Constantijn Huygens wrote to Leeuwenhoek about his observations of sperm

Date: 
December 8, 1677

This letter is lost. It is known only from Leeuwenhoek's Letter Brief 196 [113] of 17 December 1698 to Harmen van Zoelen. In it, Leeuwenhoek defended himself against the allegation of Nicolaas Hartsoeker in the latter's recently published Proeve der Deurzicht-kunde that he had been the first to observe spermatazoa, with the aid of quotations from letters that Leeuwenhoek had sent to the Royal Society in 1677 and 1678, including this one from Huygens that Leeuwenhoek copied out in its entirety.

Document: 

Haage den 8. Decemb. 1677.

Monsr. Leeuwenhoek, bysonder goede Vrient,

Hiernevens komt weder t' huys UE. laatste observatie, die ik van immers soo groote consideratie houde, als yet dat tot nog toe by Menschen Oogen of gedagten geobserveert is geweest; en salder wel geerne by occasie met UE. af discoureren. Aan mijnen zoone alleen na by mijn t' huys zijnde, heb ik niet konnen goet vinden daar over te communiceren, sullende liever hebben dat het hem van elders toekome. Ik bedank UE. voor de onderrigting aangaande de vleugel vliesen van Muggen &c. Daar op sullen wy mede al wat te seggen vinden. Ik ben

UE. Dienstwillige Vrient
C. Huygens V.Z.

Vorders moet ik tot UE. Wel Edele Gestrenge Heere seggen, dat de Koninklijke Societeyt in Londen, dese voorverhaalde ontdekkingen in hare Transactions, die zy van tijd tot tijd sijn uytgevende, hebben laten drukken als dese in de Transaction No. 142. zijnde van de Maant December 1677. en Jan. en Feb. 1678. beginnende op het blatzyde No. 1040. alwaarze aldus seggen.

Observationes D. Antonii Leeuwenhoek, de Natisé Semine genitali Animalculis.
Nec non Auctoris harum Transactionum Responsa

Observatoris Epistola Honoratiss. D.D. Vicecomiti Brounker Latine conscripta, Data Novemb. 1677. quam ipsissimis huc transmissis verbis inserendam Auctor Censuit.

Nobilissime Vir, enz.

The Hague, the 8th of December 1677.

Mr. Leeuwenhoek, special good Friend,

Enclosed I return to you your last observation, which I consider to be of as extreme interest as anything that has ever been observed by human eyes or thoughts; I shall be pleased to discuss it with you when an occasion presents itself. I have not thought fit to speak about it with my son, who alone was at home with me, since I prefer that he should learn about it from elsewhere. I thank You for the information concerning the wing membranes of gnats, etc. About this we shall also have to talk. I am

Your Dutiful Friend
C. Huygens of Zuylighem.

Further I have to tell Your Honour that the Royal Society in London had these aforesaid discoveries printed in their Transactions, which they publish from time to time, viz. these in Transaction No. 142. of December 1677. and January and February 1678, starting on page 1040, where they say as follows:

Observations of Mr. Antoni Leeuwenhoek, on the Animalcules Originating from genital Sperm,
and Reply of the Writer of these Transactions

Letter from the Observer to the Honourable Viscount Brounker, written in Latin, Dated November 1677, which the Editor thought fit to insert in the very wording in which it was sent hither.

Very Noble Sir, etc.