AUGUST 26, 1915| 
NATURE 
79 


nation or group of nations to arm themselves to such 
an extent that it or they can become a menace to the 
peace of the rest of the world. 
Organisation, 
There is another and more positive lesson for us in 
the present war. It shows the power of organisation. 
We see two Empires, but roughly one—the Germanic 
nation—at war with four other great nations, which 
has so developed its resources and organised them, 
that it can stand the strain of such a war that 
25,000,000 picked men have already been in the field. 
However deplorable this may be from ethical and 
economic points of view, it at least does show what 
science and organisation can do to-day. I suppose 
that, one way and another, 50,000,000 of the human 
race are either fighting or supplying food and muni- 
tions of war to the combatants. And no sign of 
exhaustion is as yet clearly discernible. When we 
remember that war in the olden days was conducted 
with small armies and only during a portion of the 
year, we realise the maleficent power that science has 
placed in the hands of mankind. It needs careful 
regulation. This power for evil might only have been 
potential, it might have remained undeveloped; but we 
have found to our loss that at least one nation has 
developed and organised itself, by the aid of science, 
to such an extent that it dares to declare itself inde- 
pendent not only of the power, but even of the opinions 
of the rest of the world. 
The lesson will not be lost. If the deliberate 
organisation of a single nation can result in such 
power, then every nation must organise. Not neces- 
sarily organise for war, for death; but organise for 
peace, for life. 
Laissez faire passed into twilight when the great 
war commenced. We have to turn our eyes in the 
direction of the rising sun of an organised humanity, 
of which we perceive the dawn already. Then the 
advancement of science will surely have no sinister 
meaning. We pray that the advancement of science 
will be identical with the advancement. of humanity. 
Progress of Astronomy. 
I am perhaps fortunate in belonging to a branch of 
science which has nothing to do with war. Therefore 
the astronomer can regard war with a sense of detach- 
ment; and to those who know the stars, the immen- 
sity, the eternity of the universe, its increasing 
grandeur, war seems trivial and foolish—the work of 
unbalanced minds. 
I spoke of one of the aims of science as the enlarge- 
ment of the human mind. Although every branch of 
kknowledge—a word which I take to be nearly synony- 
mous with science (science being co-ordinated know- 
ledge)—leads to the extension of the human mind, to- 
day astronomy has no other real use. We know that 
clocks are corrected through the observations of the 
stars, and that the sun and stars must be observed 
by navigators, but preparation for these practical appli- 
cations forms a very trifling portion of the activities of 
astronomers. The very perfection of that part of 
astronomy reduces it to a sort of automatism—it all 
but goes by itself. To-day the astronomer wants to 
find out the dimensions of the sidereal system—the 
extent of the universe—the structure and arrangement 
of the stars in space—their relations to each other— 
the interpretation of their spectra—the dynamics of the 
universe—the cause of variable stars. The solutions 
of any or all of these questions can scarcely have any 
material effect upon mankind—the effect is spiritual 
and emotional—man is proud to find that he can 
plumb space to its uttermost depth; he presumes that 
the germ of the future which was conceived in the past 
NO. 2391, VOL. 95] 


is taking its form to-day, and that the process is con- 
tinuous, and that as to-day he can predict tides and 
eclipses, so with greater knowledge he will in the 
future be able to predict the course of the sun amongst 
the stars and the future conditions of the planet upon 
which he has his being. 
The Distances of the Stars. 
The problem which will be more closely discussed 
in this address is that of the distance of the stars. 
The most direct way of finding these is by the 
parallactic displacement of the stars caused by the 
motion of the earth round the sun. In this inquiry 
the Union can have a local pride, as the first parallax * 
certainly found was that of a Centaurus by Hender- 
son, the Cape Astronomer. The late Sir David Gill, 
our first president, continued Henderson’s work, and 
perhaps one might say finished it in that form. Gill 
was an organiser, and when the parallax campaign, 
initiated by himself and completed with the aid of 
Dr. Elkin and others, had come to an end, it was 
apparent that the most direct* method of finding 
parallaxes which was available would only yield a 
small crop, because the stars are at such enormous 
distances from the sun that the available base-line 
for measurement, the diameter of the earth’s orbit, 
some 186,000,000 miles, or 300,000,000 kilometres, is 
vanishingly small at the distance of all but a few near 
stars. a Centaurus is the nearest known, and almost 
certainly the nearest to the sun, yet at its distance 
the diameter of the earth’s orbit subtends an angle of 
but 14 seconds of arc—an angle which is described by 
the minute hand of a clock in a four-thousandth part 
of a second of time. An angle so small is difficult to 
observe directly with accuracy, so that at best the 
measures must become differential—that is, the stars 
are measured from neighbouring stars supposed to be 
at a much greater distance away; such stars are called 
comparison stars. 
Prof. Eddington estimates that there are thirty 
stars with a parallax of 0-20" or greater, of which 
nineteen are already known. This means that within 
a distance nearly four times as great as that of 
a Centaurus there are but thirty stars in all. This is 
the limit of visual work such as was done by Gill, 
but photographic methods, especially with the enor- 
mous telescopes used in America, carry the direct 
attack further. 
The delicacy, or, if you prefer, the accuracy, of any 
measurement is limited by its probable error. The 
probable error of a parallax measured visually under 
good circumstances (such as with the Cape helio- 
meter) is about 0-10" (a tenth of a second of arc), and 
this is already, small as it is, a quantity larger than 
the quantity to be measured except in the cases of a 
hundred or so stars. The same method of parallactic 
displacement of stars on photographic plates has a 
much smaller probable error. ‘The most recent deter- 
minations made with the great telescopes of America, 
and in particular the 4o-in. refractor of the Yerkes 
Observatory, have a probable error of about o-or", or 
ten times less than the usual visual method, and Dr. 
Van Maanen, using photographs taken with the 60-in. 
reflector of the Mount Wilson Observatory, has re- 
duced this probable error to 0-006", or about a hundred 
and seventieth part of a second of arc. As regards 
the measurements of small quantities, this is a won- 
derful achievement, but delicate as these mieasure- 

2 But not the first announced. Bessel in 1833 announced the measurement 
of the parallax of 6x Cygnus two months earlier than Henderson. whose 
delay was caused by his removal to Europe. 
3 The only direct parallax found was that of a Centaurus, by Henderson. 
All other parallaxes of any certainty depend on an indirect method involving 
the assumption, nearly true, that all the stars with a few exceptions have 
very minute parallaxes. 
