Stars about ]3 Cygni. 335 



Pleiades period. The Pleiades scale value determinations were 

 made for ten plates, and depend altogether upon a comparison with 

 the Yale and Konigsberg heliometer observations.* The largest 

 scale value obtained (Plate 16) is 28".oi6j, the smallest (Plate 25) 

 is 28".oo66. The mean is 28".oi24; and if we regard all the sepa- 

 rate values as determinations of the same quantity with equal 

 weight, the probable error is ±0". 00071, which corresponds to 

 dbo".o25 per 1000". The actual error may, of course, exceed this 

 amount, but it seems only fair to conclude that the average uncer- 

 tainty of the final j3 Cygni places does not exceed about o". 15 on 

 account of scale value. It must be remembered that all the obser- 

 vations were taken by the same observers, and under precisely the 

 same instrumental conditions, as the Pleiades plates. 



An examination of table V. tends to show that slight differences 

 exist between the results obtained on the two observation dates of 

 a nature different from those that would arise from variation of 

 scale value. They appear to depend upon the position angle or 

 direction of measurement. The proper motion of j3 Cygni, which 

 is given by Auwers as — o".026 in R.A., and — o".o2o in declina- 

 tion would not account for them. If they depend upon a slightly 

 varying inclination of the plate-holder, due perhaps to using the 

 telescope at hour-angles of different sign, their effect would probably 

 tend to disappear from the mean results. We may then finally con- 

 clude that the Rutherford measures of the j3 Cygni stars furnish 

 a catalogue of places of an accuracy sufficiently high to allow a deter- 

 mination of possible relative proper motion at no very distant date. 

 It is to be hoped that such a determination will be undertaken. 



Four of the stars in the Rutherfurd list I have been unable to 

 identify in Argelander. These are : 



No.. 28 observed on 3 plates, 

 32 " " 1 plate, 



SS '' "2 plates, 

 41 " " 2 plates. 



I have examined the original negatives, and found the star images 

 in question quite distinct. I also photographed the region 1892, 

 April 19, and found numbers 32, ^^, and 41 on the negatives. No. 

 28, which is quite close to No. 27 on the Rutherfurd negatives, 

 also appears on my plates, but only as a sort of elongation of No. 27. 



* Ann. N. Y. Acad., vi, p. 270. 



