40 SECTIONAL ADDRESSES. 



approaches a definite constant value for radiation of short wave-length. 

 The value, moreover, is independent of the material. Further, scat- 

 tering is a continuous process, and there is no likelihood of any 

 saturation effect; thus for very intense streams of radiation its value is 

 maintained, whilst the true absorption may sink to comparative 

 insignificance. The difficulty in this suggestion is a numerical dis- 

 crepancy between the known theoretical scattering and the values 

 already given as deduced from the stars. The theoretical coefficient 

 is only 0"2 compared with the observed value 10 to 130. Bai'kla further 

 pointed out that the waves here concerned are not short enough to give 

 the ideal coefficient ; they would be scattered more powerfully, because 

 under their influence the electrons in any atom would all vibrate in the 

 same phase instead of haphazard phases. This might help to bridge 

 the gap, but not sufficiently. It must be remembered that many of the 

 electrons have broken loose from the atom and do not contribute to the 

 increase.-' Making all allowances for uncertainties in the data, it seems 

 clear that the astronomical opacity is definitely higher than the theoretical 

 scattering. Very i-ecently, however, a new possibility has opened up 

 which may possibly effect a reconciliation. Later in the address I shall 

 refer to it again. 



Astronomers must watch with deep interest the investigations of 

 these short waves, which are being pm'sued in the laboratory, as well 

 as the study of the conditions of ionisation both by experimental and 

 theoretical physics, and I am glad of this opportunity of bringing before 

 those who deal with these problems the astronomical bearing of their 

 work. 



I can only allude very briefly to the purely astronomical results 

 which follow from this investigation;* it is hei'e that the best oppor- 

 tunity occurs for checking the theory by comparison with observation, 

 and for finding out in what respects it may be deficient. Unfortunately, 

 the observational data are generally not very precise, and the test is not 

 so stringent as we could wish. It turns out that (the opacity being 

 constant) the total radiation of a giant star should be a function of its 

 mass only, independent of its temperature or state of diffuseness. The 

 total radiation (which is measured roughly by the luminosity) of any 

 one star thus remains constant during the whole giant stage of its 

 history. This agrees with the fundamental feature, pointed out by 

 Russell in introducing the giant and dwarf hypothesis, that giant stars 

 of eveiy spectral type have nearly the same luminosity. From the 

 range of luminosity of these stars it is now possible to find their range 

 of mass. The masses are remarkably alike — a fact already suggested 

 by work on double stars. Limits of mass in the ratio 3 : 1 would cover 

 the great majority of the giant stars. Somewhat tentatively we are able 

 to extend the investigation to dwarf stars, taking account of the 



' E.g., for iron non-ionised the theoretical scattering is 5.2, against an 

 astronomical value 120. If 16 electrons (2 rings) are broken off the theoretical 

 coefficient is 0.9 against an astronomical value 35. For different assumptions 

 as to ionisation the values chase one another^ but cannot be brought within 

 reasonable range. 



* Monthly Notices, vol. 77, pp. 16, 596; vol. 79, p. 2. 



