1877
"In April of this year I was much engaged on the subject of Mr Gill's
expedition to Ascension to observe for the determination of the
parallax of Mars at the approaching opposition of that planet.--A
large Direct-vision Spectroscope has been quite recently made by Mr
Hilger under Mr Christie's direction on a new plan, in which either
great dispersion or great purity of spectrum is obtained by the use of
'Half-prisms,' according as the incident pencil falls first on the
perpendicular or on the oblique face. In this Spectroscope either one
or two half prisms can be used at pleasure, according to the
dispersion required, and there is facility for increasing the train to
three or four half-prisms, though the dispersion with two only is
nearly double of that given by the large ten-prism Spectroscope. The
definition in this form of Spectroscope appears to be very fine.--At
the end of May 1876, spectroscopic determinations of the Sun's
rotation were made by observations of the relative displacement of the
Fraunhofer lines at the east and west limbs respectively. The results
are in close agreement with the value of the rotation found from
observations of Sun-spots.
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