ing solar prominences without the help of an
eclipse must therefore be shared between the two astronomers.

When a spectroscope is pointed to the margin of the sun so that the slit
is radial, certain short luminous lines become visible which lie exactly
in the prolongation of the corresponding dark lines in the solar
spectrum. From due consideration of the circumstances it can be shown
that the gases which form the prominences are also present as a
comparatively shallow atmospheric layer all round the great luminary.
This layer is about five or six thousand miles deep, and is situated
immediately above the dense layer of luminous clouds which forms the
visible surface of the sun and which we call the photosphere. The
gaseous envelope from which the prominences spring has been called the
chromosphere on account of the coloured lines displayed in its spectrum.
Such lines are very numerous, but those pertaining to the single
substance, hydrogen, predominate so greatly that we may say the
chromosphere consists chiefly of this element. It is, however, to be
noted that calcium and one other element are also invariably present,
while iron, manganese and magnesium are often apparent. The remarkable
element, of which we have not yet mentioned the name, has had an
astonishing history.

During the eclipse of 1868 a fine yellow line was noticed among the
lines of the prominence spectrum, and it was not unnaturally at first
assumed that it must be the yellow sodium line. But when careful
observations were afterwards made without hurry in full sunshine, and
accurate measures were obtained, it was at once remarked that this line
was not identical with either of the components of the double sodium
line. The new line was, no doubt, quite close to the sodium lines, but
slightly towards the green part of the spectrum. It was also noticed
there was not generally any corresponding line to be seen among the dark
lines in the ordinary solar spectrum, though a fine dark one has now and
then been detected, especially near a sun-spot. Sir Norman Lockyer and
Sir Edward Frankland showed that this was not produced by any known
terrestrial element. It was, therefore, supposed to be caused by some
hitherto unknown body to which the name of _helium_, or the sun element,
was given. About a dozen less conspicuous lines were gradually
identified in the spectrum of the prominences and the chromosphere,
which appeared also to be caused by this same myster