26 Programme for obseruaiion of Stars io the ninth magnitude. 



each zone. Whenever desirable or necessary, however, one or 

 more may be observed during the progress of the zone. In the 

 selection of zero stars the peculiarities of the instrument must 

 govern. In general the mean of their right ascensions should 

 fall nearly on the mean of the zone. Where this cannot be 

 attained, the observer must decide whether he can rely more 

 upon a uniform clock rate (for the determination of right ascen- 

 sions), than upon an unchanged position of the instrument m 

 reference to the meridian. With very northerly stars the selec- 

 tion should be made more with regard to the dechnations of the 

 zero stars, as an error in the clock rate has less influence here. 

 The mean daily clock rate must not be used in the reductions, 

 but the rate resulting from the mutual comparison of the zero 

 stars for the zone. 



The individual observer must judge whether a single micro- 

 scope will give the requisite security in declination. When- 

 ever the stars to be determined are observed on fewer micro- 

 scopes than the zero stars, a thorough investigation of the reduc- 

 tion between the different systems of microscopes must be made 

 throughout each zone. Where this reduction shows large 

 changes, the use of all four microscopes is advised, and also the 

 introduction of zero stars in the progress of the zone. Changes 

 in the equatorial point may be determined also by horizontal 



Where two share in the work, so that one reads the micro- 

 scopes, and especially where only one microscope is read, the 

 influence of the warmth of the observer's body must be care- 

 fully investigated and allowed for. (Consult Bonn Beob., H, 

 p. viii). 



In the observation of double stars the position of the mean 

 is to be obtained. It is therefore advisable to extract from the 

 Positiones Mediae the double stars occurring in a zone. Should 

 a star designated there as " simplex " appear double, in case oj 

 nearly equal brightness, the mean should be observed as well 

 as the brighter one. 



An ephemeris of variable stars will be published annually in 

 the Vierteljahrschrift, to furnish information in regard to the 

 most favorable times of observing them on the meridian. 



There are many stars of the 9-0™ in the D. M. which are 

 fainter, and it is much oftener so with those marked A. The 

 observer is enjoined to spare no pains in observing thc.-e star.-, 

 and should the observation not succeo*! in the course' "T the 

 zone, note should be made so that it may W attcnipt^Ml muUn- 

 more favorable circumstances. Where variability may be ^^n/^- 

 pected because a star is invisible or excessively faint, it is a(l\'i»- 

 able to immediately inform the committee or those astrono- 

 mers engaged on variable stars, that these stars may be care- 



