Hooke sent Leeuwenhoek official notice of his election as member of the Royal Society

April 22, 1680

This letter has not survived. Hooke dated it April 12, 1680 Old Style, which was April 22 New Style. It is known only from Leeuwenhoek's acknowledgment of May 13. Leeuwenhoek wrote:

I received your very welcome letter of 12 April O.S., and saw from it that the Honourable members of the Royal Society took great interest in my last observations. Also that you moved my election as a fellow of that Society in its meeting and that I was unanimously elected a fellow. A few days after the receipt of this letter, a box came to hand, enclosing the sealed Diploma, which I beheld affectionately and gratefully.

The image on the right (click to enlarge) is a detail from Verkolje's portrait of Leeuwenhoek. It has been rotated 90 degrees and shows Leeuwenhoek's hand-written parchment diploma with its red seal. Some of the words are visible. It is the only image of any of the five diplomas that the Royal Society gave out between 1664 and 1680.