of the ruling is to produce by what is
known as diffraction the required breaking up of the beam of light into
its constituent parts.

[Illustration: PLATE IV.

SOLAR PROMINENCES.

(DRAWN BY TROUVELOT AT HARVARD COLLEGE, CAMBRIDGE, U.S., IN 1872.)]

Majestic indeed are the proportions of some of those mighty prominences
which leap from the luminous surface; yet they flicker, as do our
terrestrial flames, when we allow them time comparable to their gigantic
dimensions. Drawings of the same prominence made at intervals of a few
hours, or even less, often show great changes. The magnitude of the
displacements that have been noticed sometimes attains many thousands of
miles, and the actual velocity with which such masses move frequently
exceeds 100 miles a second. Still more violent are the convulsions when,
from the surface of the chromosphere, as from a mighty furnace, vast
incandescent masses of gas are projected upwards. Plate IV. gives a view
of a number of prominences as seen by Trouvelot at Harvard College
Observatory, Cambridge, U.S.A. Trouvelot has succeeded in exhibiting in
the different pictures the wondrous variety of aspect which these
objects assume. The dimensions of the prominences may be inferred from
the scale appended to the plate. The largest of those here shown is
fully 80,000 miles high; and trustworthy observers have recorded
prominences of an altitude even much greater. The rapid changes which
these objects sometimes undergo are well illustrated in the two sketches
on the left of the lowest line, which were drawn on April 27th, 1872.
These are both drawings of the same prominence taken at an interval no
greater than twenty minutes. This mighty flame is so vast that its
length is ten times as great as the diameter of the earth, yet in this
brief period it has completely changed its aspect; the upper part of the
flame has, indeed, broken away, and is now shown in that part of the
drawing between the two figures on the line above. The same plate also
shows various instances of the remarkable spike-like objects, taken,
however, at different times and at various parts of the sun. These
spikes attain altitudes not generally greater than 20,000 miles, though
sometimes they soar aloft to stupendous distances.

We may refer to one special object of this kind, the remarkable history
of which has been chronicled by Professor Young. On October 7th, 1880, a
prominence was seen, at about 10.30 a.m., on the s