472 Sir Oliver J. Lodge [Feb. 28, 



so long as the orbit is circular and the speed therefore constant. 

 There is a recognised difference of the same sort in connection with 

 v^arjing inertia ; its value is not affected by trans^'erse accelera- 

 tion, with the speed left constant, but it is affected by longitudinal 

 acceleration, which alters the speed. 



So I am in hopes that it may be found that this latter or speed- 

 acceleration is what is responsible for radiation, and that mere 

 curvature at constant speed in a circular orbit need not radiate at all. 

 Provided always that the superposition of an external alternating 

 field of the right frequency may cause absorption. Many of the 

 difficulties connected with the stability of the astronomical atom 

 w^ould be evaded if the theory of radiation could be modified in this 

 way ; and the excitation of characteristic radiation by almost any kind 

 of perturbation of the orbit would be intelligible. 



Speculations on Radiation and Atomic Structure. 



Bohr's remarkable theory of atomic structure does not pretend to 

 be strictly dynamical ; it is partly empirical, being based on the 

 discontinuity signalised by Planck's constant, but it is very brilUant, 

 and extensively justifies itself by agreement with facts. 



His expression for the frequency of radiation emitted by any 

 element is virtually, to a fair approximation — 



n 



27r ^me^ /E\2 / 1 _^^^ 



where — is Moseley's atomic number, N the number of unbalanced 



charges in the nucleus or the number of electrons in the atom, and 

 w^here p and q are integers of which p changes from series to series, 

 while the lines in each series are sjiven by the mutations of q. For 

 heavy atoms the E in the above formula should be E minus a 

 geometrical function of all the other electrons inside the radiating 

 orbit, becau;5e they will affect the central attracting force. In this 

 w^ay outstanding discrepancies may plausibly be explained. But the 

 remarkable thing is that the formula gives the frequencies not merely 

 relatively but absolutely. For if the experimental values otherwise 

 obtained for^, m and h are inserted, the constant outside the brackets, 

 called Rydberg's constant, w^iich is spectroscopically determined and 

 know^n to be the same for all elements, comes out right. A very 

 notable fact ! 



The above expression for spectral lines not only agrees with the 

 Rydberg-Balmer known spectroscopic series, and with the kind of 

 formula given ])y many pioneer workers, but has been able to predict 

 other series which have been subsequently observed. It also accounts 

 for many extra-low-frequency lines which though not obtainable in 

 the laboratory are observable astronomically, by suggesting that 



