ASTRONOMY. 



137 



For comparison we may cite tlie followiuj? results obtained by other 

 astronomers working witli equatorials and lieliometers in the usual way: 



61 Cygni.... 



// Cassiopeso 

 Polaris ... 



Bessel, 1840 +0". 348 



Auwers, 18()3 +0 .i>M 



Ball, 1878 - +0 . 4(i8 



Hall, 1880 +0 .'2(il 



Bessel -0 .VJ 



Strnve +0 . :y-> 



Liuilenau -\-0 . 144 



Stru ve and Peters -j-0 . 1 72 



C. A. F. Peters +0 .067 



Professor Pritchard lays stress upon the fact that each i)hotographic 

 plate must be considered as carrying its own scale; and, due regard 

 beiug paid to the unavoidable, though slight, variations of scale in the 

 different plates, he is of opinion that in this delicate class of work pho- 

 tography will give as accurate results as any other known method. 



For 61 Cygni the value given rests upon the measurement of three 

 hundred and thirty plates, taken upon eighty-nine nights ; each com- 

 ponent was referred to four comparison stars. On each of fifty-three 

 nights four photographic plates were taken of p. CassiopciC, the ex- 

 posures varying from five to ten minutes. Experiments seemed to indi- 

 cate, however, that sufficiently approximate results, with a great saving 

 of labor, wouhl be obtained by confining the observations to about five 

 nights in each of four periods of the year indicated by the position of 

 the parallactic ellipse. Professor Pritchard proposes to apply this 

 method systematically to all those stars between magnitudes one and 

 one-half and two and one-half which attain at Oxford a suitable alti- 

 tude ; and he hopes to obtain good results for all of these stars that have 

 a parallax not less than the thirtieth of a second of arc. 



DOUBLE AND MULTIPLE STARS. 



Extension of the law of gravitation to stellar systems. — Professor Hall, 

 in a discussion of this question in the Astronomical Journal, after a re- 

 view of the various speculations upon the subject, says : " The weakness 

 of the proof that the ISTewtonian law governs the motions of double 

 stars arises from two sources. In the first place, the errors of obser- 

 vation have a large ratio to the quantities measured. This condition 

 makes it difficult to compute the orbits with much accuracy, or we may 

 satisfy the observations with very different elements. - - - The 

 insufficiency in the data can only be removed by further observation. 

 Since there is no theoretical difficulty in the way, the continuation of 

 the observations of double stars and the improvement of methods of ob- 

 servation will, in time, give the means for tlic accurate determination 

 of their apparent orbits. The theoretical difliculty in proving the law 

 of Newton ftn- double stars can not be overcome. But we can increase 

 the probability of the existence of this law l)y determining more orbits 



