During the seventeenth century, Europeans intensified their study of skin and skin color. For these early modern researchers, skin "color" meant dark skin, and its blackness demanded explanation in a way that whiteness never did. Europeans sought to locate, conceptually and physically, the blackness upon which new legal categories and sources of profit depended. This new "anatomy of blackness" led physicians and scholars to examine human skin through the microscope. Building on the work of Robert Boyle and Marcello Malpighi, the Dutch-German anatomist Johann Nicolas Pechlin (1646–1706) carefully observed and clearly located the physical cause of dark skin in the rete mucosum, a mesh of pigmented vessels that lay between the epidermis and the dermis. Pechlin transformed the learned consensus on the physical site of skin color, but he also showed that this pigmented layer was entirely superficial. During the 1690s Pechlin's work was referenced by members of England's Royal Society when they discussed questions of climate, race, and skin color. This empirical approach (practiced by Pechlin and championed by the Royal Society) developed under constant pressure to fix dark skin firmly in African bodies and tie it to deeper or more innate characteristics. Thus, in the Atlantic world, blackness could never be understood as a superficial phenomenon, despite the evidence revealed by the scalpel and the microscope.