278 Rutherfurd Photograjihic Measures. 



other words, if we imagine a line drawn upon the plate from the 

 Eastern impression of the central star, in the direction of position 

 angle 270 , then another line, drawn parallel to the first through the 

 Western impression of the central star, will not be directed towards 

 position angle 270 , but will make a small angle with that direction. 

 All the Western position angles of any plate will need to be cor- 

 rected by the amount of this angle. The correction, in seconds of 

 arc, is* 



— d tan 8, 

 where d is the distance of the two impressions of the central star, 

 in seconds of arc, and 5 the declination of the central star. In this 

 way the corrections required by the Western position angles upon 

 the several plates have been found to be : 



Plate. 



Corrfection. 



16 



-17" 



*7 



— 19 



18 



—25 



19 



—31 



20 



—39 



21 



—39 



22 



—44 



23 



—42 



24 



—33 



25 



-56 



Mean 



—34-5 



It is perhaps of interest to compare the Eastern position angles 

 with the Western, before the above correction has been applied. 

 This has been done for the following nine stars, which happened to 

 be the first ones finally reduced. After applying all the other cor- 

 rections, the direct differences (East minus West) were taken for 

 each star. The mean for each star from all the plates is given here. 



* This correction is of the same form as that given in Astronomical Journal, 

 No. 233, p. 130, Eq. (7). 



