The upper glass or speculum
always becomes concave, and the under glass or tool convex.
Work with straight strokes 5 or 6 in. in length; after working 5
hours hold the speculum in the sunshine and throw the rays of the
sun onto a paper; where the rays come to a point gives the focal
length. If the glass is not ground enough to bring the rays to a
point within 5 ft., the coarse grinding must be continued, unless
a longer focal length is wanted.
Have ready six large dishes, then take 2 lb. flour emery and mix
in 12 qt. of water; immediately turn the water into a clean dish
and let settle 30 seconds; then turn it into another dish and let
settle 2 minutes, then 8 minutes, 30 minutes and 90 minutes, being
careful not to turn off the coarser emery which has settled. When
dry, turn the emery from the 5 jars into 5 separate bottles, and
label. Then take a little of the coarsest powder, wetting it to
the consistency of cream, and spread on the glass, work as before
(using short straight strokes 1-1/2 or 2 in.) until the holes in
the glass left by the grain emery are ground out; next use the
finer grades until the pits left by each coarser grade are ground
out.
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