The New Transit Circle was to be erected in the Circle
Room, and considerable arrangement was necessary for continuing the
Circle Observations with the existing instruments, whilst the new
instrument was under erection. When the new Transit is completely
mounted, the old Transit Instrument may be removed, and the Transit
Room will be free for any other purpose. I propose to take it as
Private Room for the Astronomer Royal.--On May 12th I made my first
proposal of the Reflex Zenith Tube. The principle of it is as follows:
Let the micrometer be placed close to the object-glass, the frame of
the micrometer being firmly connected with the object-glass cell, and
a reflecting eye-piece being used with no material tube passing over
the object-glass: and let a basin of quicksilver be placed below the
object-glass, but in no mechanical connection with it, at a distance
equal to half the focal length of the object-glass. Such an
instrument would at least be free from all uncertainties of twist of
plumb-line, viscosity of water, attachment of upper plumb-line
microscope, attachment of lower plumb-line microscope, and the
observations connected with them: and might be expected, as a result
of this extreme simplicity, to give accurate results.
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