AUGUST 22, 1912] 
since one that is composed of a thousand stars only 
must certainly arrive to the perfection of its form 
sooner than another which takes in a range of a 
million. Youth and age are comparative expressions ; 
and an oak of a certain age may be called very young, 
while a contemporary shrub is already on the verge 
of its decay. The method of judging with some assur- 
ance of the condition of any sidereal system may 
perhaps not improperly be drawn from the standard 
laid down earlier; so that, for instance, a cluster or 
nebula which is very gradually more compressed and 
bright towards the middle may be in the perfection 
of its growth, when another which approaches to the 
condition pointed out by a more equal compression, 
such as the nebulz I have called Planetary seem to 
present us with, may be looked upon as very aged, 
and drawing on towards a period of change, or disso- 
lution. This has been before surmised, when in a 
former paper I considered the uncommon degree of 
compression that must prevail in a nebula to give it 
a planetary aspect; but the argument, which is now 
drawn from the powers that have collected the 
formerly scattered stars to the form we find they 
have assumed, must greatly corroborate that sentiment. 
“This method of viewing the heavens seems to 
throw them into a new kind of light. They now are 
seen to resemble a luxuriant garden, which contains 
the greatest variety of productions, in different 
flourishing beds; and one advantage we may at least 
reap from it is, that we can, as it were, extend the 
range of our experience to an immense duration. For, 
to continue the simile I have borrowed from the 
vegetable kingdom, is it not almost the same thing, 
whether we live successively to witness the germina- 
tion, blooming, foliage, fecundity, fading, withering, 
and corruption of a plant, or whether a vast number 
of specimens, selected from every stage through which 
the plant passes in the course of its existence, be 
brought at once to our view?” 
I now turn to another line of discovery of which 
I cannot show any pictures, but which, to me at any 
rate, is more interesting. Until 1838—that is to say, 
until sixteen years after Herschel’s death—no one had 
succeeded in determining the distance of a single fixed 
star, but in that year Henderson and Bessel almost 
simultaneously attained success in the cases of the 
two stars 2 Centauri and 61 Cygni. The attempts at 
this measurement had already been numerous, and 
Herschel amongst others had failed, but his failure 
was a glorious one, for he made incidentally a dis- 
covery of another kind and of at least equal interest. 
The earth moves round the sun at a distance of 
93 million miles, so that in six months we shift our 
position by 186 million miles. 
stars of which one is relatively near to and the other 
far from the sun, but so situated as to appear to us 
very close together, the near one ought to shift its 
position relatively to the distant one in the course of 
each six months. The amount of this change of 
position, called by astronomers annual parallax, 
should furnish the distance of the nearer of the pair, 
provided that the other is very far off. This idea is 
as old as the time of Galileo, but no one had been 
able to make successful use of it. 
As I have already said, the only general test of 
the distance of a star is its brightness, and therefore 
Herschel chose pairs of stars of very different bril- 
liancy. He thought, at least at first, that it was 
mere chance which brought the stars so near to one 
another, and there are undoubtedly such pairs now 
known as “optically double stars.” But Herschel’s 
mode of attack was bound to fail if the seemingly 
neighbouring stars were really so, and were linked 
together “by their mutual gravitation. Already as 
early as 1707 Michel had suggested the existence of 
NO. 2234, VOL. 89] 
If, then, there are two | 
NATURE 
647 
such true double stars, but it was Herschel who 
proved their existence. His first catalogues of double 
stars, published in 1782, contained 203 cases of such 
doublets, and he already suspected a community in 
their motions explicable only by their real association ; 
but by 1802 he had become certain. In many cases 
the two components of a binary pair were found to 
be moving in nearly the same direction and at the 
same speed, but superposed on this motion of the 
system as a whole there was an orbital motion of one 
star round the other. Herschel even lived long 
enough to see some of his pairs of stars perform 
half a revolution about one another. 
After his death Savary took the matter one stage 
further, and showed that the revolution was governed 
by the laws of gravity, and thereby confirmed the 
truth of Herschel’s belief. Thus the failure to 
measure the distance of stars led to the proof that 
gravity reigns amongst the stars as in the solar 
system. 
Arago thought that of all Herschel’s discoveries 
this was the one that had the greatest future, and 
his prophecy has proved singularly correct. Every 
year adds to the number of double stars the orbits 
of which are now accurately determinable. These 
systems are found to be very unlike our own solar 
system, for the component stars are, in many cases, 
far larger than the sun, and revolve about one another 
in periods which, in various cases, may be either 
many years or only a few hours. 
The spectroscope has, moreover, added enormously 
to our knowledge, for the speed of approach or reces- 
sion of a star from the sun can now be determined 
as so many kilometres per second. Thus that com- 
ponent of the motion of a star which was concealed 
from Herschel is now known with the greater cer- 
tainty. Moreover, being ignorant of the distance of 
the stars, he could only express the transverse com- 
ponent of motion in seconds of arc. 
A wonderful corollary also results from the use of 
the spectroscope, namely the existence of many stars 
kkxnown as “‘spectroscopic binaries.’ As seen even 
with the most powerful telescope such a star is a 
single point of light, but if the spectral lines are 
duplicated we know that the source of light is double, 
and that one component is approaching us and the 
other receding from us. In this way the orbits and 
relative masses of these visually inseparable stars are 
determinable. The number of known double stars, 
including both visual and spectroscopic ones, is already 
large, and Campbell, of Lick Observatory, has ex- 
pressed his opinion that one star in six is double. 
Some of them revolve so near to one another and in 
such a plane that they partially eclipse one another 
as they revolve, and thus produce a winking light 
like that of a lighthouse. It would seem that we 
can now even tell something of the shapes of a pair 
of stars visually inseparable from one another. But 
I must not go further into this subject, and will only 
repeat Arago’s saying, that this discovery of Her- 
schel’s has ‘“‘le plus d’avenir.”’ 
It is a figure of speech to refer to the stars as 
fixed, for a large number of them possess a measur- 
able amount of ‘“‘ proper motion” relatively to their 
neighbours. The existence of double stars was dis- 
covered by the observation of their movements, and 
thus the study of proper motions is linked to the 
subject of which I have just been speaking. Some 
few proper motions had been observed by earlier 
astronomers, but when Herschel took up the subject 
proper motion had not been accurately measured in 
anv case. 
If a man is walking through a wood the trees in 
front of him seem to be opening out before him, whilst 
those behind seem to be closing together. In the 
