16 PRESIDENT’S ADDRESS. 
of 0-76, or is distant about 44 light-years. Its mass is independently 
known to be almost exactly equal to that of our Sun; and its spectrum 
being also identical with that of our Sun, we may reasonably assume that 
it appears to us of the same magnitude as would our Sun if removed to 
the distance of a, Centauri. 
But the average star of the same apparent magnitude as a, Centauri 
was found to have a parallax of only 0'10, so that either a, Centauri or 
our Sun, if removed to a distance equal to that of the average fixed star of 
the first magnitude would appear to us but little brighter than a star of 
the fifth magnitude. 
Again, there is a star of only 8} magnitude! which has the remarkable 
annual proper motion of nearly 83 seconds of arc—one of those so-called 
runaway stars—which moves witha velocity of 80 miles per second at right 
angles to the line of sight (we do not know with what velocity in the line 
of sight). It is at about the same distance from us as Sirius, but it emits 
but one ten-thousandth part of the light energy of that brilliant star. 
Sirius itself emits about thirty times the light-energy of our Sun, but it in 
turn sinks into insignificance when compared with the giant Canopus, 
which emits at least 10,000 times the light-energy of our Sun. 
Truly ‘one star differs from another star in glory.’ Proper motion 
rather than apparent brightness is the truer indication of a star’s probable 
proximity to the Sun. Every star of considerable proper motion yet 
examined has proved to have a measurable parallax. 
This fact at once suggests the idea, Why should not the apparent 
parallactic motions of the stars, as produced by the Sun’s motion in space, 
be utilised as a means of determining stellar parallax ? 
Secular Parallactic Motion of Stars. 
The strength of such determinations, unlike those made by the method 
of annual parallax, would grow with time. It is true that the process 
cannot be applied to the determination of the parallax of individual 
stars, because the peculiar motion of a particular star cannot be separated 
from that part of its apparent motion which is due to parallactic displace- 
ment. But what we specially want is not to ascertain the parallax of the 
individual star, but the mean parallax of a particular group or class of 
stars, and for this research the method is specially applicable, provided 
we may assume that the peculiar motions are distributed at random, so 
that they have no systematic tendency in any direction ; in other words, 
that the centre of gravity of any extensive group of stars will remain fixed 
in space. 
This assumption is, of course, but a working hypothesis, and one which 
from the paper on star-streaming communicated by Professor Kapteyn of 
Groningen to the Johannesburg meeting of the Association two years ago 
we already know to be inexact.? Kapteyn’s results were quite recently 
! Gould’s Zones, V" 243. 2 Rep. Brit. Assoe., 1905, p. 257. 
