198 Prsesepe Group; Measurement and Reduction 



On the next page is given a specimen of the recording sheets ; 

 all the measurements relating to the same star for any one plate 

 are recorded on one sheet. The forms are not designed for the sepa- 

 rate reduction of the two impressions of a group which appear on 

 each plate, but the mean of the measurements on the two images 

 of a star is taken at once ; that is, each star is treated as though 

 it occupied the middle point between its two images. The place 

 of this point is known when we have given the lines of the scale 

 used in the measurement and the quantity \ m, which is obtained 

 thus: froni the reading on the scale we subtract the reading on 

 the star, and the mean of these differences for both images is 

 taken, giving " Mean of Diff's ; " as the scale is one of millimetres 

 and as two complete turns of the screw correspond to a space on 

 the scale we divide by 2 and get -J m. In reading on the scale 

 it was made a rule to select the line having the next lower num- 

 ber ; consequently the place of a star is obtained by adding ^ m 

 to the mean of the numbers of the lines used ; in rare cases, how- 

 ever, the next higher line was employed when it came nearly op- 

 posite to a star, as on March 24 ; this has been indicated b}^ affix- 

 ing a minus sign to i m. In the y measurements the same line 

 was always used for both images, since the latter differ only in 

 apparent right ascension. 



Table III gives the uncorrected observations ; it is only neces- 

 sary to set down the numbers of the lines used and \ m, for these 

 not only fix the place of the star but, as we shall see later, they 

 are sufficient for the application of all instrumental corrections. 

 The numbers of the stars are in the order of increasing right as- 

 censions and are those given by Professor Schur,* excepting 7 A and 

 23.4, which do not appear in his triangulation of the group. The 

 table gives the observations of \m by both observers, followed by 

 the difference reduced to seconds of arc in order to facilitate a 

 comparison. 



* " Astronomische Mittheilungen der K. SternwartezuGottingen," part IV. 



