Wrote Letter L-204 of 1689-01-12 to the members of the Royal Society about his observation of the circulation of the blood in a variety of fish as well as a description of the construction of his ‘eel spy-glass’ and the slime on the skin of an eel

Text of the letter in the original Dutch and in English translation from Alle de Brieven. The Collected Letters at the DBNL - De Digitale Bibliotheek voor de Nederlandse Letteren

Leeuwenhoek wrote this letter about his observation of the circulation of the blood in the capillaries of the tail-fin and the pectoral fins of young and older eel, also of the coagulation of the blood in those vessels. Leeuwenhoek had demonstrated all these phenomena to a number of authorities. Description of the construction of the ‘eel spy-glass’, made by Leeuwenhoek in two types. The slime on the skin of the eel. Circulation of the blood in the tail-fin of the perch. The diameter of the blood-vessels and on the thickness of their wall. Circulation of the blood in the tailfin of the pike, the roach and the carp and in the tail and the end of the legs of the shrimp. The nature of the blood of the shrimp. Circulation of the blood in the fins of the flounder and the plaice.