At age 52, his 25th article in Philosophical Transactions
Date:
February 23, 1685
At age 52, his 25th article in Philosophical Transactions, volume 15, number 168 (February 1685): An Abstract of a Letter of Mr. Anthony Leewenhoek Fellow of the R. Society; Concerning the Parts of the Brain of severall Animals; The Chalk Stones of the Gout; The Leprosy; And the Scales of Eeles.
Because some letters were split into two articles and other articles contained two letters, Letter 42 of 1684-07-25 (AB 81) to Members of the Royal Society is the 24th letter that Leeuwenhoek had published in Philosophical Transactions.