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Transparent Cards
Transparent cards (also called Hofzinser cards) should really have been called “translucent cards”, but the misnomer seems here to stay. Transparent cards appear normal, front and back, in everyday light, but they are gimmicked in a way that, when one is held up to a bright light, a card shines through its back that is different from the one on its face.
Seeing secret images in paper when held to the light began with Italian paper makers c. 1282, who developed the technique of watermarking their paper. Around the turn of the 19th century, the idea evolved to gluing a secret image between two otherwise normal layers of paper. In Trevor Denning's treatise on the subject, Translucent Playing-cards, 1976, Denning describes a set of zodiac cards from c. 1800 that when held to the light, displayed pornographic images. From there, similar pornographic images began appearing inside playing cards.
Pioneering this technology for conjuring, two magicians hit on the idea seemingly independently around the same time: Théodore Massy and Johann Hofzinser. The translucent playing card gaff appeared in Jean-Nicolas Ponsin's Nouvelle Magie Blanche Dévoileé, 1853, p. 158, in a trick titled Faire voir une carte, la poser sur la table, et la faire changer sans y toucher, n’ayant pas d’autres cartes dans les mains. In it, Ponsin credits the gaff to Théodore Massy.
Hofzinser also developed a similar card gaff, but as with most of his material, he didn’t publish the details himself. It didn’t appear in print under his name until Der Zauberspiegel, Vol. 3 No. 7, Jan. 1898, p. 106. But surviving letters between him and Carl von Pospischil, dated January 7th, 1853 (Magic Christian, Non Plus Ultra, Vol. 1, 2013, p. 272, or p. 247 of the German original from 1998), show Hofzinser mentioning his “transparent cards,” and in a later letter to Carlo Marchini, dated March 15th, 1872 (p. 290 of the English edition, or p. 276 of the German one), he explicitly claims the gaff as his own invention. Magic Christian dates Hofzinser’s gaffs as early as 1847, based on mentions in earlier letters to Pospischil (see this claim in Non Plus Ultra, Vol. 2, 2013, p. 72, or p. 64 of the German edition from 2004). But when these letters are read, it is seen that Hofzinser only mentions “special cards” (“gewisse Karten”) without any further information or context as to what these special cards are or what they do. Hofzinser made an incredible number of different types of card gaffs, so trying to claim that “special cards” means translucent cards is speculative.
It isn’t known when Massy developed his version, so who first created the gaff remains unclear. An indication that it was independent invention is that each man constructed his gaff in an entirely different way. Massy’s method was essentially what card gaffers still use today: he split cards and reassembled them in the familiar order of face, reversed face, and back, all glued together. Hofzinser instead hand-painted a face onto both sides of a single piece of paper and then glued a back onto one of those sides (the word “back” is loose here, since cards often had plain white backs in that period, so magicians would simply glue on a sheet of white paper; Ponsin explicitly mentions the blank back). It’s worth noting that the issue of Der Zauberspiegel mentioned above describes Hofzinser’s gaffs as being made by the card-splitting method. But this contradicts Hofzinser’s own letters.