A method of making a photograph which can be made to appear at will is
thus described in _Les Recreations Photographiques_.
Take a convex watch crystal, V, or any similar larger glass if
desired--for instance, those used for colored photographs; clean the
glass well, place it perfectly level, convex side down, and fill it
even full with a mixture of white wax and hog's lard. When it has
solidified, apply to the back a flat glass plate, P, cut exactly to
the largest dimensions of the convex glass, secure the glasses
together with a strip, B, of gold-beater's skin, fastened by strong
glue as shown in the figure. Now mount a portrait, with the front
towards the convex glass, on the plate P. The combination is now
ready; by heating it the wax between the two glasses melts and becomes
transparent, allowing the portrait to be seen; on cooling it will lose
its transparence and the portrait will disappear.
[Illustration: FIG. 36.]