196 Permanerce of the Rutherfurd Photographic Plates. 



The cluster of stars most convenient for the present research 

 is, of course, the group of the Pleiades. This group was fre- 

 quently photographed by Rutherfurd, and ten of his best plates 

 were carefully measured under his direction by Miss Ida C. Mar- 

 tin. Each of these plates has two impressions. The results of 

 all these measures b}' Miss Martin have been published.* It has 

 seemed very appropriate that the first work to be undertaken 

 with the Repsold photographic measuring machine presented to 

 the Observatory of Columbia University by Mr. Rutherfurd 

 Stu3'-vesant should be the remeasurement of these old Pleiades 

 plates, for the purpose of deciding the question of their perma- 

 nence. This question once settled, the measurement of the other 

 clusters photographed by Rutherfurd can be commenced. Many 

 of these clusters have never been micrometrically determined. 

 The only ones that have been measured with the heliomcter, so 

 far as we know, are the Pleiades and the Praesepe.f The latter 

 cluster has been very carefully measured with the heliometer of 

 the Gottingen Observatory, and Dr. Schur has only verj^ re- 

 cently published his most exhaustive discussion of these observa- 

 tions. He has not failed to point out the great desirability of 

 publishing the results of Rutherfurd's later work on this cluster. 

 But he has not emphasized the fact that his own heliometer ob- 

 servations are of the greatest importance in the reduction of all 

 the Rutherfurd work. Since the appearance of the Gottingen 

 triangulation of the Praesepe it has become possible to get a very 

 complete check upon the scale- value and other constants of reduc- 

 tion of the Rutherfurd plates. These constants have heretofore de- 

 pended altogether on Dr. Elkin's work on the Pleiades. It is to 

 be hoped that, after the Praesepe plates have been measured, a 

 comparison with the Gottingen triangulation will throw much 

 light upon several points which have not as yet been made quite 

 clear. The measurement of the Praesepe plates will therefore be 

 the next work undertaken in connection with the Rutherfurd pho- 

 tographs. 



2. The Repsold machine for the measurement of astronomical 

 photographs belonging to the observatory of Columbia Univer- 

 sity is almost exactly similar to the one constructed by the same 



* Annals N. Y. Acad. Sci., Vol. VI, p. 239. 



t While the present paper was being printed, the Observatory of Yale Uni- 

 versity published a heliometric triangulation of the cluster in Coma Berenices, 

 by Dr. Chase. This cluster is also among those photographed by Kutherfurd. 



