718 
NATURE 
[|AuGusT 26, I915 

ing the total number of stars in the sky, arranged 
according to magnitudes :— 
Magnitude Number Magnitude Number 
2-0 38 10:0 272,006 
3:0 II 11-0 698,000 
4:0 300 12-0 1,660,000 
5:0 g50 13:0 3,080,000 
6:0 3,150 14-0 7,050,000 
7:0 9,510 15:0 15,500,000 
3-0 32,360 16-0 29,500,000 
9:0 97,400 17-0 54,900,000 
So actually, the Franklin-Adams plates locate for 
reference at any time about roo million stars, and 
these may be said to be all the stars known to astro- 
nomers. Special plates taken with the largest tele- 
scopes indicate a much larger number of stars—per- 
haps ten to fifteen hundred million in all. It will be 
noticed that the ratio from one magnitude to another, 
which is larger than 3 at the beginning of the table, 
progressively decreases, and is already less than 2 for 
the 15-16 magnitude; hence the authors conclude 
‘that modern photographic telescopes penetrate to a 
distance at which the stars begin to thin out fairly 
quickly either really or by absorption.” 
Variation of Latitude. 
Since March, 1910, and until December, 1914, the 
Union Observatory has, aided for some years by a 
subsidy from the International Geodetic Bureau, taken 
part in a scheme of observations for measuring the 
variation of latitude. I must be brief, and will only 
say that the question at issue was: “Is this variation 
common to the whole globe, or is it in part or wholly 
due to the elasticity of the earth, so that the deforma- 
tion in the northern hemisphere might be different 
from that of the southern hemisphere?” The result 
of our observations to March, 1913, proves that in 
the variation of latitude the earth moves as a solid. 
In Dr. Albrecht’s own words :— 
‘From this series of observations we can deduce 
an interesting confirmation of the result, previously 
obtained, that the values of the quantities x, y, and 
s deduced from observations made in the northern 
hemisphere, can be applied without any modification 
to the variation of the latitude in the southern hemi- 
sphere.”’ * 
Gravitation. 
For upwards of half a century it has been known 
that the law of gravitation seems to be insufficient to 
account for all the planetary motions—the most con- 
spicuous exception being the motion of the perihelion 
of Mercury’s orbit—and it has been found more 
recently that it is impossible to reconcile the moon’s 
motion with gravitation. Recently Sir J. Larmor and 
Mr. H. Glauert have proved that a certain amount 
of these irregularities are due to variations in the 
length of the day; Glauert finding that the length of 
the day has increased by a hundredth of a second in 
a third of a century. This means that as compared 
with a third of a century ago, the year will appear 
to be about 33 seconds longer. Such a change, be- 
cause of our methods of determining time, will be 
most clearly reflected in the motion of the first satellite 
of Jupiter, the eclipses of which can be observed with 
an accuracy of about one second, and the motion of 
which is the most purely periodic that is known. 
Since 1908, every visible eclipse of this satellite has 
been observed at the Union Observatory, so that in 
the course of time we may expect that our observa- 
tions may assist in the solution of an obscure problem. 
In dealing with the structure of the sidereal universe, 
or in a smaller way with the dynamics of a star- 
cluster, it is often tacitly assumed that gravitation 
is the only force at work. That gravitation is not 
5 Rapport sur les Travanx du Rvreau Central en 1914, p. 6. 
NO. 2391, VOL. 95] 


universally applicable we see in the solar system in 
the phenomena of comets’ tails, and even more so in 
the disintegration and disappearance of periodic comets 
such as those of Biela and Holmes. Many double 
stars are undoubtedly subject to the law of gravita- 
tion in all its purity, but in far many more gravitation 
appears to be at most only a secondary (thus in the 
case of double stars of which both components are 
of the helium type, there do not appear to be any 
signs of gravitative action between the two stars). 
It is true that stars with variable radial velocities have 
been found spectroscopically, and their orbits deduced 
by purely gravitational principles, but in many of these 
cases it is not indubitably certain that the shift in the 
lines of the spectrum is due to recession or approach. 
The difficulty is that in the so-called earlier type of 
stars, it is found that the H and K lines of calcium 
| do not share in the variable motion on which the 
binary orbit is based. The interpretation of spectra— 
the contradictory behaviour of different lines, their 
thickness and intensities—still provides problems to 
be solved. In this connection one must refer to the 
illuminating papers by Dr. Nicholson on the relation 
between atomic structure and the lines in the spec- 
trum. Nicholson’s work makes much use of the 
spectra of nebule, in which we see matter under 
simpler conditions than is possible on earth. At this 
meeting Prof. Malherbe is reading a paper upon 
“Atoms, Old and New,” which will go further into 
this subject than is possible here. 
Organisation of Astronomy. 
In the earlier part of this address I dwelt upon the 
power of organisation under scientific direction. I 
am tempted to develop the subject, limiting my 
example of organisation to the science of astronomy, 
which is truly international in its aims. Astronomers 
are scattered all over the world, and pursue their 
work independently of the people amongst whom they 
live, and who provide the money necessary for their 
existence. The people are not ungenerous, but they 
cannot be critical. The astronomer is on his honour 
as it were, and this is nearly good enough, but, not 
quite. If the astronomer is a man of sufficient 
initiative and energy with a regulated imagination, 
he will not require much supervision, but he may feel 
that without the co-operation of his colleagues spread 
over the world his work may be one-sided. He sees 
the need for organisation, and such organisation is 
not quite unknown, and has been found beneficial. 
Such occasional events as the transits of Venus and 
total eclipses of the sun generally lead to some loose 
co-operation. More organised affairs were the Star 
Catalogue of the Astronomische Gesellschaft (a society 
having its headquarters in Germany, but with inter- 
national aims). It divided the sky into zones, and 
allotted these to certain observatories, which were 
willing to co-operate. The catalogues actually pub- 
lished have been contributed by Austria, England, 
Holland, Germany, Norway, Sweden, Russia, and the 
United States. This organised effort, started in 1868, 
is still going on. The other and more important 
organisation is that of the Carte du Ciel, started in 
1887, and in which our first president took a leading 
part—he was connected with it from its inception, 
and when he died he was the president of the Com- 
mission. The scheme for the variation of latitude 
observations is also an international organisation. 
All these organisations were voluntary. In every 
way they were useful. The problem is whether we 
can extend the organisation to the whole body of 
astronomers, and yet not destroy their initiative. A 
control, however light, which would destroy initiative 
would be fatal. At present many observatories furnish 
an annual report. Thus the Royal Astronomical 

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