Group of the Pleiades. 269 



v. 



SCALE VALUE. 



It is of course of the highest importance to obtain an accurate 

 determination of the "scale value," or number of seconds of arc 

 corresponding to one division of the measuring micrometer. The 

 values of this quantity used in the present paper depend entirely 

 upon a comparison with the Konigsberg and Yale heliometer meas- 

 ures of the Pleiades. It is proper to call attention to the fact that 

 the reduction of the other clusters measured by Rtjtherfurd must 

 be made with the scale values deduced here: and this is another 

 reason for letting the reduction of the Pleiades precede that of 

 the other clusters. Accordingly, six "standard stars" have been 

 selected, and the scale value for each plate has been determined so 

 as to make the sum of the six distances of these stars from 24 p 

 equal to the sum of the same distances as furnished by the helio- 

 meter observations. For this purpose, the heliometer places have 

 been interpolated to 1873.0 by means of the Yale and Konigsberg 

 measures, using the proper motions given by Elkin.* The condi- 

 tions governing the selection of the standard stars have been the 

 following : 



I. The stars must have been observed both at Yale and Konigs- 

 berg. 

 II. The magnitudes must be between 7.0 and 8.2, so as to be 

 bright enough to be heliometrically well determined, and 

 at the same time not so bright as to blur the photographic 

 images. 

 III. The stars must be symmetrically situated in position angle 

 about 24 p. This will free the resulting scale value from 

 the various errors depending on the direction of measure- 

 ment. It will also remove the effect of such inaccuracy as 

 may exist in the heliometer place of the central star 24 p. 



* Transactions of the Astr. Observatory of Yale University, vol. i, p. 99. 



