Wrote Letter L-419 of mid-February 1704 to Siewert Centen to refute his contention that cochineal must be a plant, not an animal

February 15, 1704
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This letter is known only by reference in another letter.

In this reply to a letter from Siewert Centen, Leeuwenhoek summarized an article from Philosophical Transactions on cochineal and added his own fresh observations, with figures, to refute Centen’s claim that cochineal must be a plant, not an animal.

Letter L-422 of 21 March 1704 to the Royal Society

I sent almost the whole of the aforesaid account, with the figures, to the Amsterdam merchant, in order that he might be able to see what my discoveries were.

Leeuwenhoek was replying to Centen’s Letter L-417 of 7 February 1704, in which Centen argued that cochineal comes from a plant. The “aforesaid account” and ten figures comprise most of the long Letter L-422 of 21 March 1704, in which Leeuwenhoek re-examined cochineal to affirmed that it came from an insect. Centen’s Letter L-420 of late February or early March 1704 reported on further observations that confirm his original claim in spite of Leeuwenhoek’s evidence and reasoning.