The Rutherfurd Photographs. 235 



coordinates of some of the stars with meridian observations. For 

 a star which appears on all the plates we have thus eight determi- 

 nations of its right ascension, and eight of its declination, and a 

 catalogue of the group may now be formed by taking the means. 

 Finally a least square solution was made to determine how much 

 the constants would have to be changed on the average, so as to 

 secure the best possible agreement with the heliometer places. 

 It is evident that if we now apply to our catalogue positions the 

 corrections which result from these average changes in the con- 

 stants we shall obtain the same results as though we had used the 

 heliometer places to determine the constants for the separate 

 plates, and had then taken the means. 



The meridian observations used are those quoted by Professor 

 Schur in the work mentioned above ; they were used by him to 

 help fix the place of his triangulation in the sky. Five stars were 

 observed, the central star and four others distributed S3 r mmetric- 

 ally over the plate ; they are admirably fitted for our purpose, 

 being at sufficiently large distances from the central star to insure 

 accurate determinations of the scale- value and orientation, and 

 yet not so distant as to have their photographic images much 

 distorted. Their magnitudes are such that they appear on all of 

 the plates with good images. The stars were observed both at 

 Berlin and Gottingen ;* the positions obtained at the former ob- 

 servatory and reduced to the A. G. catalogue system are as fol- 

 lows : 



Star. 



Epoch. 



Equinox of 1890.0. 



4 



1890.26 



8 h 3i m 28. s 444, 



+ i9°39'oo // .65 



5 



•51 



32 02. 359 



20 35 28 .43 



15 



.26 



33 23. 463 



20 09 55 .38 



40 



•51 



35 00. 573 



19 39 04 .35 



44 



.26 



35 33- 4^7 



20 3303 .28 



Each star was observed four times, and the probable error of a 

 single observation is given as 



±o. s oi2 in right ascension, 

 ±o."25 in declination. 



At Gottingen each star was observed six times with these re- 

 sults : 



* " Astronomische Mittheilungen von der K. Sternwarte zu Gottingen," 

 Part IV, pages 139 et seq. 



