not the case, and there can equally occur absorption of rays with larger frequencies belonging to other spectral lines in the sodium atom and corresponding to other possible transitions between stationary states in the sodium atom. If the temperature of the sodium vapour is sufficiently low, in which case almost all the atoms are in the normal state, it is evident that in the absorption only those lines will appear which correspond to transitions from the normal state, and which therefore form only a part of all the lines of the sodium spectrum. We thus obtain an explanation of the previously enigmatical circumstance that not all spectral lines which can appear in emission will be found in absorption. At the same time we get, in absorption experiments, valuable information about the structure of the atom beyond what the observations in the emission spectra are able to give.