646 
NATURE 
[ AUGUST. 22, 1912 
must have started on its journey towards us thousands 
or even millions of years ago. The celestial museum 
therefore exhibits to us the remotest past alongside 
with the present, and we have in this way the means 
of reconstructing to some extent the processes of 
evolution in the heavens. In photography the modern 
astronomer possesses an enormous advantage, but 
Herschel laid the foundation of this branch of astro- 
nomy without it. 
The most conspicuous and the most wonderful 
object in the heavens is the Milky Way. It runs all 
round the skies in a great band, with a conspicuous 
rent in it forming a streamer which runs through 
many degrees. To the naked eye it shines with a 
milky light, but Herschel was able to show that it 
consists of countless stars in which there lie embedded 
many fleecy nebula. There is good reason to believe 
that the Milky Way on the whole consists of stars 
which are younger than those in the other parts of 
space, for the stars in it are whiter and hotter, and 
the nebula are mostly fleecy clouds. On the other 
hand, the-spiral and planetary nebule are more fre- 
quent away from the Milky Way, and these are 
presumably older than the cloudy and _flocculent 
nebula. The shape of the Milky Way seems to 
resemble a huge millstone or disk of stars, and since 
it forms a complete circuit in the heavens the sun 
must lie somewhere towards its middle. It is probable 
that we look much further out into space along this 
tract than elsewhere, although it happens that by 
far the nearest of all the stars—namely, « Centauri— 
hes in the line of the Milky Way. — 
_ This great congregation of stars is far from uniform 
in density, for there are places in it where there are 
but few stars or none at all. Caroline Herschel, 
writing to Sir John Herschel at- the Cape of Good 
Hope, in 1833, mentions that her brother, when 
examining the constellation of the Scorpion (which 
lies at best low down on our horizon), had exclaimed, 
“after a long awful silence, ‘ Hier ist wahrhaftig ein 
Loch im Himmel.’”. And her nephew, as he said, 
rummaged Scorpio with the telescope and found many 
blank spaces without the smallest star. ; 
It will explain some of the deductions which 
Herschel drew from his star-gauges, and will at the 
same time furnish a good example of his style, if I 
read a passage from a paper of his written in 1789.7 
He points out that the sun is merely a star, and, 
referring to the stars, he continues thus :— 
“These suns, every one of which is probably as 
of much consequence to a system of planets, satellites, 
and comets as our own sun, are now to be considered, 
in their turn, as the minute parts of a proportionally 
greater whole. I need not repeat that by my analysis 
1t appears that the heavens consist of regions where 
Suns are gathered into separate systems, and that the 
catalogues I have given comprehend a list of such 
systems; but may we not hope that our knowledge 
will not stop short at the bare enumeration of 
phenomena capable of giving us so much instruction 2 
Why should we be less inquisitive than the natural 
philosopher, who sometimes, even from an incon- 
siderable number of specimens of a plant, or an 
animal, is enabled to present us with the history of 
its rise, progress, and decav? Let us then compare 
together, and class some of these numerous sidereal 
groups, that we may trace the operations of natural 
causes so far as we can perceive their agency. The 
most simple form. in which we can view a sidereal 
system, is that of being globular. This also, very 
favourably to our design, is that which has presented 
itself most freauently, and of which I have given 
the greatest collection. 
2 Phil. Trans., vol. Ixxix., p. 212. 
NO. 2234, VOL. 89] 
“But, first of all, it will be necessary to explain 
what is our idea of a cluster of stars, and by what 
means we have obtained it. For an instance I shall 
take the phenomenon which presents itself in many 
clusters. It is that of a number of lucid spots, of 
equal lustre, scattered over a circular space, in such 
a manner as to appear gradually more compressed 
towards the middle, and which compression, in the 
clusters to which I allude, is generally carried so far, 
as, by imperceptible degrees, to end in a luminous 
centre of an irresolvable blaze of light. ‘To solve this 
appearance it may be conjectured that stars of any 
given very unequal magnitudes may easily be so 
arranged, in scattered, much extended, irregular rows, 
as to produce the above described picture; or, that 
stars, scattered about almost promiscuously within 
the frustum of a given cone, may be assigned of such 
properly diversified magnitudes as also to form the 
same picture. But who, that is acquainted with the 
doctrine of chances, can seriously maintain such im- 
probable conjectures ¢" 
Later in the same paper he continues :— 
“Since then almost all the nebulz and clusters of 
stars I have seen, the number of which is not less 
than three and twenty hundred, are more condensed 
and brighter in the middle; and since, from every 
form, it is now equally apparent that the central 
accumulation or brightness must be the result of 
central powers, we may venture to affirm that this 
theory is no longer an unfounded hypothesis, but is 
fully established on grounds which cannot be over- 
turned. 
“Let us endeavour to make some use of this im- 
portant view of the constructing cause, which can 
thus model sidereal systems. Perhaps, by placing 
before us the very extensive and varied collection of 
clusters and nebulae furnished by my catalogues, we 
may be able to trace the progress of its operation 
in the great laboratory of the universe. 
“Tf these clusters and nebulz were all of the same 
shape, and had the same gradual condensation, we 
should make but little progress in this inquiry; but 
as we find so great a variety in their appearances, 
we shall be much sooner at a loss how to account 
for such various phenomena, than be in want of 
materials upon which to exercise our inquisitive 
endeavours. 
“Let us, then, continue to turn our view to the 
power which is moulding the different assortments 
of stars into spherical clusters. Any force, that acts 
uninterruptedly, must produce effects proportional to 
the time of its action. Now, as it has been shown 
that the spherical figure of a cluster of stars is owing 
to central powers, it follows that those clusters which, 
ceteris paribus, are the most complete in this figure, 
must have been the longest exposed to the action of 
these causes. This will admit of various points of 
view. Suppose, for instance, that 5000 stars had been 
once in a certain scattered situation, and that other 
5000 equal stars had been in the same situation, then 
that of the two clusters which had been longest 
exposed to the action of the modelling power, we 
suppose would be most condensed, and more advanced 
to the maturity of its figure. An obvious consequence 
that may be drawn from this consideration is that we 
are enabled to judge of the relative age, maturity, 
or climax of a sidereal system, from the disposition 
of its component parts; and, making the degrees of 
brightness in nebulz stand for the different accumula- 
tion of stars in clusters, the same conclusions will 
extend to them all. But we are not to conclude from 
what has been said that every spherical cluster is of 
an eaual standing in regard to absolute duration, 
