Unless new documents emerge there can be no definitive answer to this question, and it is only possible to speculate. There are however a number of plausible candidates among 17th century students of optics. The prime candidate for the role of Vermeer's 'optical consultant' is Antony van Leeuwenhoek, the pioneer of microscopy, who was Vermeer's exact contemporary, a fellow-citizen of Delft, and who disposed of Vermeer's estate when he died. Some critics have even wondered whether Leeuwenhoek was the sitter for Vermeer's two paintings of scholars in their studies, 'The Geographer' and 'The Astronomer'. Two other distinguished contemporaries are known to have visited Vermeer: Constantijn Huygens, and the French savant and traveller Balthasar de Monconys. Both men combined a love of art with deep interests in the telescope and the microscope.

Vermeer, 'The Geographer' c.1668-69, Städelsches Kunstinstitut, Frankfurt, oil on canvas 56 x 47.5 cm.