PHOTOGRAPHIC MEASURES OF THE CLUSTER χ PERSEI
Sir Robert Ball and Mr. Arthur Rambaut have published in Vol. XXX. , Part IV, of the " Transactions of the Royal Irish Society," the relative positions of 223 stars in this cluster as deduced from photographic measures. Th e negative was exposed for 10 minutes, and rectangular co-.ordinates of the stars were measured in a micrometer similar to that used by Prof. Pritchard at Oxford. On comparing the declinations with those of Vogel, a small difference is apparent owing to a slightly altered position of the fundamental star. There is a slight systematic difference between the right ascensions as determined by Pihl and those of the Irish astronomers, probably due to the different methods employed for finding the parallels. Th e memoir also includes a map showing the position of the stars plotted directly from the x and y co-.ordinates.
The astro-.photograph used was a 15-inch silver-on-glass reflector mounted according to Cooke's standard equatorial pattern.