130 RECORD OF SCIENCE FOR 1«87 AND 1888. 



brighter stars determined by Bessel and Elkin, thus furnishing further 

 data for testing in the future any movement that may be going on iu 

 the system. 



Since the discovery of the nebula in the Pleiades around the star 

 Maia, the Henrys have beeu at work perfecting their apparatus, and 

 upon repeating their •examination of the Pleiades with au exposure of 

 four hours, and very sensitive plates, they have defined with considera- 

 ble detail a great mass of cosmic matter covering a large part of the 

 group. The most interesting detail is a straight nebulous filament 35' 

 to 40' long and only 3" to 4" wide projecting from the main mass in an 

 east and west direction. This filament passes over seven stars, which 

 it seems to connect like beads on a string; a slight change in direction 

 takes place where it meets the largest star. The plate contains nearly 

 twice as many stars as the flfst plate — about 2,000 down to the eight- 

 eenth magnitude. 



Excellent photographs of the Pleiades have also been taken by Mr- 

 Roberts near Liverpool with an 18-iuch silvered-glass reflector. 



# 



ASTRONOMICAL CONSTANTS. 



Constant of precegsion. — Dr. Ludwig Struve has deduced a new value 

 of the constant of precession and the motion of the solar system iu space 

 from an elaborate comparison of recent Pulkowa catalogues with Brad- 

 ley's observations as reduced by Auwers, thus obtaining an interval of 

 a century — 1755.0 to 1855.0— for the determination of proper motions. 

 These i^roper motions, computed with O. Struve's precession constant 

 (of 1841), were affected by the apparent displacement due to the motion 

 of the solar system in space and by the error of the assumed precession 

 constant. 



They thus furnished a means of determining these two quantities. 

 After rejecting seven stars which seem to be exceptionally near us, the 

 remaining 2,509 are divided into 120 groups, forming 240 equations of 

 condition to be solved by least squares for the determination of the five 

 unknowns, the co-ordinates X, Y, Z of the sun's "goal" (or point iu 

 space towards which the sun is traveling, to adopt the term introduced 

 by Professor ISfewton) and the corrections Am and Ati to Bessel's con- 

 stants. The following table shows the resulting value of the luni-solar 

 precession compared with that of previous calculators: 



Bessel 50."3635 Bolte 50."3584 



O. Struve ....50. 3798 Bolto 50. 3570 



Nyr6n 50. 3269 Bolto 50. 3621 



Dreyer 5(T. 3820 L. Struve 50. 3514 



At the end of the paper the author treats of the planetary precession 

 and the secular variation, and gives a list of stars whose proper mo- 

 tions as found by him differ from those deduced by Auwers from Green- 

 wich and Berlin observations. The results obtained for the motion of 

 the solar system are quoted elsewhere in this report. 



