spectrum is not uniformly darkened as it would be if the
absorption were caused by floating particles. In the course of
examination of many large and quiescent spots, he perceived that the
middle green part of the spectrum was crossed by countless fine, dark
lines, generally touching each other, but here and there separated by
bright intervals. Each line is thicker in the middle (corresponding to
the centre of the spot) and tapers to a fine thread at each end; indeed,
most of these lines can be traced across the spectrum of the penumbra
and out on to that of the solar surface. The absorption would therefore
seem to be caused by gases at a much lower temperature than that of the
gases present outside the spot.

In the red and yellow parts of the spot-spectrum, which have been
specially studied for many years by Sir Norman Lockyer at the South
Kensington Observatory, interesting details are found which confirm this
conclusion. Many of the dark lines are not thicker and darker in the
spot than they are in the ordinary sun-spectrum, while others are very
much thickened in the spot-spectrum, such as the lines of iron, calcium,
and sodium. The sodium lines are sometimes both widened and doubly
reversed--that is, on the thick dark line a bright line is superposed.
The same peculiarity is not seldom seen in the notable calcium lines H
and K at the violet end of the spectrum. These facts indicate the
presence of great masses of the vapours of sodium and calcium over the
nucleus. The observations at South Kensington have also brought to light
another interesting peculiarity of the spot-spectra. At the time of
minimum frequency of spots the lines of iron and other terrestrial
elements are prominent among the most widened lines; at the maxima these
almost vanish, and the widening is found only amongst lines of unknown
origin.

The spectroscope has given us the means of studying other interesting
features on the sun, which are so faint that in the full blaze of
sunlight they cannot be readily observed with a mere telescope. We can,
however, see them easily enough when the brilliant body of the sun is
obscured during the rare occurrence of a total eclipse. The conditions
necessary for the occurrence of an eclipse will be more fully
considered in the next chapter. For the present it will be sufficient to
observe that by the movement of the moon it may so happen that the moon
completely hides the sun, and thus for certain parts of th