The definition of the stars near
the zenith is extremely good: with a high power (as 800) they are
points or very nearly so--indeed I believe quite so--so that it is
clear that the whole light from the great 6-feet mirror is collected
into a space not bigger than the point of a needle. But in other
positions of the telescope the definition is not good: and we must
look to-day to see what is the cause of this fault. It is not a fault
in the telescope, properly so-called, but it is either a tilt of the
mirror, or an edge-pressure upon the mirror when the telescope points
lower down which distorts its figure, or something of that kind. So I
could not see Saturn at all well, for which I was sorry, as I could so
well have compared his appearance with what I have seen before. I
shall be very much pleased if we can make out what is the fault of
adjustment, and so correct it as to get good images everywhere. It is
evident that the figuring of the mirror, the polishing, and the
general arrangement, are perfectly managed.
THE CASTLE, PARSONSTOWN,
_1848, Aug. 30_.
Yesterday we were employed entirely about the Great Telescope,
beginning rather late.
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