ead 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. When the two last grades are used shorten the strokes to less
than 2 in. When done the glass should be semi-transparent, and is
ready for polishing.

When polishing the speculum, paste a strip of paper 1-1/3 in. wide
around the convex glass or tool, melt 1 lb. of pitch and turn on
to it and press with the wet speculum. Mold the pitch while hot
into squares of 1 in., with 1/4-in. spaces, as in Fig. 1. Then
warm and press again with the speculum, being careful to have all
the squares touch the speculum, or it will not polish evenly. Trim
the paper from the edge with a sharp knife, and paint the squares
separately with jeweler's rouge, wet till soft like paint. Use a
binger to spread it on with. Work the speculum over the tool the
same as when grinding, using straight strokes 2 in. or less.

When the glass is polished enough to reflect some light, it should
be tested with the knife-edge test. In a dark room, set the
speculum against the wall, and a large lamp, L, Fig. 2, twice the
focal length away. Place a large sheet of pasteboard, A, Fig. 2,
with a small needle hole opposite the blaze, by the side of the
lamp, so the light

[Illustration: Detail of Telescope Construction]

from the blaze will shine onto the glass. Place the speculum S,
Fig. 2, so the rays from the needle hole will be thrown to the
left side of the lamp (facing the speculum), with the knife
mounted in a block of wood and edgeways to the lamp, as in K, Fig.
2. The knife should not be more than 6 in. from the lamp. Now move
the knife across the rays from left to right, and look at the
speculum with the eye on the right side of the blade. When the
focus is found, if the speculum is ground and polished evenly it
will darken evenly over the surface as the knife shuts off the
light from the needle hole. If not, the speculum will show some
dark rings, or hills. If the glass seems to have a deep hollow in
the center, shorter strokes should be used in polishing; if a hill
in the center, longer strokes. The polishing and testing done, the
speculum is ready to be silvered. Two glass or earthenware dishes,
large enough to hold the speculum and 2 in. deep, must be
procured. With pitch, cement a strip of board 8 in. long to the
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