652 Professor A. Fowler [March 31 



accordance with Rydberg's theory. They seemed to have a close 

 numerical relation to the hydrogen series, and no relation whatever to 

 the lines of helium. Moreover, there was evidence that helium aided 

 the development of the ultra-violet lines of hydrogen, and it was 

 reasonable to suppose that it might also facilitate the production of the 

 lines under consideration. Nevertheless, the idea that the new lines 

 were due to hydrogen was not wholly satisfactory, as the occurrence 

 of lines not anticipated by Rydberg broke the analogy with the series 

 of other elements. It therefore seemed desirable to seek for other 

 examples of snch series, in order to test the value of the numerical 

 evidence on which the assignment of the new lines to hydrogen was 

 still so largely dependent. 



A series of the same character as the 4686 series, consisting of 

 spark-lines of magnesium, had already been discovered, when fresh 

 light was thrown on the subject from another quarter. Not from 

 the stars this time, but from mathematical physics. In July, 1913, 

 Dr. Bohr published the first instalment of his now famous theoretical 

 investigation of the constitution of the atom in relation to spectral 

 emission, and the lines of the 4686 and Pickering series became of 

 great importance in this connexion. 



Beginning with the Rutherford model of the atom, and intro- 

 ducing Planck's conception of the qnantum of energy. Dr. Bohr 

 quantitatively derived Balmer's formula for the hydrogen spectrum, 

 and in such a form as to exclude the 4686 and Pickering series 

 entirely. On this theory, the atom of hydrogen is supposed to 

 consist of a relatively massive, but very minute, nucleus which is 

 positively charged, and a single electron in orbital motion around it. 

 The electron may revolve in one of certain specified orbits, subject to 

 ordinary dynamical laws, exactly like a planet in motion around the 

 sun, and in these so-called " stationary states " it radiates no light. 

 When under the influence of electric discharge, we are to suppose 

 til at the electron is removed from the nucleus, and that on its return 

 it jumps from one stationary state to another ; in so jumping it 

 radiates definite amounts of energy which determine the frequency, 

 and therefore the positions, of the spectral lines. At any instant a 

 given atom only contributes to one line of the series, but the sum- 

 mation of the radiations from a large number of atoms accounts for 

 the whole series. 



The helium atom is regarded as consisting of a doubly-charged 

 nucleus of four times the mass of the hydrogen atom, and two 

 electrons revolving round it. Moderate discharges are only supposed 

 capable of detaching one of the electrons, and the ordinary spectrum 

 of helium is emitted when this returns to the system ; the theory 

 of this spectrum, however, remains incomplete. Under the action 

 of sufficiently strong discharges, both electrons are considered to be 

 removed from the nucleus. The theory is simpler in this case, and 

 Dr. Bohr derived a formula for the series of lines which should be 



