ETHER AND GRAVITATIONAL MATTER. 



229 



seen as a star of something between the first and second magnitude. 

 For a moment suppose each of our 1,000,000.1 km) suns, while of the 

 same mass as our own sun. to have just such brightness as to make it 

 a star of the first magnitude at distance corresponding to parallax 

 1"'0. The brightness at distance r (5) corresponding to parallax 

 0"*001 would be one one-millionth of this, and the most distant of our 

 assumed stars would be visible through powerful telescopes as stars of 

 the sixteenth magnitude. Newcomb (Popular Astronomy, 1883. p. 

 424) estimated between 30,000,000 and 50,000,000 as the number of 

 stars visible in modern telescopes. Young (General Astronomy, p. 

 448) goes beyond this reckoning and estimates at 100,000,000 the total 

 number of stars visible through the Lick telescope. This is only the 

 tenth of our assumed number. It is nevertheless probable enough 

 that there may be as many as 1,000,000,000 stars within the distance r 

 (5), but many of them may be extinct and dark, and nine-tenths of 

 them, though not all dark, may be not bright enough to be seen by us 

 at their actual distances. 



Sec. 21. I need scarcely repeat that our assumption of equable dis- 

 tribution is perfectly arbitrary. How far from being like the truth is 

 illustrated by Herschel's view of the form of the universe as shown in 

 Newcomb's Popular Astronomy, page 469. It is quite certain that the 

 real visible stars within the distance /' (5) from us are very much more 

 crowded in some parts of the whole sphere than in others. It is also 

 certain that instead of being all equally luminous, as we have taken 

 them, they differ largely in this respect from one another. It is also 

 certain that the masses of some are much greater than the masses of 

 others, as will be seen from the following table, which has been com- 

 piled for me by Professor Becker from Andre's Traite d'Astronomie 

 Stellaire, showing the sums of the masses of the components of some 

 double stars, and the data from which these have been determined. 



a Centauri... 



61 Cygni 



Sirius 



Proeyon 



o- Eridani . . . 

 7j Cassiopeia- 

 p Ophiuchi . . 



One-half major axis- 



Parallax. 



0.75 

 .44 



.39 

 .27 

 .19 



.15 

 .15 



In 

 seconds. 



18.17 

 29. 48 

 8.31 

 5.84 

 5. 72 

 8.20 

 4.60 



In terms of 



semimajor 



axis of 



earth's 



orbit. 



Period in 

 years. 



783 

 52 

 40 

 176 

 190 

 88 



M+M' 

 in units 

 of the 

 sun's 

 mass. 



2.0 

 0.5 

 3.2 

 6.3 

 0.9 

 4.3 

 3.6 



■y Virginis 

 V Leonis . . 



. 05 

 . 02 



3.99 

 1.98 



79t> 



102b 



194 



407 



15 

 6.5 



"Parallax calculated from dynamical determinations of ratio of semimajor axis of double star's 

 orbit to semimajor axis of earth's orbit. 

 b From spectroscopic observations by Belopolsky of Poulcowa, combined with elements of orbit. 



