Volumes 18, 19, and 20 of Alle de Brieven / Collected Letters became available at the end of 2024, completing the series begun in 1939. Download for free.
Which number?
Master list (pdf) of all 602 L-numbers, their Leeuwenhoek number, and their Collected Letters number
Wrote Letter L-556 of 1717-08-19 to Adriaan Swalmius about membranes inside an eggshell, salt crystals from an eggshell, and how birds and oysters produce shells