The principal objects had relation to the fault
of definition when the telescope is pointed low (which I had remarked
on the preceding night), and were, to make ourselves acquainted with
the mechanism of the mirror's mounting generally, and to measure in
various ways whether the mirror actually does shift its place when the
telescope is set to different angles of elevation. For the latter we
found that the mirror actually does tilt 1/4 of an inch when the tube
points low. This of itself will not account for the fault but it
indicates that the lower part is held fast in a way that may cause a
strain which would produce the fault. These operations and reasonings
took a good deal of time. Lord Rosse is disposed to make an alteration
in the mounting for the purpose of correcting this possible strain.
THE CASTLE, PARSONSTOWN,
_1848, Aug. 31_.
The weather here is still vexatious: but not absolutely repulsive.
Yesterday morning Lord Rosse arranged a new method of suspending the
great mirror, so as to take its edgewise pressure in a manner that
allowed the springy supports of its flat back to act.
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