STARS IN COMA BERENICES. 449 



factor only. Its whole effect will therefore be included in the 

 scale-value and orientation corrections, when these are obtained 

 by the method now to be described. 



Constants of the Plates. — Four quantities must be known 

 for each plate, in order that we may determine the absolute po- 

 sitions of the stars whose coordinates have been measured. 

 They are : the right ascension a^ and the declination o^ of the 

 center, or origin ; the value in seconds of arc of one division of 

 the scale ; and the angle made by the axes to which the meas- 

 urements are referred, with the axes of reference in the celestial 

 sphere. To obtain them, we must compare the measured coor- 

 dinates of certain stars ("standards") with the corresponding 

 distances of the same stars, from the same point as origin, ob- 

 tained from meridian observations. Matters will be greatly 

 facilitated by a knowledge of approximate values for these con- 

 stants. As regards my plates, such information was available. 

 The position of the center which coincides with star 14 {\2e 

 Comae), was accurately known ; the approximate scale-value was. 



placed at 



I mm. = 5 2^''. 87, 



that being the result of a previous reduction of Rutherfurd's 

 photographs of the Pleiades ; while the orientation correction, 

 due to the rotation of the axes, would be necessarily small, 

 owing to the manner in which the plate was adjusted in the 

 measuring machine. 



We must now obtain the distances Ja = « — a^, and Jo = <5 . 

 — o^j, for certain stars on the plates. Let us return to Part I, 

 Section III of this paper. We find there (p. 396) a catalogue of 

 twelve stars suitable for this purpose. Of these, the following 

 eleven are sufficiently well observed to serve as " standards " : 2, 

 3, 5, 6, 10, II, d, 14, 18, 19, 20. These are Chase's numbers; 

 they correspond to my numbers i, 2, 4, 5, 9, 10, 14, 15, 21, 22, 

 23, respectively. In the following I shall designate them by the 

 latter numbers only. But before proceeding to obtain da and 

 Jo for these stars, we must apply a correction for proper motion. 



(109) 



