After this had been done he commenced the rough grinding. Using the large
checkerboard tool, steel shot, and levigated emery Dr. Peate ground out
a rough hollow. This took only a few days. George Howard stated that the
depth of the concavity was about ⅝ inch and the shape correct to within
about 1/10,000 inch. The calculated concavity of the mirror would be
6/10 inch. Peate evidently used the usual method in polishing the large
mirror, that is, he covered the tool face with pitch and used rouge (iron
oxide) as the abrasive. This method had been used for many years before
this time and is still in use today.
The figuring, which consists of removing high spots to achieve a truly
parabolic contour, probably took the longest time to complete. A mirror
must be continually tested as this polishing is being done, and since
the polishing warms the glass and distorts its shape, it is necessary to
allow a long time for the glass to cool before it can be tested. Peate
estimated that polishing and figuring the mirror took 750 hours.[36]
We do not have a really accurate account of how he tested the mirror.
Unfortunately none of the eyewitnesses to these tests had any knowledge
of optics or of standard testing procedure. The information of those who
had such knowledge is all at least secondhand and sometimes even more
remote. J. W. Fecker, successor to Brashear,[37] who was one of a group
that examined the mirror in 1923, states that Peate did not use the knife
edge test but that he did use a pin with a hole in its head in one of the
tests used at that time.
A variety of different tests and diversions with the mirror have been
reported. Dr. Peate would entertain visitors in various ways. One of
these was to train the mirror on an apple orchard in a valley a few miles
away. In another Peate would pull out one of his whiskers and hang it
on a fence nearly a quarter of a mile away. Peate himself tells of the
time spent in testing the mirror, but does not go into detail about the
procedure. He does mention a testing table that stood about 75 feet away
from the revolving table on which the mirror rested. He says further that
the mirror was tested “in all ways known, in the shop and on a pin and a
watch dial a thousand feet distant.” Of these only the pin test seems to
have been a conventional one.[38]
After the polishing, the mirror was silvered. Said Peate: “It was
silvered and tried on the heavens in the starless region under Corvus,
and under the very imperfect management of the mirror on telescopic
stars, the report was as good as could be expected.”[39] Dr. Peate must
have spent some time testing it on the stars. The mirror was evidently
completed sometime late in summer of 1897, and when Peate was satisfied
that it was as perfect as possible, he made arrangements to send it to
American University. He also designed the shipping case to protect it
on the trip to Washington. It is described in the University paper as
follows:[40]
Public-domain text, read in full here on John Shaqi.
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