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. [Illustration: A B C D, the Plate Glass; _e_ straight lines engraved on silvered side and gradually increasing in thickness from _e_ to _f_.] The engraved silver glass plate moved through a groove in the woodwork at the top of the chamber, and was supported below on a beautiful carriage, the wheels of which were covered with vulcanised indiarubber rings, and moved on a tramway below the floor of the room, perpendicularly. The glass could be made to slide at an angle of forty-five degrees, and as it always made a rumbling noise while moving, the music of the band concealed that defect. The ground plan of the apartment is shown on the opposite page. Some idea of the cost of making a full-sized apparatus, with hangings and curtains and engraved glass, may be gathered from the fact that the author’s outfit for Australia with a certain number of dresses cost £327 12s. 1d. Whilst the author was travelling through Australia Mr. James Walker, with his great inventive genius, made a further improvement, by which the concealed figure at K was done away with, and the whole apartment thrown open to the public gaze. This was done to illustrate a clever sketch written by Mr. Burnand called “Curried Prawns.” [Illustration: Ground plan of chamber. A A′A″A‴, floor of the apartment; B B, groove at an angle of 45°, in which the glass moved; A″ to C groove continued outside of the apartment used when the glass was moved away; E F G H, short flight of three or four steps, as the room must stand some distance from the floor to allow of carriage moving on tramway.