At each of these points the communication with Earth is made
by a copper plate 2 feet square. The straight line connecting the
extreme points of the first station intersects that connecting the two
points of the second station, nearly at right angles, and at little
distance from the Observatory.--The question of dependence of the
measurable amount of sidereal aberration upon the thickness of glass
or other transparent material in the telescope (a question which
involves, theoretically, one of the most delicate points in the
Undulatory Theory of Light) has lately been agitated on the Continent
with much earnestness. I have calculated the curvatures of the lenses
of crown and flint glass (the flint being exterior) for correcting
spherical and chromatic aberration in a telescope whose tube is filled
with water, and have instructed Mr Simms to proceed with the
preparation of an instrument carrying such a telescope. I have not
finally decided whether to rely on Zenith-distances of gamma Draconis
or on right-ascensions of Polaris. In any form the experiment will
probably be troublesome.--The transit of Mercury on 1868, Nov. 4th,
was observed by six observers. The atmospheric conditions were
favourable; and the singular appearances usually presented in a
planetary transit were well seen.
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