NATURE 
221 

THURSDAY, JANUARY 19, 1871 

THE MEDITERRANEAN ECLIPSE, 1870 
LOUD in Sicily, cloud in Spain, cloud in Africa. 
Such at first sight might seem to be the only result 
of all the observations made on the eclipsed sun of 1870 ; 
such the reception given by Nature to those who wooed 
her as she had never been wooed before, who approached 
her full of the rarest gifts which Science has placed at 
man’s disposal. 
But, after all, has the oracle been silent? I think not. 
Dare we, however, say that the great problem of the 
Corona, that one among the many still outstanding diffi- 
culties which the eclipse was invoked to settle, is settled ? 
This, perhaps, would be saying too much, but still, I 
think, a step in advance has been made. The oracle has 
spoken darkly perhaps, but it Zas spoken. 
Let me endeavour to put the question as it stood a few | 
weeks ago as briefly as possible. 
Beginning the story some few years back we find the 
corona, a halo of white light round the moon, with a height 
sometimes represented as equal to the moon’s diameter, 
sometimes more, sometimes less, with a border @ discrétion, 
—so much did the drawings vary—regarded as the solar 
atmosphere. 
Some thought the red prominences to be mountains, 
other observers called them clouds. 
The polariscope was brought up with a view of de- 
termining whether the corona shone by reflected light or 
not. The result of this new method of observation was 
doubtful. 
In the Indian eclipse of 1868 M. Janssen, by means of 
the spectroscope, still another aid, determined that the 
prominences were masses of hydrogen gas, but there was 
no final word about the corona. Major Tennant observed 
that its spectrum wascontinuous. Later in the same year 
Dr. Frankland and myself approximately determined the 
pressure of the prominence gases by means of the new 
method and laboratory experiments, and at once stated 
our conviction that the extensive corona which had been 
depicted and represented by Kirchhoff and others to be 
the solar atmosphere must be something else. This 
was our idea. I cannot quote our words, for I am 
writing in Venice and have no copies of our paper 
with me. 
In the American Eclipse of 1869 the problem was 
advanced considerably, perhaps even more considerably 
than we can yet form an idea of, writing as we must 
still do doubtfully. I do not refer to the drawings, for 
they varied considerably, but to the observation that the 
light of the outer corona, like that of the prominences, 
gave a bright-line spectrum. But as at least some of the 
observers gave positions doubtfully, “near C” and “near 
E,” I thought that the explanation was still possible which 
regarded the corona as of terrestrial origin ; that is, which 
assumed it to be an appearance due to the presence of 
light in our own atmosphere. The problem was one of 
such difficulty that there seemed a possibility that, by 
some unexplained cause, some of the solar light might 
be diffused and beat out of its course, and then, mixing 
VOL. III. 
up with the light of the chromosphere, give us a 
sort of continuous spectrum, with the hydrogen bright 
| lines superposed upon it; in other words, that as the eye 

perceives a bright, irregular region or glare around the un- 
eclipsed sun, an effect due to our atmosphere, so also the 
eye might perceive a bright, irregular region or glare 
round the wneclipsed chromosphere during eclipses, due 
also to our atmosphere. 
One word here about the Chromosphere, the name I 
have given to the bright-line-giving region outside the 
photosphere. It has long been clear that the spectroscopic 
method of observing it when the sun is not eclipsed is not 
totally effective ; that is to say, that we only see a per- 
centage of it—perhaps only a relatively small per-centage— 
| but the glowing prominences, that is, those in which there 
| is no evidence of the rapid motion of ejection from the 
sun, the ejection taking place at all angles from the line 
of sight, afford evidence that there is probably a layer of 
cooler hydrogen susceptible of being rendered visible 
above the ordinary level. Now as these prominences may 
be 5’ high, it is not unreasonable to suppose that the chro- 
mosphere may even extend to that distance, or even a 
little beyond it.* 
Hence it was that in the Instructions to Observers, 
drawn up by Professor Stokes and myself, and approved 
by the Organising Committee for this 1870 Eclipse, it is 
stated that— 
“The PRINCIPAL OBJECT to be obtained is to deter- 
mine whether it is possible to differentiate the outer 
layers of irregular outline and the streamers (of the 
corona) from a stratum, say some 5’ or 6’ high, round the 
sun, which may possibly be the limit of the gaseous en- 
velopes above the photosphere.” 
The spectroscopic observers, therefore, were enjoined— 
a. “To determine the actual height of the chromo- 
sphere as seen with an eclipsed sun; that is, when the 
atmospheric illumination, the effect of which is doubtless 
only partially got rid of by the Janssen-Lockyer method, 
is removed. If the method were totally effective, the C 
line, the line of high temperature, should hardly increase 
in height ; but there can be little doubt that the method 
is not totally effective, so the increase in height should 
be carefully noted.” 
6. “ To determine if there exists cooler hydrogen above 
and around the vividly incandescent layers and pro- 
minences.” 
And the polarisers— 
“To examine a detached and selected part of the 
corona about 6’ from the limb of the sun, and say about 
8’ in diameter.” 
* Here is what I wrote on this point a year ag>:—‘‘ [ next come to the 
obliterating effect of the illuminat’on of our atmosphere on the spectrum 
of the chromosphere. This is considerable ; in fact, the evidences of it are 
very much stronger than one could have wished, but hardly more decided 
than I had anticipated. Professor Winlock’s evidence on this point, in a 
letter to myself, is as follows :—* I examined the principal protuberance be- 
fore, during, and after totality. I saw three lines (C, near D, and F) before 
and after totality, and eleven during totality; eight were instantly extin- 
guished on the first appearance of sunlight”” This effect was observed 
with two flint prisms and 7 inches aperture. Professor Young, with five 
prisms of 45° and 4 inches aperture, found the same result in the part of the 
spectrum he was examining at the end «f the totality He writes :—‘I had 
just completed the measurements of 2602, when the totality ended. 7hzs dime 
disappeared instantly, but 2796 [the hydrogen line near G] was nearly a 
minute in resuming its usual faintness.’ These observations [ consider 
among the most important ones made during the eclipse ; for they show most 
unmistakeablv that, as I have already reported to the Secretary of the 
Government-Grant Committee, the new method to be employed under the 
best conditions must be used with large apertures and large dispersion,” 
(Proc. R. S. 1870, p, 181.) 
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