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
back of the speculum, and lay the speculum face down in one of the
dishes; fill the dish with distilled water, and clean the face of
the speculum with nitric acid, until the water will stick to it in
an unbroken film.
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