YI. — The Prsesepe Group; Measurement and Reduction of the 

 Rutherfurd Photographs. 



By Fkank Schlesinger. 



Read April 4, 1898. 

 I. 



Description of the Plates. 



The collection of astronomical photographs presented by the 

 late Lewis M. Rutherfurd to the Observatory of Columbia Uni- 

 versity contains eleven photographs of Prsesepe taken with his 

 larger and improved instrument ; only eight of these were meas- 

 ured and reduced, three having been judged inferior to the rest. 

 According to Rutherfurd 's invariable practice, each plate shows 

 two complete pictures of the group separated by about a milli- 

 metre in right ascension, the driving clock of the instrument 

 having been stopped for a few seconds after the completion of the 

 first or eastern impression. Near the west edge of the plate still 

 a third image of each of the brighter stars in the group is found, 

 separated by about forty millimetres from the two other impres- 

 sions, the driving clock having been stopped for an interval of 

 about three minutes after the completion of the second impres- 

 sion, and then started again and allowed to run long enough to 

 permit the brighter stars to leave well-defined images. The ob- 

 ject in securing these third impressions or trails was to afford 

 means for orienting the group, but in the present work they were 

 not used for this purpose, the orientation having been effected in 

 another way. It is important, however, to know how accurately 

 the use of trails will give the orientation, and they were therefore 

 completely measured and reduced, and the results compared with 

 those obtained by the method actually emploj'ed, which is of un- 

 questioned accuracy but may not be always available. 



A perpendicular to the plate passing through the optical centre 

 of the object glass pierces the plate at a point whose approximate 

 position must be known in order to reduce the measures of the 

 stars to right ascensions and declinations. Rutherfurd so ad- 

 justed his plate holder that this point coincides approximately 

 with the image of the central star of the group, numbered 15 in 

 the following pages. 



189 



Annals N. Y. Acad. Sci., X, May, 1893—13. 



