414 
SCIENTIFIC SUMMARY. 
ASTRONOMY. 
D R. DE LA RUE’S address,* to Section A. of the British Association, was 
one of the features of the recent meeting. We quote from it the fol- 
lowing passages : — 
The Solar Corona. — “ The great problem of the solar origin of that portion 
of the corona which extends more than a million of miles beyond the body 
of the sun has been by the photographic observations of Colonel Tennant 
and Lord Lindsay in 1871 set finally at rest,” says Dr. De la Rue, u after 
having been the subject of a great amount of discussion for some years. 
The spectroscopic discovery in 1869 of the now famous green line, 1474 K, 
demonstrated undoubtedly the self-luminosity, and hence the solar origin of 
part of the corona. Those who denied the possibility of any extensive 
atmosphere above the chromosphere received the observation with great 
suspicion ; but in 1870 and again in 1871 it was fully verified. So far, 
therefore, the testimony of spectroscopic observations was in favour of the 
solar origin of the inner corona. Indeed the observations of 1871 have 
proved hydrogen to be also an essential constituent of the 1 coronal atmo- 
sphere,’ as Janssen proposes to call it — hydrogen at a lower temperature 
and density, of course, than in the chromosphere. Janssen was further so 
fortunate as to catch glimpses of some of the dark lines of the solar spec- 
trum in the coronal light, an observation which goes far to show that in the 
upper atmosphere of the sun there are also solid or liquid particles, like 
smoke or cloud, which reflect the sunlight from below. Many problems, 
however, even with reference to the admittedly solar part of the corona, are 
unsettled. The first relates to the nature of the substance which produces 
the line 1474 K. Since it coincides with a line in the spectrum of iron, it 
is by many considered due to that metal ; but then we must suppose either 
that iron vapour is less dense than hydrogen gas, or that it is subject to 
some peculiar solar repulsion which maintains it at its elevation, or other 
hypotheses may be suggested for explaining the fact. Since the line is one 
of the least conspicuous in the spectrum of iron and the shortest, and as 
none of the others are found associated with it in the coronal spectrum, it 
seems natural, as many have done, to assume at once that it is due to some 
* Dr. De la Rue on u Recent Astronomical Progress.” 
