e (under spectroscopic scrutiny) those very rays which now seem
wanting. There would be a spectrum of multitudinous bright lines,
instead of a rainbow-tinted spectrum crossed by multitudinous dark
lines. It is, indeed, only by contrast that the dark lines appear dark,
just as it is only by contrast that the solar spots seem dark. Not only
the penumbra but the umbra of a sun-spot, not only the umbra but the
nucleus, not only the nucleus but the deeper black which seems to lie at
the core of the nucleus, shine really with a lustre far exceeding that
of the electric light, though by contrast with the rest of the sun's
surface the penumbra looks dark, the umbra darker still, the nucleus
deep black, and the core of the nucleus jet black. So the dark lines
across the solar spectrum mark where certain rays are relatively faint,
though in reality intensely lustrous. Conceive another change than that
just imagined. Conceive the sun's globe to remain as at present, but the
atmosphere to be excited to many times its present degree of light and
splendour: then would all these dark lines become bright, and the
rainbow-tinted background would be dull or even quite dark by contrast.
This is not a mere fancy. At times, local disturbances take place in the
sun which produce just such a change in certain constituents of the
sun's atmosphere, causing the hydrogen, for example, to glow with so
intense a heat that, instead of its lines appearing dark, they stand out
as bright lines. Occasionally, too, the magnesium in the solar
atmosphere (over certain limited regions only, be it remembered) has
been known to behave in this manner. It was so during the intensely hot
summer of 1872, insomuch that the Italian observer Tacchini, who noticed
the phenomenon, attributed to such local overheating of the sun's
magnesium vapour the remarkable heat from which we then for a time
suffered.

Now, the stars are suns, and the spectrum of a star is simply a
miniature of the solar spectrum. Of course, there are characteristic
differences. One star has more hydrogen, at least more hydrogen at work
absorbing its rays, and thus has the hydrogen lines more strongly
marked than they are in the solar spectrum. Another star shows the lines
of various metals more conspicuously, indicating that the glowing
vapours of such elements, iron, copper, mercury, tin, and so forth,
either hang more densely in the star's atmosphere than in our sun's, or,
being cooler, absorb