Leeuwenhoek began this letter by recounting, as he often did, what spurred the investigations he was about to report. In this case, as so often, he was responding to someone else's request.
Mr. Antoni Heinsius, LL. D., Pensionary and Counsellor of this City, one-time Envoy Extraordinary tot His Royal Majesty of France, and then Commissioner of this country to the Court of His Royal Majesty of Great Britain, wrote to me from Westminster on 24th July/3rd August, 1685 that The Right Honourable Robert Boijle would be pleased if I examined, among other things, the cochineal.
I thereupon replied to the abovementioned Mr. Heinsius, on the 10th of August, as follows:
Next came a long passage from his letter to Heinsius followed by Heinsius' reply of August 21/31, reporting Boyle's reaction, again quoted at length.
The question was the origin of cochineal. Was it crushed seeds or crushed insects? After excerpting Heinsius's letter, Leeuwenhoek excerpted his reply to that:
I thereupon wrote to the said Mr. Heinsius, on the 21st September, 1685, as follows with respect to cochineal.
After that long introduction, the reader was caught up and Leeuwenhoek could proceed with his recent observations. He had first seen the cochineal as crushed seeds. Because Boyle disagreed, Leeuwenhoek looked again, and saw that it was crushed shells that also contained other bits of the insect's bodies.
From the tone of these letters, it was not obvious that Heinsius and Leeuwenhoek were old friends. In 1667, Heinsius, then in his mid-20's, became Delft's city attorney after Leeuwenhoek, ten years older, had already been working at the Stadhuis since 1660. They worked together for twelve years until Heinsius became became pensionary for Delft in the States of Holland, the same year that Leeuwenhoek was appointed the city's wine gauger. By the time they wrote these letters in the mid-1680's, they had become the two people from Delft most prominent in the larger world, Heinsius for his diplomatic skills and Leeuwenhoek for his research skills.
