of the Rutherfurd Photographs. 275 



dinate is independent of errors in the determination of the division 

 corrections, because the two observers always used the same 

 lines; but not so with the differences of the right ascensions 

 or declinations as derived from different plates. Similarly, the 

 corrections for temperature and straightness of the scale, which 

 we have neglected, do not affect the agreement of the measured 

 coordinates. Possibly too, there have been distortions of the 

 film, but the smallness of the probable errors on the whole must 

 rather be taken as evidence against such distortions. It is im- 

 portant to note that the close agreement of the right ascen- 

 sions and declinations for different plates affords a striking con- 

 firmation of the permanence of the Rutherfurd plates, which in 

 the present case have been measured a quarter of a century after 

 they were made.* 



If we consider the probable errors of the measured coordinates, 

 we see that the uncertainty is considerably greater upon some 

 plates than upon others. Notwithstanding, equal weights have 

 been assigned to all the plates, since it appears that the uncer- 

 tainty in a measured coordinate forms only a small part of the 

 uncertainty in the corresponding right ascension or declination. 



Let us now compare the photographic results with those of the 

 heliometer. In his memoir upon the group, Professor Schur has 

 given the places of forty-five stars referred to the mean equinox 

 and epoch of 1875.0, which are the same as those used in the 

 present paper. Of these stars all but five appear on the photo- 

 graphs. The following table gives first the uncorrected or direct 

 differences obtained by subtracting the right ascension, declina- 

 tion and proper motion of each star in our catalogue, from the 

 corresponding quantities in Schur's. The differences in right 

 ascension and in proper motion in right ascension have been 

 multiplied by cos 8 Q to reduce them to arc of a great circle. 



*See, in this connection, "On the Permanence of the Kutherfurd Photo- 

 graphs," by Harold Jacoby, Annals of the N. Y. Acad, of Sciences, Vol. IX. 



