CHAPTER IX.
AT GREENWICH OBSERVATORY FROM JANUARY 1ST, 1876,
TO HIS RESIGNATION OF OFFICE ON AUGUST 15TH,
1881.
1876
"At the door from the Front Court to the staircase of the Octagon Room
(the original entrance to the Observatory as erected by Sir
Christopher Wren), a small porch-shelter has been often desired. I
proposed to fix there a fan-roof of quadrantal form, covering the
upper flat stone of the external steps.--On a critical examination of
the micrometer-screws of the Transit Circle it was found that the
corrections, which range from -1 deg.38" to +0 deg.76", indicate considerable
wear in the screws; and it was found that as much as one-hundreth part
of an inch had been worn away from some of the threads. The old screws
were consequently discarded, and new ones were made by Mr Simms.--The
adjustment of the Spectroscope has occupied a great deal of
attention. There was astigmatism of the prisms; and false light
reflected from the base of the prisms, causing loss both of light and
of definition. The latter defect was corrected by altering the angles,
and then astigmatism was corrected by a cylindrical lens near the
slit.
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