Take two level pieces of glass, (plate glass is the best,) about three
inches long and four wide, exactly of the same size; lay one on theother, and leave a space between them by pasting a piece of card, or two or three small pieces of thick paper, at each corner.
Join these glasses together at the edges by a composition of lime
slaked by exposure to the air, and white of an egg. Cover all theedges of these glasses with parchment or bladder, except at one end,which is to be left open to admit the following composition.
Dissolve, by a slow fire, six ounces of hogs'-lard, with half an ounce
of white wax; to which you may add an ounce of clear linseed oil.
This must be poured in a liquid state, and before a fire, between the
glasses, by the space left in the sides, and which you are then toclose up. Wipe the glasses clean, and hold them before the fire, to
see that the composition will not run out at any part.
Then fasten with gum a picture or print, painted on very thin paper,
with its face to one of the glasses, and, if you like, you may fix thewhole in a frame.
While the mixture between the glasses is cold, the picture will be
quite concealed, but become transparent when held to the fire; and, as the composition cools, it will gradually disappear.