The inside of test tubes, bulbs, &c., are silvered by putting the solutions into them, no second vessel being then required.
Throughout the whole operation the most scrupulous cleanliness is the grand essential.
100 cc. are equal to rather more than 3½ fluid ounces.
- 1 gramme = 15·432 grains.
- 1 litre = 35¼ fluid ounces.
The plate of glass being thus carefully silvered is allowed to dry thoroughly, and is finally varnished with a good thick varnish, containing plenty of red lead, so that the back surface of the silver mirror has a smooth and red appearance, while the varnish protects the delicate film of metallic silver.
An ordinary photographic picture on glass is really represented by precipitated metallic silver, but the metal in this case is in minute particles, which do not shine or reflect light.
The silvered plate glass is now engraved in the following simple manner. Being placed in a support or rack against the wall, and quite upright, a chisel—or rather, a series of chisels—are drawn across the surface in straight lines, and perpendicular, by the use of a large T-square. Every time the chisel is drawn with pressure across the varnished back of the glass a portion of the silver is removed, leaving a straight line quite clear or transparent, and, in fact, laying bare the surface of the plate glass.
The lines were ruled in three degrees of comparison: thick, thicker, thickest; and considerable skill and experience—which no description can teach—were required to get these correctly engraved.
a b c d, the Plate Glass;
e straight lines engraved on silvered side and
gradually increasing in thickness from e to f.