152 
NATURE 
[ApRIL II, 1912 
of Vavau, and there the duration of totality was com- 
puted to be three minutes thirty-seven seconds, or 217 
seconds. 
In the earlier days of eclipse expeditions those who 
took part in them had to be content with eye observa- 
tions alone. The discovery of and rapid advance made 
in the sensitive photographic plate, and its successful 
application in 1860 to eclipse work, revolutionised 
eclipse programmes altogether, so that an abundance 
of facts may now be photographed in a brief interval 
of time, and these be examined at leisure at a less 
exciting moment. 
It is well to remember that many inquiries, which 
in the earlier days formed part of eclipse programmes, 
need attention no longer. Thus, for example, the 
corona was first thought to be the illuminated lunar 
atmosphere until observations proved it to be a solar 
appendage. Further, during eclipses the corona was 
supposed to be either quickly rotating or pulsating | 
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the main solar inquiries will be able to be conducted 
without waiting for their occurrence. 
In recent years, among the most important work of 
eclipse expeditions, that of the study of the form-and 
chemistry of the chromosphere and corona has taken 
first place. 
Even now the research on the chemistry of the 
chromosphere is in process of being divorced from 
eclipse work. This is due to the magnificent work 
that is being carried on at the Mount Wilson Solar 
Observatory with large-scale instruments. At that 
observatory the chromospheric spectrum has been 
photographed in full sunlight. The method employed, 
while surpassing in accuracy of wave-length measures 
those made from eclipse spectra, may in time equal; 
or even possibly exceed, them in detail. 
Thus the chemistry and form of the corona are 
practically the only large inquiries which are re- 
stricted to eclipses, and probably we may not have 
long to wait before even these form 
part of the daily routine of solar physics 
observatories situated in good observ- 
ing localities. 
Time will not permit me to tell you 
even briefly how the special results 
obtained during eclipses help the ad- 
vancement of solar and _ celestial 
physics. 
When it is remembered, however, 
that our sun has a temperature of about 
7ooo° at its surface, and perhaps 
several hundreds of thousands of de- 
grees at its centre—that the very sun- 
spots which appear to us as black 
spots on its surface are brighter than 
the brightest arc lamp—then the im- 
portance of the study of every attain- 
able part of this very effective group 
of furnaces in and out of eclipse is 
imperative for the advancement. of 
knowledge. 
If one be permitted to refer briefly 
to the progress of our knowledge of the 
form, origin, and chemistry of the 
corona, you are well aware that. its 
100 * ©2200 
Fic. 1.—Curves to show that while the different forms of the corona exhibit a regular variation, 
c 1 1 sun-spot areas and prominence trequency, it is the promin- 
ences (which, unlike the spo s, are not limited to any latitude) that are responsible for the 
corresponding in-time to those 
varying systematic changes of form of the corona. 
visibly, but subsequent observations have shown that 
during those times it is apparently as rigid and 
stationary as an Indian order suspended in the sky. 
The prominences, those ruddy, brilliant tree-lilke 
forms which appear during totality at the edge of the 
moon’s limb, were also considered as belonging to 
our satellite, until observations in 1860 demonstrated 
them as belonging to the sun. While we know that 
they are solar, there is even now no necessity to waste 
time during eclipses on either the study of their 
forms, positions, or chemistry. The reasons for this 
are that in 1868 a method was devised by which they 
can be individually studied visually any day when the 
sun shines, and in 1891 a means was afforded of 
photographing in a few minutes, on one plate, all the 
prominences situated on the sun’s limb. 
The solution of these and other problems which 
might be mentioned are gradually reducing the 
importance of observing eclipses, and it is well within 
the bounds of possibility that in the near future all 
NO. 2215, VOL. 89] 
shape is not the same at every eclipse, 
but that there seems to be a systematic 
ps | aa change going on, extending | over 
1300 wwe several years (Fig. .1). A study of 
these forms has shown that the 
changes repeat themselves about every 
eleven years, and since the mean daily 
areas of sun-spots are known to have 
a periodicity of this length of time, 
their close association is generally conceded. It 
happens, however, that when the coronal streamers 
are most prominent in highest solar latitudes, and 
when at the same epochs the mean daily spotted area 
is at a maximum, the mean latitude of the spotted 
area is very low, being only about 15°. Thus there 
seems reason to question the conclusion that sun-spots 
at such a low latitude can originate coronal streamers 
so distant as the solar poles. 
It must not be forgotten that a study of the fre- 
quency of solar prominences has disclosed the fact 
that not only have these phenomena a periodicity of 
about eleven years, synchronising exactly with the 
spotted area, but that when their frequency is at a 
maximum they are conspicuous at the highest solar 
latitudes. Thus there occur at the same time promin- 
ences and coronal streamers near the solar poles, a 
very possible and probable condition for cause and 
effect. 
In the eclipses of roo1 and 1905 several striking 
