Introduction: a "mathematical jewel"On March 21, 1930, readers of the Belvidere Daily Republican, a local newspaper from Belvidere, Illinois, were presented with a short note titled "400 Years Old." Prominently illustrating the note was a photograph of the astronomer Philip Fox staging an observation with a large antique astronomical instrument (Figure 1.1). The caption read "Professor Fox, chief of the Adler Planetarium at Chicago, using an astrolabe, an instrument to tell latitude, longitude, and time of the day from the sun, which was made in England in the year 1500 and is still workable. It has been acquired for the collection of astronomical and navigational instruments of the planetarium" (Belvidere Daily Republican, March 21, 1930). 1 The instrument, a planispheric astrolabe, 2 was part of a lot containing roughly 500 instruments, which benefactor Max Adler and Philip Fox had secured for the Adler Planetarium from Anton W. M. Mensing (1866Mensing ( -1936, a Dutch antiques dealer based in Amsterdam. Though sold and shipped from the Netherlands, the so-called Mensing collection included scientific instruments originating in places as varied as the already mentioned England plus Germany, Italy, France, and other European countries, as well as North Africa, the Middle East, and the Indian subcontinent. The bulk of the collection as such had previously traveled from France. It was amassed by the Paris-based collector and dealer Raoul Heillbroner, then sequestered by the French government during the Great War (Heilbronner was German, and thus regarded as foe), and finally purchased by Mensing, for most of it to end up in Chicago.The acquisition of the Mensing collection established the core of the holdings of the Adler Planetarium, which opened to the public on May 12, 1930, becoming the first institution of its kind in the Western Hemisphere (Raposo, 2019;Taub, 2005). The Mensing collection was used to seal the character of the Planetarium as a proper museum of astronomy, which was