May 15, 1896.] 



SCIENCE. 



721 



Stumpe, of Bonn. The results agree as well 

 as we could expect at present in fixing 

 ' the brilliant Vega ' as ' the center round 

 which the new determined apexes tend 

 loosely to group themselves.' The general 

 direction of the solar motion is thus fairly 

 well determined. The velocity of this 

 motion has not yet been accurately worked 

 out. Sixteen miles a second is given by 

 some astronomers as a probable value. 



The distances of the stars have always 

 excited the curiosity of man. During this 

 century the reiined methods for obtaining 

 reliable values have been worked out. Only 

 within the last twenty years have the most 

 accurate values been determined. The 

 solar system to our finite minds seems iso- 

 lated in space, the nearest star being so far 

 away that light traveling at the rate of 

 186,330 miles in each second of time con- 

 sumes 4.35 years in reaching the earth. 

 The parallax of a Centauri is 0."75, i. e., 

 the distance separating the earth from the 

 sun, over 90,000,000 miles, would appear 

 to the eye of an observer on a Centauri, as 

 small as -j^ of an inch appears to our eyes 

 at a distance of one mile. This nearest 

 star, a Centauri, is at the head of a list of 

 less than 60 stars whose parallaxes have 

 been determined with all the accuracy, 

 very nearly, at present possible. But the 

 laborious search for measurable stellar par- 

 allaxes has not been extensive enough 

 among the millions of stars to make us feel 

 that astronomers have determined certainly 

 even the nearest star. Perhaps it will be 

 found among some of the fainter telescopic 

 stars, or even on the photographic plates, 

 that with long exposure show us stars so 

 faint that we can never expect to see them. 

 Photography has proved itself to be a most 

 valuable aid in this investigation, and from 

 the plates specially made much more is to 

 be expected in the future. 



The telescope shows numerous cases in 

 which two stars are so close to each other 



that they can be separated only by a high 

 magnifying power. These are ' double 

 stars.' The catalogues now enumerate more 

 than 10,000 such couples, and the number 

 known to us is increasing quite rapidly. 

 One of the chief pieces of work in which 

 the largest telescopes are used is in detect- 

 ing new cases of very faint and exceedingly 

 close doubles. A careful examination has 

 revealed the fact that some 200 or more 

 cases of double stars show that the com- 

 ponents are physically connected. 



The components revolve about the com- 

 mon center of gravity of the system. When 

 one of the stars is much greater in mass 

 than the other, the second star, usually the 

 fainter, revolves about the larger one. 

 Many of these binary stars as they are 

 called are of great interest. Their times of 

 revolution range from 14 years to 1,500 

 years. The orbits are comparable with the 

 larger orbits of the solar system, some of 

 them being twice as large as that of the 

 planet Neptune, which, as you will remem- 

 ber, moves in an orbit having a radius of 

 about 2,800,000,000 of miles, and revolves 

 about our sun in 165 years nearly. 



There are cases of multiple stars. Ep- 

 silon Lyrae is a beautiful quadruple star, 

 composed of two pairs. Each pair makes a 

 slow revolution in a period of over 200 

 years. It is thought that there is evidence 

 that the two pairs revolve about the com- 

 mon center of gravity of the four stars. 



Peters found in 1851 that the apj)arent 

 irregularities in the movements of the bril- 

 liant Dog star Sirius could be fully ex- 

 plained by an orbital revolution in a period 

 of fifty years. Bessel had announced in 

 1844 that the two bright Dog stars Pro- 

 cyon and Sirius moved in seeming irregu- 

 lar paths, because of the presence of unseen 

 bodies near them. Peters thus vindicated 

 Bessel's prediction. On the 31st of Jan- 

 uary, 1862, while testing the new 18-inch 

 object glass ordered by the late Pres. Barn- 



