SOME CONTEMPORARY ADVANCES IN PHYSICS— X 97 



kind; or by absorbing radiation it may pass from one Stationary 

 State to another of higher energy; or it may pass spontaneously 

 from one Stationary State to another of lower energy. In this last 

 case, it emits radiation of which the frequency v is related to the 

 difference A ^between the energy-values of the initial Stationary State 

 and the final one by the equation 



v=AU/h, (h = 6.56. 10-" erg/sec.) 



The same equation governs the last case but one, in which it con- 

 nects the frequency of the absorbed radiation wnth the energy-differ- 

 ence between the two Stationary States from and into which the atom 

 passes. On this equation is founded the method of analyzing spectra 

 which is the most accurate and most widely applicable method of 

 determining the energy-values of Stationary States. The other 

 ways in which atoms are caused to pass from one State to another 

 lead to methods of determining these states, which are almost useless 

 for accurate measurements, but invaluable as controls. 



The energy-values of the various Stationary States of an atom 

 are interrelated, and sometimes it is possible to express a long sequence 

 of them by means of a simple or a not very complicated formula. 

 There are also interrelations between the distinctive energy-values 

 for different elements; and this statement is meant to apply also to 

 atoms from which electrons, one or more, have been detached, which 

 should be considered as distinct though not as stable elements. There 

 are unmistakable numerical relations among the Stationary States 

 which come into being when atoms are subjected to electric or to 

 magnetic fields. Finally there is the important principle that the 

 spontaneous transitions between various pairs of states, which re- 

 sult in spectrum lines, do not occur equally often; and yet the rela- 

 tive oftenness or seldomness of their occurrence is itself regulated 

 by laws. One finds instances in which transitions from a state A 

 to a state Bi are just twice as common as transitions from ^ to a state 

 So close to Bi. One finds instances in which transitions from a state 

 A to a state B do not occur at all under usual conditions and an atom 

 in state A cannot get into state B without touching at some state C 

 from which A and B are both accessible. It is possible so to arrange 

 the Stationary States of an atom that by looking at the situations 

 of any two states in the arrangement, one can tell immediately whether 

 direct transitions between them do occur or do not; and this arrange- 

 ment is found to be suited to, even to be demanded by the numerical 

 interrelations to which I alluded above. Upon these facts the classi- 



