Volumes 1 through 20 of Alle de Brieven / Collected Letters, the complete series. Download for free.
Master list (pdf) of all 602 L-numbers, their Leeuwenhoek number, and their Collected Letters number
cousin Maerten Huijchs Leeuwenhoek (1631-1694) baptized in Oude Kerk
DTB Delft inv. 8, fol. 92