I912-] . DEPTH OF THE MILKY WAY. 13 



.t- = 2,512, corresponding to an enfeeblement of 17 magnitudes, from 

 mere increase of distance alone, the extinction of light varies from 

 almost total loss, with Struve's coefficient, to no loss whatever, on 

 Herschel's tacit hypothesis of zero extinction. This latter view, 

 however, certainly is extreme, and probably all modern astronomers 

 agree that there is extinction of light due to cosmical dust in space. 

 A hazy background of dust is shown on the photographs of the 

 Milky Way and other portions of the sky, and proved to pervade 

 the solar system by the universal prevalence of meteors. 



Since, however, both comets and nebulae are found to be extremely 

 tenuous bodies, and observed to transmit the light of stars with but 

 excessively slight enfeeblement, it is obvious that the general extinc- 

 tion will be much smaller still, but yet appreciable. I have therefore 

 adopted a coefficient of 0.999, about one hundredth larger than 

 Struve's, as best harmonizing all known phenomena. This value, 

 it is true, is much nearer to Herschel's than to Struve's coefficient, 

 yet it admits an extinction of light which becomes appreciable at 

 great distances, while for moderate distances it is nearly insensible ; 

 and I believe this to correspond closely with all the known facts of 

 the sidereal universe. 



An enfeeblement of one twelfth at a distance appropriate to 

 stars 17 magnitudes fainter, could easily be compensated for by a 

 corresponding abnormal brilliancy of the remotest stars, which on 

 several grounds seems to be highly probable. Thus our procedure 

 involves no extravagant assumptions as to the great brightness of the 

 most distant stars, or as to large extinction of light, while on the 

 other hand it avoids Herschel's tacit hypothesis of zero extinction, 

 which certainly is unjustifiable.^ 



^ In an important paper read to the Bavarian Academy of Sciences, June 

 10, 191 1, p. 459, Professor H. von Seeliger likewise reaches the conclusion 

 that the absorption is very small, amounting to 0.34 of a magnitude at 780 

 times the distance of Sirius, which Seeliger takes for the border of the 

 sidereal system. 



