Lambert van Velthuysen wrote to Leeuwenhoek about some of his observations

April 12, 1679

This letter is lost. It is known by reference in Leeuwenhoek's Letter 45 of 11 May 1679, his first to Van Verthuysen, which refers to "The very courteous and pleasant letter of Your Honour of the 12th of April," so Van Velthuysen must have initiated the correspondence.