620 SCIENCE PROGRESS 



1 316, 1320), and a paper on a certain linear congruence by 

 H.J. van Veen (Nieuw Archie} voor Wiskunde, 1915, 11, 232). 



ASTRONOMY. By H. Spencer Jones, M.A., B.Sc, Royal Observatory, 

 Greenwich. 



Stellar Dynamics. — There is a considerable degree of probability, 

 supported by various considerations, that the light reaching 

 us from the stars has suffered a small amount of absorption 

 or scattering in its path, due to the existence of matter in 

 interstellar space. The density of this matter is, of course, 

 exceedingly small and not necessarily uniform. Recent investi- 

 gations by J. C. Kapteyn (Ap. J. xxx. 1909, p. 284) and 

 H. S. Jones (M.N., R.A.S., Ixxv. 1914, p. 4) assign a fairly 

 definite value to this absorption, and a somewhat smaller 

 value has been assigned by P. J. van Rhijn (Doctor's Disser- 

 tation, Groningen, 191 5). Its smallness may be judged from 

 the fact that in order to lose one-tenth of its original intensity, 

 light from the stars must travel through space for from five to 

 ten centuries. It would have been thought d priori that matter 

 of such tenuity would possess merely a theoretical importance, 

 but Dr. Louis Vessot King has shown in Trans. Roy. Soc. 

 Canada, Ser. Ill, ix. p. 99, 191 5, that if an absorption of even 

 this small amount is substantiated by future researches, we 

 must recast some of our fundamental conceptions. Using 

 Rayleigh's law of molecular scattering, which seems justifiable, 

 he calculates that the amount of matter necessary to cause 

 this scattering is equivalent to about 1*3 x io* hydrogen 

 molecules per cubic centimetre. The highest vacuum which 

 can be produced artificially contains a density of matter 

 greatly exceeding this, yet it means that in a cubic parsec 

 there is an amount of dust whose mass is 6,300 times the 

 Sun's mass ; in the same volume, the average mass of the 

 lucid stars is estimated at only o'02 of the Sun's mass. Thus 

 there is about 300,000 times as much dust as there is matter 

 condensed into stars. The result is truly startling, and even 

 if the scattering or absorption of light in space is considerably 

 less than present estimates, and if there are a large number of 

 dark stars, it would seem that this residual gas should be taken 

 account of in stellar dynamics, for it will exert an attraction 

 far exceeding that of the stars themselves. 



