252 
THE SOLAR ECLIPSE OF 1898 . 
a continuous band of colour, but a spectrum consisting of a number of 
bright lines, varying in position and intensity according to the different 
elements present in the vapour. On the other hand, if behind such a 
mass of vapour we place a body emitting a strong white light, we shall 
find that these vapours now absorb just those rays that they formerly 
emitted, and we shall get a spectrum of a bright background crossed 
by dark lines, these dark lines being in exactly the same position as 
the bright lines were, and therefore giving us information as to the 
particular vapours through which the white light has passed. 
This is exactly the case with the sun. We have the bright photo¬ 
sphere, emitting white light, above that we have a region of vapours 
which absorb certain rays and cause the dark lines in the spectrum. 
This has been called the “ reversing layer.” Merging into this layer 
is a region where we get the lighter vapours, such as hydrogen and 
the metals magnesium and calcium, which, on account of its colour 
when seen during a total eclipse, has been called the chromosphere. 
It is here that the well-known prominences or red flames have their 
origin, which we now know are composed principally of hydrogen 
and light metals and also the gas helium. These sometimes rise to an 
immense height above the solar surface. 
Formerly these prominences could only be seen during a total eclipse 
—the light of the sun quite overpowering them at other times—but wo 
are now enabled by means of the spectroscope to see and photograph 
them at any time the sun is visible. This is done by making use of 
the fact that their spectrum is one of bright lines, and if therefore we 
use a spectroscope of great dispersion we are able, so to speak, to 
throw away the light we do not want and select only that ray in 
which the prominence is visible. By an ingenious application of this 
principle, due to M. Deslandres, of Paris, and Professor Hale, of 
Chicago, it is possible to photograph the prominences, not only where 
they appear on the edge or limb of the sun, but actually also on the 
bright disc. 
Outside the prominence region is the corona, which can only be seen 
during a total eclipse, when it forms an aureole of silver white light 
extending often a distance of more than the sun's diameter from the 
limb. 
In former times a total eclipse was important because it enabled us 
to see the prominences. How this particular usefulness has dis¬ 
appeared, though it is still undoubtedly true that we can study this 
region better during an eclipse than at any other time. The corona, 
however, we can never see at ordinary times, and it is to this most 
remarkable and fascinating phenomenon that the attentions of eclipse 
observers are particularly directed. 
At the present time the observations made during eclipses are 
almost entirely photographic. The time available is so short and the 
conditions such that even the best observers seem to lose their heads. 
This is particularly the case with regard to drawings of the corona, 
and we may fairly say that these are of practically no value. The 
corona is not an easy object to photograph. The difference in intensity 
