190 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1924 



placed by the nucleus of the helium atom, which is four times as 

 heavy as that of hydrogen, the common center of gravity is still 

 closer to the nucleus so that the helium nucleus describes a much 

 smaller circle than does the hydrogen nucleus. This situation is re- 

 sponsible for a certain slight but accurately predictable difference in 

 the energies of the two orbits which should cause the lines produced 

 by electron jumps to these two different orbits to be slightly displaced 

 from one another. This displacement is actually found between the 

 corresponding hydrogen and helium lines, and the ratio of the mass 

 of the electron to the mass of the hydrogen atom computed from this 

 displacement agrees with other determinations of this ratio to within 

 a small fraction of a per cent. 



The third amazing quantitative success of the orbit theory came 

 when Sommerfeld showed that the Bohr orbit theory ought to 

 demand two different hydrogen orbits corresponding to the second 

 quantum state, one a circle and one an ellipse. And by applying 

 the relativity theory to the change in mass of the electron with its 

 change in speed, as it moves through the different portions of its 

 orbit, he showed that these two orbits should have slightly different 

 energies, and consequently that both the hydrogen and the helium 

 lines should be doublets. 



Now not only is this found to be the fact, but the measured 

 separation of these two doublet lines agrees precisely with the 

 predicted value, so that this again constitutes an extraordinary 

 bit of quantitative evidence for the validity of the orbit conceptions 

 underlying the computation. 



The fourth quantitative argument was introduced by Epstein 

 when he applied his extraordinary grasp of orbit theory to the exceed- 

 ingly difficult problem of computing the perturbations in electron 

 orbits and hence the change in energy of each due to exciting 

 hydrogen and helium atoms to radiate in an electrostatic field. 

 He thus predicted the whole complex character of what we call the 

 Stark effect, showing just how many new lines were to be expected 

 and where each one should fall, and then the spectroscope yielded, 

 in practically every detail, precisely the result which the Epstein 

 theory had foretold. 



The fifth quantitative success of the orbit theory is one which 

 Mr. I. S. Bowen and myself at the California Institute have just 

 brought to light. Through creating what we call " hot sparks " in 

 extreme vacuum, we have succeeded in stripping in succession one, 

 two, three, four, five, six, and seven of the valence, or outer, electrons 

 from the atoms studied. In going from lithium through beryllium, 

 boron and carbon to nitrogen, we have thus been able to play with 

 stripped atoms of all these substances. Now the stripped atoms 



