h velocities even so great as from 500 to
600 miles a second. It has been noticed that the eruptive prominences
are mostly found in those portions of the sun where spots usually
appear, namely, in the regions near the solar equator. The quiescent
prominences, on the other hand, are confined, as a rule, to the
neighbourhood of the sun's poles.

Prominences were at first never visible except during total eclipses of
the sun. But in the year 1868, as we have already seen, a method of
employing the spectroscope was devised, by means of which they could be
observed and studied at any time, without the necessity of waiting for
an eclipse.

A still further development of the spectroscope, the
_Spectroheliograph_, an instrument invented almost simultaneously by
Professor Hale and the French astronomer, M. Deslandres, permits of
photographs being taken of the sun, with the light emanating from _only
one_ of its glowing gases at a time. For instance, we can thus obtain a
record of what the glowing hydrogen alone is doing on the solar body at
any particular moment. With this instrument it is also possible to
obtain a series of photographs, showing what is taking place upon the
sun at various levels. This is very useful in connection with the study
of the spots; for we are, in consequence, enabled to gather more
evidence on the subject of their actual form than is given us by their
highly foreshortened appearances when observed directly in the
telescope.

V. CORONA. (Latin, _a Crown_.)

This marvellous halo of pearly-white light, which displays itself to our
view only during the total phase of an eclipse of the sun, is by no
means a layer like those other envelopments of the sun of which we have
just been treating. It appears, on the other hand, to be composed of
filmy matter, radiating outwards in every direction, and fading away
gradually into space. Its structure is noted to bear a strong
resemblance to the tails of comets, or the streamers of the aurora
borealis.

Our knowledge concerning the corona has, however, advanced very slowly.
We have not, so far, been as fortunate with regard to it as with regard
to the prominences; and, for all we can gather concerning it, we are
still entirely dependent upon the changes and chances of total solar
eclipses. All attempts, in fact, to apply the spectroscopic method, so
as to observe the corona at leisure in full sunlight in the way in which
the prominences can be observed, have