Group of the Pleiades. 



273 



Table of Zero Corrections in Position Angle. 



Plate. 



Correction. 



16 East 



+9' 42" 



16 West 



+9 43 



17 East 



+9 47 



17 West 



+9 48 



18 East 



+7 53 



18 West 



+7 54 



19 East 



+4 40 



19 West 



+4 43 



20 East 



+6 32 



20 West 



+6 34 



21 East 



+7 31 



21 West 



+7 32 



22 East 



+6 46 



22 West 



+6 47 



23 East 



+6 50 



23 West 



+6 51 



24 East 



+6 13 



24 West 



+6 16 



25 East 



+8 15 



25 West 



+8 16 



EMPIRICAL CORRECTIONS. 



When we compare the various values of any position angle with 

 the mean of all, we find that certain systematic errors seem to attach 

 to each plate. For instance, all the position angles of some particu- 

 lar plate will exceed their mean values. This can only be ascribed 

 to a systematic error in the zero reading of that plate. In fact, it 

 would be perfectly proper to subtract from all the angles of any 

 plate a constant quantity equal to the mean of the excesses of the 

 various angles of that plate over their respective mean values. We 

 might even derive such a correction from a selected number of 

 special stars whose magnitudes were such as to ensure good photo- 

 graphic images.* But I have preferred not to do this for the fol- 

 lowing reasons : 



Let n = number of plates. 



s = number of stars on each plate. 



* This process was employed by Gould. Memoirs of the National Academy, 

 vol. iv, third memoir, p. 184, and fourth memoir, p. 194. 



