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tographing the sun’s surface in monochro-
matic light, such as the light given off as one
of the bright lines of hydrogen or of calcium,
by means of the ”Spectroheliograph.” The
spectroscope is placed with its slit in the
focus of an equatoreal telescope, pointed to
the sun, so that the circular image of the
sun falls on the slit. At the other end of
the spectroscope is the photographic plate.
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