Anthonie Heinsius wrote to Leeuwenhoek about Robert Boyle's request for an examination of cochineal

July 24, 1685

We know of this letter only from Leeuwenhoek's Letter 105 [60] of 28 November 1687 to the members of the Royal Society.

Mr. Antoni Heinsius, ... 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.

August 10 in Delft would have been July 31 in England. Heinsius, in turn, spoke to Boyle and replied to Leeuwenhoek on August 23.