394 Dr. N. Bolir on the Quantum Theory of 



thickness in an arc. The area of each curve is a function of 

 the number of vibrating centres in the layer and of the 

 amplitude of vibration, while its shape and the position of 

 its maximum ordinate are governed by the combined effect 

 of pressure and the density of the material responsible for 

 the spectrum line; since the latter factor is more important 

 in the central zones than in the outer strata, we have the 

 differences in the shapes of the curves a, b, c in the diagram, 

 in which a represents the effect of the central core and b and 

 c the effect of two of the outer envelopes. The position of 

 the maximum ordinate of the resultant composite line (dotted) 

 does not correspond to that of the light emitted by the core, 

 but to some intermediate layer b in the diagram. Further 

 differences in the shapes of a, b, and c are occasioned by the 

 scattering of light by the layers outside the one considered. 

 Had the temperature and density gradients and the scattering- 

 power been different, it is obvious that the resultant curve 

 would have differed both in its shape and in the position of 

 its maximum ordinate. We consequently see that bright 

 lines as well as reversals are likely to show their dependence 

 upon the density gradients and the scattering power in the 

 various layers in the arc. It becomes increasingly evident 

 that the anomalies which have been encountered in estimating 

 the pressure of the solar atmosphere arise from the fact that 

 the density and temperature gradients in the sun and in the 

 sources of light with which his spectrum has been compared, 

 are different; it is not strictly legitimate, for example, to 

 compare solar displacements with those found when an arc 

 is subjected to external pressure, because the two sources are 

 radically dissimilar in structure. 



Physical Laboratory, 



University College, Reading, 

 May 19, 1915. 



XLII. On the Quantum Theory of Radiation and the Struc- 

 ture of the Atom. By N. Bohr, Dr. phil. Copenhagen ; 

 p. t. Reader in Mathematical Physics at the University of 



Manchester 



IN a series of papers in this periodical f the present writer 

 has attempted to give the outlines of a theory of the 

 constitution of atoms and molecules by help of a certain 



* Communicated bv Sir Ernest Rutherford, F.R.S. 

 t Phil. Mag. xxvi* pp. 1, 476, 857 (1913) and xxvii. p. 506 (1914). 

 These papers will be referred to as I., II., III., & IV. respectively. 



