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The BSB released a facsimile this week of VD16 M 6547, Kalender / vonn allerhandt artzney durch anzeigung der siben Planeten / zwöff Zeychen / und der XXXVi. Bilder deß himels / sampt iren Figuren und gestirnen (Augsburg: Heinrich Steiner, 1539), attributed to the fifteenth-century astronomer Regiomontanus. I haven't inspected the contents carefully, but I would guess at first sight that relatively little of this compilation is actually derived from the works of Regionmontanus. Rather than a calendar or almanac, the work is most similar to the "planet books," explanations of the principles of astronomy and (mostly) astrology that were popular in the sixteenth century.
Imagine my surprise when I found that the BSB had covered part of one page with a crumpled piece of black paper.
Then I turned the page and the mystery only deepened.
But I would not so easily be dissuaded. While recovering the secrets of fol. 46r-v in VD16 M 6547 may be beyond my skill, I knew that similar editions of the same work may have already been digitized. How deep did the conspiracy go?
Turning to VD16, I quickly discovered that another edition was available, VD16 M 6545, published in Strasbourg in 1537. I quickly located the arcane knowledge that the BSB did not want me to see:
catalog would help?
And there we find the explanation: "Ex. unvollst.: Blatt XLVI mit starkem Textverlust beschnitten."
In other words, I had been staring at an optical illusion. I wasn't seeing a black piece of paper placed over the page, but a black piece of paper that had been placed behind the page to show where a section had been cut out at some time in the past.