130 KECOKD OF SCIHNCE FOli 1887 AND 1888. 



bri.iiliter stars deterniiiied by Bcssel and Elkiu, thus fuinisliing further 

 data for testing in the future anj- movement that may be going on in 

 the system. 



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

 Maia, the Henrys have been at work i)erfecting their apparatus, and 

 upon repeating their examination of the Pleiades with an 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 first plate — about 2,000 down to the eight- 

 eenth magnitude. 



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

 Koberts near Liverpool with an 18-inch silvered-glass reflector. 



ASTRONOMICAL CONSTANTS. 



Constant of precession. — Dr. Liidwig Struve has deduced a new value 

 of the constant of precession and the motion of the solar system in 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 proper motions, computed with O. Struve's precession constant 

 (of 1811), 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 210 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 in 

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

 by Professor Newton) and the corrections Aw and An to Bessel's con- 

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

 precession compared with that of previous calculators: 



Bessol 50. "3635 Bolte 50. "3584 



O.Stnive ....50. 3798 Bolte 50. 3570 



Nyr6n 50. 3269 Bolte 50. 3621 



Dreyer 50. 3820 L. Strnve 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. 



