278 Praesepe Group ; Measurement and Reduction. 



of the plates, we can only conclude that the somewhat large value 

 of dr is due to the fact that the relative positions of these stars 

 with respect to the rest of the group have been differently deter- 

 mined by the photographs on the one hand, and the heliometers 

 on the other. This explanation is borne out by the comparatively 

 large values of the corrected differences for the comparison stars, 

 numbers 4, 5, 15, 40 and 44. 



The large value for dk, or the systematic correction in declina- 

 tion, was to be expected. We have already remarked (see page 

 236) that the proper motions used for the comparison stars were 

 not derived by Schur from the direct differences between the two 

 heliometer determinations of the places of these stars, but that 

 systematic corrections, 



4- o s .ooo3 and — o // .039 



were added to the proper motions in right ascension and declina- 

 tion respectivel}'. Hence we must expect the photographic places 

 to differ from those of the heliometer for the epoch of 1875.0, by 



+ c/'.oyi and — (y.612, 



the proper motion having been used for an interval of 15.7 years. 

 These corrections agree quite well with the values of dk and dc 

 respectively, as obtained above. 



