PART II. 



MEASUREMENT AND REDUCTION OF THE 



PLATES. 



I. The Plates: Description and Measurement. 



Description. — The photographs of Coma Berenices were 

 taken with Rutherfurd's large telescope in the years 1870, 1875 

 and 1876. They differ in no particular from his other star 

 plates. There are always two images of each star, obtained by 

 stopping the driving clock a few seconds after the first expo- 

 sure had been made, and then starting it again, leaving, mean- 

 while, the plate in position so that another impression could be 

 made. A third image (or " trail") of the brightest stars is usu- 

 ally found at the distance of about 3 5 mm. from the second image, 

 obtained in a similar manner, except that the clock was stopped for 

 a longer time than in the previous case. These trails were intended 

 to give an independent means of orienting the plate. I have 

 not used them otherwise than to place the photograph correctly 

 in the measuring machine; for Dr. Schlesinger^ has shown 

 that no reliance can be placed on the trails for other purposes. 



The plates are by no means uniform in quality, some of them 

 giving a much sharper picture than others. Especially notice- 

 able, and at the same time rather annoying, is the elongation of 

 the images on some of the photographs due to irregularities in 

 the clock, which failed to keep pace exactly with the diurnal mo- 



1 " The Praesepe Group, Measurement and Reduction of the Rutherfurd Photo- 

 graphs " by Frank Schlesinger. Annals of the N. Y. Academy of Sciences, Vol. 

 X. The page referred to is 282. 



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