ASTRONOMY. 5 



observations, Bredichin gives these: (A) 5003.9 1.2, (B) 

 4957.9 11.4, (C) 4859.23.1. 



Professor Holden has made a determination of the relative 

 brightness of the different parts of the nebula of Orio?i, and 

 for this purpose has used a photometer devised by Dr. Has- 

 tings, of the Johns-Hopkins University. These photometric 

 determinations show that this instrument is capable of giv- 

 ing excellent results. 



Dr. Yogel, of the Potsdam Observatory, has published the 

 results of measures on the cluster x JPersei, made in 1867-70 

 by means of the 8-inch refractor at Leipsic, with the object 

 of fixing the relative positions (and magnitudes) of the stars 

 of this cluster, so that any future change may not pass unde- 

 tected. 176 stars, in all, have been fixed in position by the 

 filar micrometer. The field was bright, and a magnifying 

 power of 145 diameters was employed throughout. The va- 

 rious sections of this work of 36 quarto pages treat of the 

 following subjects : 



1. T lie position of the instrument; the determination of 

 the parallel. 



2 contains an investigation of the position-circle , and of 

 the value of the revolution of the micrometer. The zero of 

 the micrometer is dependent upon the position of the instru- 

 ment, and also upon the kind of illumination of the thread. 

 The value of the revolution is found from transits on twenty 

 nights, from November, 1867, to May, 1870. During all this 

 time the reticle was left at the same distance from the objec- 

 tive, and the thermometric coefficient resulted +0.001581" t 

 in Reaumur's scale. The magnitude and the sign of this 

 Dr. Yogel explains by the fact that the focal point was not 

 determined each night (as he says is usual), and lie correctly 

 points out the necessity of leaving the focus unchanged for 

 such observations, and for determining the value of the screw 

 during the series itself. 



8 3 deals with the methods of observation and reduction, 

 The brighter stars less than 10 magnitude were determined 

 from measures of p and s with four selected stars of the 

 group. These four were connected by measures of p and s 

 and also Aa and Ac; and they were further connected witli 

 two stars of the cluster h Persei, which had been observed 

 with the Bonn meridian-circle. For each pair of the brighter 



