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.