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aid. Moreover, the stars investigated have been in general selected on 
the grounds of some a priori probability of their possessing a measurable 
parallax, either on account of apparent brightness or on account of their 
large apparent motion ; and for this reason they can scarcely be regarded 
as typical of the generality of stars. 
In order, then, to gauge the depths of the visible universe it would 
appear imperative that our base-line must in some manner be extended. 
The distance of 186,000,000 miles, through which we are carried in 
the course of a single half-year by the orbital motion of our planet round 
the sun, is so small in comparison with interstellar distances as to give 
rise to changes in the apparent relative positions of stars which, except in 
the most pronounced instances, are so insignificant in amount as to 
defy detection even by the most refined processes of measurement we 
possess. 
How, then, can such an extension of our base-line be attained ? I have 
already pointed out that the so-called "fixed stars" are not truly 
fixed," but that on close observation it is found that each star has 
an apparent motion either peculiar to itself or shared by other neighbour- 
ing stars which, with it, constitute an independent system. I refer 
primarily to the visible motion transverse to the line of sight. 
If then our sun, as we may reasonably suppose, is itself a member of 
the stellar universe, it may be anticipated that it too will not be at rest, 
but will be moving forward in space, and the visible motions will be 
those due to the combined effects of the motion of the sun and stars. 
That the apparent motions of the stars were not entirely fortuitous 
but that they could at least partially be co-ordinated throughout the sky 
as the visible manifestations of a single phenomenon, viz., a translatory 
motion of the sun with its system of planets through interstellar space 
was first pointed out by Sir William Herschel, who further indicated that 
the point of space to which this motion was directed was situated in 
the constellation "Hercules." 
Before proceeding to the further consideration of this solar motion, 
I wish first to point out to you how its existence at once suggests 
a means of " extending our base-line " for the purpose of gauging these 
interstellar depths. I have refrained from any numerical estimates of the 
amount of this motion, as this involves philosophical questions into which 
I do not desire to enter to-night. But in order to fix our ideas it is 
necessary for me to give you some notion at least of the order of 
magnitude. It is now possible to state with some certainty that the 
speed of the sun's motion relatively to the stars as a whole amounts 
to about 20 kilometres per second, and that the space traversed in a single 
day therefore amounts to rather over a 1,000,000 miles, that in a year to 
about 400,000,000 miles. Thus the stars, as seen on two occasions a 
