SOME CONTEMPORARY ADVANCES IN PHYSICS-X 137 



Absorption of an X-ray quantum by an atom, then, results in a 

 transition of the atom from its normal state to one of the "X-ray 

 Stationary States." Emission of an X-ray quantum by an atom 

 results from a transition of the atom from one into another of its 

 "X-ray Stationary States"— a transition which begins in a condition 

 in which the atom lacks one electron, and ends in another condition 

 in which the atom lacks one electron. To take instances: radiation 

 of an adequate frequency falling upon an atom in its normal state 

 may put it over into an X-ray Stationary State known as the Lu state, 

 an electron being extruded. Radiation of an adequate frequency 

 (a higher frequency will be required) falling upon a similar atom in 

 its normal state may put it over into another X-ray Stationary State 

 of higher energy, known as the K state, an electron being extruded. 

 The atom in the K state may then spontaneously pass over into the Lu 

 state, emitting a radiation belonging to the X-ray emission-spectrum, 

 its frequency being l/h times the energy-difference between the K 

 state and the Lu state. Later the atom may pass into still another 

 state, such as the Mi state, by emitting radiation of some frequency 

 v'\ the energy of the Mi state is therefore less than that of the Lu 

 state by the amount A/; calculating it thus, and then applying to nor- 

 mal atoms a stream of electrons or of quanta having energy just 

 adequate to put them over into this ilfj state, we find that this 

 efifect is duly produced. 



Thus there is a thoroughgoing analogy between the genesis of 

 optical spectra by transitions between the optical Stationary States, 

 and the genesis of X-ray spectra by transitions between the X-ray 

 Stationary States. The differences between the two kinds of spectra 

 seem all to derive from the one fundamental difference between the 

 two kinds of Stationary States; the former do not involve the absence 

 of an electron from an atom, the latter do. In the optical region, 

 for instance, we find that an atom in the normal state cannot be put 

 into a particular excited state by any radiation except one of just 

 the right frequency Vo for which hvo is equal to the energy-difference 

 between the normal state and the excited state in question. In 

 the X-ray region, we find that an atom can be put into the X-state 

 (for instance) by any radiation of frequency equal to or exceeding 

 that critical frequency Vo for which hvo is equal to the energy-difference 

 between the normal state and the K state. This difference in be- 

 havior occurs because in the former case a quantum of radiation 

 having frequency v exceeding vo would have no place to put the left- 

 over energy h{v-Vo), whereas in the latter case this extra energy 

 can be and is delivered over to the extracted electron as kinetic 



