a
quicker rate, when an "image of the sun's surface, with the faculae and
spots, is formed on the plate exactly within the image of the
chromosphere formed during the first exposure. The whole operation,"
Professor Hale continues, "is completed in less than a minute, and the
resulting photographs give the first true pictures of the sun, showing
all of the various phenomena at its surface."[609] Most of these novel
researches were, by a remarkable coincidence, pursued independently and
contemporaneously by M. Deslandres, of the Paris Observatory.[610]

The ultra-violet prominence spectrum was photographed for the first time
from an uneclipsed sun, in June, 1891, at Chicago. Besides H and K, four
members of the Huggins-series of hydrogen-lines imprinted themselves on
the plate.[611] Meanwhile M. Deslandres was enabled, by fitting quartz
lenses to his spectroscope, and substituting a reflecting for a
refracting telescope, to get rid of the obstructive action of glass upon
the shorter light-waves, and thus to widen the scope of his inquiry into
the peculiarities of those derived from prominences.[612] As the result,
not only all the nine white-star lines were photographed from a
brilliant sun-flame, but five additional ones were found to continue the
series upward. The wave-lengths of these last had, moreover, been
calculated beforehand with singular exactness, from a simple formula
known as "Balmer's Law."[613] The new lines, accordingly, filled places
in a manner already prepared for them, and were thus unmistakably
associated with the hydrogen-spectrum. This is now known to be
represented in prominences by twenty-seven lines,[614] forming a kind of
harmonic progression, only four of which are visibly darkened in the
Fraunhofer spectrum of the sun.

PLATE I.

[Illustration: Photographs of the Solar Chromosphere and Prominences.

Taken with the Spectroheliograph of the Kenwood Observatory, Chicago, by
Professor George E. Hale.]

The chemistry of "cloud-prominences" is simple. Hydrogen, helium, and
calcium are their chief constituents. "Flame-prominences," on the other
hand, show, in addition, the characteristic rays of a number of metals,
among which iron, titanium, barium, strontium, sodium, and magnesium are
conspicuous. They are intensely brilliant; sharply defined in their
varying forms of jets, spikes, fountains, waterspouts; of rapid
formation and speedy dissolution, seldom attaining to the vast
dimensions of