14 PRESIDENT’S ADDRESS. 
Tn all discussions of the solar motion in space, from that of Herschel 
down till a recent date, it has been assumed that the peculiar motions of 
the stars are arranged at random, and may therefore be considered zero 
in the mean of a considerable number of them. It is then possible to find 
such a value for the Precession, and such a common apex for the solar 
motion as shall leave the residual peculiar motions of the stars under 
discussion to be in the mean = zero. That is to say, we refer the 
motion of the Sun in space to the centre of gravity of all the stars con- 
sidered in the discussion, and regard that centre of gravity as immovable 
in space. 
In order to proceed rigorously, and especially to determine the 
amount as well as the direction of the Sun’s motion in space, we ought to 
know the parallax of every star employed in the discussion, as well as its 
proper motion. In the absence of such data it has been usual to start 
from some such assumption as the following: the stars of a particular 
magnitude are roughly at the same distance ; those of different classes 
of magnitude may be derived from the hypothesis that on the average 
they have all equal absolute luminosity. 
The assumption is not a legitimate one — 
1. Because of the extreme difference in the absolute luminosity of 
stars. 
2, Because it implies that the average absolute luminosity of stars 
is the same in all regions of space. 
The investigation has been carried out by many successive astronomers 
on these lines with fairly accordant results as to the position of the solar 
apex, but with very unsatisfactory results as to the distances of the fixed 
stars.! In order to judge how far the magnitude (or brightness) of a star 
is an index of its probable distance, we must have evidence from direct 
determinations of stellar parallax. 
1 Argelander, Mém. présentés a l’Acad. Imp. des Sciences St. Pétersbourg, 
tome iii. 
Lundahl, Astron. Nachrichten, 398, 209. 
Argelander, Astron. Nachrichten, 898, 210. 
Otto Struve, Mém. Acad. des Sciences St. Pétersbowrg, vi° série, Math. et Phys., 
tome iii. p. 17. 
Galloway, Phil. Trans., 1847, p. 79. 
Madler, Dorpat Observations, vol. xiv., and Ast. Nach., 566, 213. 
Airy, Mem. R.A.S., vol. xxviii. p. 143. 
Dunkin, Mem. R.A.S., vol. xxxii. p. 19. 
Stone, Monthly Notices R.A.S., vol. xxiv. p. 36. 
De Ball, Inaugural Dissertation, Bonn, 1877. 
Rancken, Astron. Nachrichten, 2482, 149. 
Bischoff, Inaugural Dissertation, Bonn, 1884. 
Ludwig Struve, Mém. Acad. St. Pétersbourg, viit série, tome xxxv. No. 3. 
