ASTRONOMY. 



137 



For comparison we may cite tlic followiiii;' results obtained by otber 

 astro no mors workin^i;- with e<j[uatorials anil helionieters in Mie usual way: 



61 Cygni 



// Cassiopeao 



Polaris 



Bossel, 1840 +0". 



Auwors. 18t;:J +0 . 



Hall, 1878 +0 . 



Hall, 18H0 +0 . 



]{ess(!l -0 . 



Struve +0 . 



Liiidenan -|-0 . 



Struve and PctiTs -j-O . 



C.A.F. Peters +0 . 



•MS 

 ntil 



4()8 



2(;i 

 i-i 



•M2 

 144 

 172 

 067 



Professor Pritcbard lays stress upon the fact tliat each photographic 

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

 being paid to the unavoidable, though slight, variations of scale iu the 

 ditlerent plates, he is of opinion that iu this delicate class of work jiho- 

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



For Gl 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 c^iiparison stars. On each of fifty-three 

 nights four photographic plates were taken of /j. Cassiope», 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, would be obtained by confining the observations to about five 

 nights in each of four periods of the year indicated b^' the position of 

 the parallactic ellipse. Professor Pritchard proposes to apply this 

 method s^'stematically 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 laic of fjravitation to stellar systems. — Professor Ilall, 

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

 view of the various speculations ui)on the subject, says : " The weakness 

 of the proof that the Newtonian 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 difierent elements. - - - The 

 insufiiciency 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 the accurate determination 

 of their ai)parent orbits. The theoretical difliculty in proving the law 

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

 the probability of the existence of this law by determining more orbits 



