Steel has
been almost exclusively used in the mounting. Recommended as the
material for the objective cell by its quality of changing volume
under variations of temperature nearly _paripassu_ with glass, its
employment was extended to the telescope tube and other portions of
the mechanism. The optical part of the work was done by Merz, Alvan
Clark having declined the responsibility of dividing the object lens.
Its segments are separable to the extent of 2 deg., and through the
contrivance of cylindrical slides (originally suggested by Bessel)
perfect definition is preserved in all positions, giving a range of
accurate measurement just six times that with a filar micrometer.
(Gill, "Encyc. Brit.," vol. xvi., p. 253; Fischer, _Sirius_, vol.
xvii., p. 145.)
This beautiful engine of research was in 1883 placed in the already
practiced and skillful hands of Dr. Elkin. He lost no time in fixing
upon a task suited both to test the powers of the new instrument and
to employ them to the highest advantage.
The stars of the Pleiades have, from the earliest times, attracted the
special notice of observers, whether savage or civilized. Hence, on
the one hand, their prominence in stellar mythology all over the
world; on the other, their unique interest for purposes of scientific
study and comparison.
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