A. — ^MATHEMATICS AMD PHYSICS. 39 



Now this is of the same order of magnitude as the absorjjtiou of 

 ,T-rays measured in the lalwratory. I think the result is in itself of 

 some interest, that in such widely different investigations we should 

 approach the same kind of value of the opacity of matter to radiation. 

 The penetrating power of the radiation in the star is much hke that of 

 a;-rays; more than half is absorbed in a path of 20 cms. at atmospheric 

 density. Incidentally, this very high opacity explains why a star is so 

 nearly heat tight, and can store vast supplies of heat with comparatively 

 little leakage. 



So far this agrees with what might have been anticipated ; but there 

 is another conclusion which physicists would probably not have foreseen. 

 The giant series comprises stars differing widely in their densities and 

 temperatures, those at one end of the series being on the average 

 about ten times hotter throughout than those at the other end. By 

 the present investigation we can compare directly the opacity of the 

 hottest stars with that of the coolest stars. The rather surprising 

 result emerges that the opacity is the same for all; at any rate there 

 is no difference large enough for us to detect. There seems no room 

 for doubt that at these high temperatures the absorption-coefficient is 

 approaching a limiting value, so that over a wide range it remains 

 practically constant. With regard to this constancy, it is to be noted 

 that the temperature is concerned twice over : it determines the character 

 and wave-length of the radiation to be absorbed, as well as the physical 

 condition of the material which is absorbing. From the experimental 

 knowledge of a;-rays we should have expected the absorption to vary 

 very rapidly with the wave length, and therefore with the temperature. 

 It is surprising, therefore, to find a nearly constant value. 



The result becomes a little less mysterious when we consider more 

 closely the nature of absorption. Absorption is not a continuous 

 pixxiess, and after an atom has absorbed its quantum it is put out of 

 action for a time until it can recover its original state. We know 

 very little of what determines the rate of recovery of the atom, but it 

 seems clear that there is a limit to the amount of absorption that can 

 be performed by an atom in a given time. When that limit is reached 

 no increase in the intensity of the incident radiation will lead to any 

 more absorption. There is in fact a saturation effect. In the 

 laboratory experiments the radiation used is extremely weak ; the atom 

 is practically never caught unprepared, and the absorption is propor- 

 tional to the incident radiation. But in the stars the radiation is very 

 intense and the saturation effect comes in. 



Even, granting that the problem of absorption in the stars involves 

 this saturation effect, which does not affect laboratory experiments, it 

 is not very easy to understand theoretically how the various conditions 

 combine to give a constant absorption-coefficient independent of tem- 

 perature and wave-length. But the astronomical results seem con- 

 clusive. Perhaps the most hopeful suggestion is one made to me a 

 few years ago by C. G. Barkla. He suggested that the opacity of 

 the stars may depend mainly on scattering ratlier than on true atomic 

 absorption. In that case the constancy has a simple explanation, for 

 it is known that the coefficient of scattering (unlike true absorption) 



