The rough grinding was effected
by means of a tool of cast iron about the same size as the mirror,
which was moved by suitable machinery both backwards and forwards,
and round and round, plenty of sand and water being supplied between
the mirror and the tool to produce the necessary attrition. As the
process proceeded and as the surface became smooth, emery was used
instead of sand; and when this stage was complete, the grinding tool
was removed and the polishing tool was substituted. The essential
part of this was a surface of pitch, which, having been temporarily
softened by heat, was then placed on the mirror, and accepted from
the mirror the proper form. Rouge was then introduced as the
polishing powder, and the operation was continued about nine hours,
by which time the great mirror had acquired the appearance of highly
polished silver. When completed, the disc of speculum metal was
about six feet across and four inches thick. The depression in the
centre was about half an inch. Mounted on a little truck, the great
speculum was then conveyed to the instrument, to be placed in its
receptacle at the bottom of the tube, the length of which was sixty
feet, this being the focal distance of the mirror. Another small
reflector was inserted in the great tube sideways, so as to direct
the gaze of the observer down upon the great reflector.
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