en, sodium,
carbon, nickel, magnesium, cobalt, aluminium, chromium, strontium,
manganese, copper, zinc, cadmium, silver, tin, lead, potassium. Some of
the elements which are of the greatest importance on the earth would
appear to be missing from the sun. Sulphur, phosphorus, mercury, gold,
nitrogen may be mentioned among the elements which have hitherto given
no indication of their being solar constituents.

It is also possible that the lines of a substance in the sun's
atmosphere may be so very bright that the light of the continuous
spectrum, on which they are superposed, is not able to "reverse"
them--_i.e._ turn them into dark lines. We know, for instance, that the
bright lines of sodium vapour may be made so intensely bright that the
spectrum of an incandescent lime-cylinder placed behind the sodium
vapour does not reverse these lines. If, then, we make the sodium lines
fainter, they may be reduced to exactly the intensity prevailing in that
part of the spectrum of the lime-light, in which case the lines, of
course, could not be distinguished. The question as to what elements are
really missing from the sun must therefore, like many other questions
concerning our great luminary, at present be considered an open one. We
shall shortly see that an element previously unknown has actually been
discovered by means of a line representing it in the solar spectrum.

Let us now return to the sun-spots and see what the spectroscope can
teach us as to their nature. We attach a powerful spectroscope to the
eye-end of a telescope in order to get as much light as possible
concentrated on the slit; the latter has therefore to be placed exactly
at the focus of the object-glass. The instrument is then pointed to a
spot, so that its image falls on the slit, and the presence of the dark
central part called the _umbra_ reveals itself by a darkish stripe which
traverses the ordinary sun-spectrum from end to end. It is bordered on
both sides by the spectrum of the _penumbra_, which is much brighter
than that of the umbra, but fainter than that of the adjoining regions
of the sun.

From the fact that the spectrum is darkened we learn that there is
considerable general absorption of light in the umbra. This absorption
is not, however, such as would be caused by the presence of volumes of
minute solid or liquid particles like those which constitute smoke or
cloud. This is indicated by the fact, first discovered by Young in
1883, that the