A short history of two nineteenth-century German instruments at the Bologna Observatory: the 16-cm Steinheil refractor and the Ertel & Sohn meridian circle

Recent work to restore and set up the materials exhibited at the Museo della Specola of the University of Bologna provided an opportunity to review the history of two important German instruments from the mid-nineteenth century, an Ertel & Sohn meridian circle and a Steinheil refractor. Purchased by the Directors of the Bologna Observatory to revitalise local astronomical research, which had gradually declined over the years, both instruments have intriguing histories because, despite the fact that they were essentially underused, they also contributed to two important research projects. Lorenzo Respighi used one of them—the Ertel & Sohn meridian circle—for an experiment in physical optics related to the debate on whether light was undulatory or corpuscular, and it was essentially a forerunner of ‘water-filled telescopes’. The other, a Steinheil refractor to which a Tauber spectroscope was attached, was the largest and most important instrument used by the Italian expedition to India, organised by Pietro Tacchini to observe the transit of Venus across the Sun in 1874.