Stevin’s Signa Hermetis, “Signs of Hermes,” the Decanic Spirits
“We are not in the position anymore to verify what exactly Stevin saw when he wrote of the monstrous figures, because these decorations perished in a fire twenty years later. On the basis of the written source material, however, it becomes possible to reconstruct what the “signs of Hermes” may have looked like. The zoomorphic creatures composed of various different animals described by the Flemish savant are most probably the depictions of certain planetary figures, the so-called decanic spirits.
These demons of Egyptian origin are supposed to rule the decans, arcs of ten degrees, thirty-six of which constitute the ecliptic. The artist may have had various different astrological models for his work, since the concept of decans— originally introduced in Europe through Arabic astral magic—was quite wide- spread; it appears not only in works of image magic of Arabic origin, such as the Picatrix, but also in the astrological writings of various Latin authors.
However, it is worth underlining that it is exactly the Picatrix which introduces the decans with the following words: “Modo sequuntur imagines Hermetis,” and it is exactly a Kraków copy of the Picatrix, which—singularly—not only describes the decanic spirits but even provides pictures of them.
Therefore, it is highly possible that the Signa Hermetis, after which the zoomorphic figures of the Wawel were modeled, were exactly the decanic figures of the Kraków Picatrix, extant in the collection of the University of Kraków, on the folios of the codex BJ 793.2″
–Benedek Lang, Unlocked Books: Manuscripts of Learned Magic in the Medieval Libraries of Central Europe, 2008. Pp. 79-80.