Programme for observation of Stars to the nint/i magnitude. 27 



fully attended to. A notice to the Bonn observatory would 

 render it possible to scrutinize the original papers for any error. 



Unless special circumstances prevent, the magnitude is to be 

 carefully estimated at the observation of every star. It is desir- 

 able that the participants conform their scale of magnitudes to 

 that of the D. M., unless some other scale is already m use; 

 in that case its relation to the D. M. scale must be determined. 

 (Consult Bonn Beob., Ill, p. xviii, ff.) To guard against a dif- 

 ference between observations of bnght and faint stars, observ- 

 ers are recommended to observe variable stars in diflerent stages 

 of their brightness. 



The choice of wire intervals both vertical and horizontal re- 

 quires mature deliberation. The former must be arranged ac- 

 cording to the declination ; rather wide for equatorial zones, 

 and rather narrow for northern zones. For eye and ear observ- 

 ing, wire intervals of 10« to 12« are recommended. Equidistant 

 wires and in zones near the equator intervals of a whole num- 

 ber of seconds are to be avoided. With the chronographic 

 method intervals of 2=* -5 to 8« will suffice. The choice of nar- 

 rower intervals cannot be recommended. The horizontal wires 

 should not be too close, because the fainter stars cannot be seen 

 distinctly through them. A definite precept cannot be given, 

 as the magnifying power employed is closely connected with 

 the question. The observer wdl do well to ascertain what dis- 

 tance suits him best. Bessel used at first a power of 66 tunes 

 and afterwards one of 107 times ; a third of 119 times, which 

 was perhaps more proper, was not employed on account ot its 

 small field of view, a difficulty not to be encountered m the 

 present work, as each star can be directly set upon. The dis- 

 tance of the horizontal wires was 8", which is probably rather 

 narrow. The Bonn northern zones were observed with a power 

 of 108 times, those published in vol. vi with one of loO times. 

 The horizontal wire interval at Bonn is 14", which is perhaps 

 too wide ; 10" to 12" may perhaps be more advantageous. 



The final catalogue will be reduced to the equinox of IbJoU 

 It is earnestly recommended to make the reduction of the 

 observations as soon as possible, so as not to be pressed at a 

 later period by the work. Of course a definitive reduction is 

 possible only when the accurate determinations of the zero stars 

 are completed at Pulkova, which will take several vears. Mean- 

 while the provisional places given at present wi 1 be so nearly 

 correct that only small corrections will need to be introduced. 

 This reduction will aftbrd an opportunity of jndgmg what se- 

 curity of position is arrived at and where perhaps a third ob- 

 idded. It will, it is hoped, appear from aU 



! observations that the mean difi"erence between 



) observa- 



tions of the same star will not exceed O^'l in R. A. (for u 



