likely
to maintain, a practical monopoly in the construction of heliometers.
That completed by them for the observatory of Yale College in 1882
leaves so little to be desired as to show excellence not to be the
exclusive result of competition. In mere size it does not indeed take
the highest rank. Its aperture is of only six inches, while that of
the Oxford heliometer is of seven and a half; but the perfection of
the arrangements adapting it to the twofold function of equatorial and
micrometer stamps it as a model not easy to be surpassed. 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. They constitute an undoubted cluster; that is to
say, they are really, and not simply in appearance, grouped together
in space, so as to fall under the sway of prevailing mutual
influences. And since there is, perhaps, no other stellar cluster so
near the sun, the chance of perceptible displacements among them in a
moderate lapse of time is greater than in any other similar case.
Authentic data regarding them, besides, have now been so long garnered
that their fruit may confidently be expected at least to begin to
ripen.

Dr. Elkin determined, accordingly, to repeat the survey of the
Pleiades e