ates. To counteract its mischievous effects, a second
slit,[607] besides the usual narrow one in front of the collimator, was
placed on guard, as it were, behind the dispersing apparatus, so as to
shut out from the sensitised surface all light save that of the required
quality. The sun's image being then allowed to drift across the outer
slit, while the plate holder was kept moving at the same rate, the
successive sectional impressions thus rapidly obtained finally "built
up" a complete picture of the prominence. Another expedient was soon
afterwards contrived.[608] The H and K rays of calcium are always, as we
have seen, bright in the spectrum of prominences. They are besides fine
and sharp, while the corresponding absorption-lines in the ordinary
solar spectrum are wide and diffuse. Hence, prominences formed by the
spectroscope out of these particular qualities of violet light, can be
photographed entire and at once, for the simple reason that they are
projected upon a naturally darkened background. Atmospheric glare is
abolished by local absorption. This beautiful method was first realised
by Professor Hale in June, 1891.

A "spectroheliograph," consisting of a spectroscopic and a photographic
apparatus of special type, attached to the eye-end of an equatoreal
twelve inches in aperture, was erected at Kenwood in March, 1891; and
with its aid, Professor Hale entered upon original researches of high
promise for the advancement of solar physics. Noteworthy above all is
his achievement of photographing both prominences and faculae on the very
face of the sun. The latter had, until then, been very imperfectly
observed. They were only visible, in fact, when relieved by their
brilliancy against the dusky edge of the solar disc. Their convenient
emission of calcium light, however, makes it possible to photograph them
in all positions, and emphasises their close relationship to
prominences. The simultaneous picturing, moreover, of the entire
chromospheric ring, with whatever trees or fountains of fire chance to
be at the moment issuing from it, has been accomplished by a very simple
device. The disc of the sun itself having been screened with a circular
metallic diaphragm, it is only necessary to cause the slit to traverse
the virtually eclipsed luminary, in order to get an impression of the
whole round of its fringing appendages. And the record can be extended
to the disc by removing the screen, and carrying the slit back at