618 Proceedings of the Royal Irish Aeadony. 



a clock-correction derived from four or five standard stars, the dif- 

 ferent estimation of bright and faint stars will cause a constant 

 fault of all the right ascensions of the zone. 



In order to explain the deviations of ITyren's constant of pre- 

 cession from those of Bessel and Struve,* we might seek the origin 

 of the undeniable difference: Schjellerup -Bessel -AVeisse = - 0'-095 

 in such a different estimation in one of the two observers, which 

 would be more likely to have existed in Bessel's case, as Schjellerup's 

 right ascensions can hardly be affected with any large constant error, 

 considering their excellent agreement with those of the Gottingen 

 zones,! which are founded on the same standard stars. Before leav- 

 ing the Copenhagen Observatory, I tried to find whether the above 

 difference could be explained in this way. From the original obser- 

 vations of Schjellerup's and the " Konigsberger Beobachtungen," I 

 wished to derive differences of right ascension between zone stars ap- 

 pearing in both catalogues, and standard stars, which the two observers 

 would have had in common, if such had been observed together on one 

 night in Konigsberg, as well as in Copenhagen. But it appeared 

 soon that any certain and reliable result could not be obtained in this 

 way, as Schjellerup nearly always had used other standard stars (one 

 or two classes fainter) than Bessel, so that only a very small number 

 of zone stars occurring in both catalogues, could be compared with 

 one and the same standard star. However, the difference between 

 the right ascensions of the two catalogues has probably another and a 

 deeper origin, as neither of the two constants of precession used now-a- 

 days seems derived in quite a satisfactory way.;]; Besides, according to 

 Argelander's comparison between Bessel's zones and Struve's " Posi- 

 tiones mediae," it does not seem likely that the former are affected 

 with any constant error as the one suggested above. § 



It cannot, however, be doubted that a different estimation of 

 transits of bright and faint stars may exist, || and an example has been 

 found by Argelander by comparing Santini's fifth catalogue, contain- 

 ing positions of stars between 90° and 93° JST. P. D., from observations 



* Determination du coefficient constant cle la precession au moyen d'etoiles 

 de faible eclat. Par M. Nyren (Bulletin de rAcademie Imp. des Sciences de 

 St. Petersbourg, 1869). 



t Gottingen- Scbiellenip =- 0^-005 (R. Copeland und C. Borgen: Mittlere 

 Oerter der in den Zonen — 0" und — 1° enthaltenen Sterne : Gottingen, 1869 



X Bessel has, for instance, used Lindenau's constant of nutation, wliicli is 

 0"-25 too small, while 0. Struve has founded his researches on his father's deter- 

 mination of the relation between the mean distances of stars from different classes. 



§ Viei'teljahrsschrift der Astron. Gesellschaft, vii., p. 17. 



II Bessel has already suspected this (Briefwechsel zwischen Olhers und Bessel, ii., 

 p. S58). 



