424 BELL SYSTEM TECHNICAL JOURSAL 



energy; on the right, they are measureci from the state of ilii- idiii/cd 

 atom (which is the more common practice;". 



R 2. Bohr's Inter pretalion of Spectra 



in 1!I12. ilie e\icience to wiiich the foregoing section is de\()ted was 

 still eniireh' undiscovered, the Stationary States were unknown. 

 That evidence was sought and found because N'icls Bohr had divined 

 the Stationary States in de\eloping a new and brilliant interpretation 

 of spectra. Until then, all physicists had wished to interpret the fre- 

 quencies forming the spectrum of an atom as the natural resonance- 

 frequencies of an elastic system. Bohr supplanted this idea with 

 an idea of his own, one of the most no\el, fecund and potent in all 

 the long evolution of physics. Several of the ideas incorporated in 

 the contemporary atom-model are due to Bohr; among them all this 

 is I In- [jrimary and fimdamental one, and certainly the most secure. 



Consider the spectrum of hydrogen. In the visible region, this 

 spectrum consists of a "line-scries" — that is to say, a procession of 

 lines converging upon a limit, falling at intervals ever narrower and 

 narrower, these intervals so smoothly diminishing that they bear 

 witness lo m common character and a mutual origin of all the lines. 



E 



I 



■ 



Fig. 



. n( liius ill I lie hydnigin s|>( el i urn. 

 Koote & Mohler, "Origin of Spectra") 



rli-s, hdrn 



This line-series is shown in I'ig. 2. Not only to the eye is it of a 



wonderful regularit\-; the fre(|uencies of its consecutive lines are bound 



together in a simple numerical law. They are equal successively to 



vii^-R/V; vii„-R/A^, vii„-R/r>\ viir,-R/Q>-, etc. (1) 



" This method of l<K-atiiiK stationary states by observing transfers of energy from 

 electrons at atoms is called I he melhoil of inelastic impacts; for the impacts of electrons 

 against atoms are elastic (by definition) so long as there is no transfer of energy 

 into the internal economy of the atom, and are inelastic when such transfers occur. 

 When an atom returns into its normal state from an e.\cited state, it usually emits 

 radiation; hence a method for detecting the first commencement of radiation is 

 usually (|)erhaps not always) equivalent to a method for detecting the first com- 

 mencement of inelastic impacts. As it is generally easier lo set up apparatus for 

 iletecting radiation than to seek evidence for elastic impacts, tlirect ol)scrvations 

 upon Ihesi' last are not so abundant. as they should be. Nobody really knows how 

 many stationary states mi^;ht be discovered by the method of inelastic impacts, 

 although Francic and Kinsporn detected over a dozen for mercury (of which those 

 given in Table II are some). In fact they detected more than could conveniently 

 be a.scrilH'd to mercury atoms, so that it was necessary to attribute some of them 

 to molecules. 



