This tonometer is an extraordinary feat of engineering and medical precision and is the direct evolution of Vierordt's idea, put in practice by Von Recklinghausen in 1905 (thank you Fritz-Dieter Söhn for the history lesson). It goes to show that there are in fact very few evolutions, and that by definition most, if not all, innovations are in fact natural evolutions of previous technologies. Purchased in Strasbourg in 2017.