Section III., 1915 [99] Trans. R.S.C. 



On the Density of Molecules in Interstellar Space. 



By Louis Vessot King, M.A. (Cantab.), D.Sc. (McGill), F.R.S.C. 

 Associate Professor of Physics, McGill University, Montreal. 



(Read May Meeting, 1915). 



In recent years evidence has been brought forward by several 

 investigators^ indicating that light from distant stars suffers a slight 

 attenuation in travelling through interstellar space. In particular a 

 recent investigation by Jones^ assigns definite numerical values to 

 coefficients of attenuation corresponding to "photographic" and 

 "visual" light from stars of known proper motions and spectral types 

 whose magnitudes had been carefully measured by Parkhurst^ for light 

 of these wave-lengths. If, as seems reasonable, this extinction is 

 assumed to be due to attenuation by scattering in travelling through 

 a "residual" gas occupying interstellar space, we are enabled to esti- 

 mate the average density of molecules in the intervening regions, 

 following a method due originally to Larmor* for assigning an upper 

 limit to the density of matter in comets' tails. 



If we denote by K the coefficient of attenuation corresponding 

 to wave-length A, radiation of this wave-length originally of intensity 

 Eo is R=Eo e~^* reduced, after travelling a distance x, to the value 

 given by E = Eo e~^*. According to Rayleigh's Law of molecular 

 scattering K is given in terms of the refractive index n and the mole- 

 cular density of the medium n by the relation K=%7i^ (fi^—iyX'^/n. 

 The ability of Rayleigh's Law to account almost completely for the 

 attenuation of solar radiation in travelling through the earth's 

 atmosphere was first pointed out by Schuster^: a later investigation 

 by the writer^ based on the results of the Smithsonian Astrophysical 

 Observatory indicated that formulae based on this law were competent 

 to explain atmospheric extinction as well as to account quantitatively 



J Kapteyn, J. C, Astrophysical Journal, 29 (1909), pp. 46-54: 30 (1909), pp. 

 284-317 and correction p. 398. Turner, H. H., Monthly Notices Roy. Ast. Soc, 69 

 (1908), p. 61- . King, E. S., Harvard Annals, 59, No. VI., p. 179, April, 1911: 

 Harvard Annals, 76, No. I., pp. 1-10, 1913. Brown, F. S., Monthly Notices, 72 (1912), 

 p. 195- also p. 718. 



2 Jones, H. S., Monthly Notices Roy. Ast. Soc, 75 (1914), pp. 4-16. 



3 Parkhurst, J. A., "Yerkes Actinometry," Astrophysical J.; 36, (1912), p. 169-. 

 ^ Larmor, Sir J., Lectures, Cambridge, 1908. 



» Schuster, A., "Nature," July 22, 1909: "Optics," 2nd Ed., 1909, p. 329. 

 « King, L. v., Phil. Trans. Roy. Soc, 212A (1912), pp. 375-433. 



