y 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. A similar determination has also been made
in the case of Jupiter, with equally satisfactory results.--An
Electrometer on Sir William Thomson's plan, for continuous
photographic registration of atmospheric electricity has been received
from Mr White of Glasgow. It was mounted in December.--The computation
of the photographic records of the barometer from 1854 to 1873 has so
far advanced that we can assert positively that there is no trace of
lunar tide in the atmosphere; but that there is a strongly marked
semi-diurnal solar tide, accompanied with a smaller diurnal tide. We
are at present engaged in comparing the barometric measures with the
directions of the wind.--Regarding the distribution of the printed
observations: There is no extensive wish for separate magnetic
observations, but general magnetic results are in great demand,
especially for mining operations, and to meet this a map of magnetic
declination is furnished in the newspaper called the 'Colliery
Guardian.'--As regards the operations for the Transit of Venus: The
computing staff has by degrees been reduced to two junior computers
within the Observatory; and one or two computers external to the
Observatory, who are employed on large groups of systematic
calculations. The principal part of the calculations remaining at the
date of the last Report was that applying to the determination of the
geographical longitudes of fundamental stations. At the moment of my
writing, the last of these (the longitude of Observatory Bay,
Kerguelen) is not absolutely finis