Visited by John Locke
Under suspicion of plotting to assassinate King Charles II, John Locke fled England in 1683. He lived in Holland -- Utrecht, Amsterdam, and Rotterdam -- until he accompanied the future Queen Mary back to England in February 1689. 
While he was in Holland, Locke became acquainted with the leading Dutch intellectuals and wrote some of his major works, including Essay Concerning Human Understanding.
He also kept a journal. The entry for June 22, 1685, tells of his visit to Leeuwenhoek's house and laboratory in Delft. Leeuwenhoek showed Locke red blood cells, a human tooth, and dog sperm. Locke regretted that he did not see the best lenses.
After his return to England, Locke published Essay Concerning Human Understanding. It emphasizes the importance of knowledge coming from direct sensory perception, so looking through Leeuwenhoek's lenses must have fascinated Locke. The Essay contains a passage that does not mention Leeuwenhoek by name, but seems to have been rooted in his visit.
The portrait of Locke on the right was drawn by Locke’s manservant Sylvester Brounower around 1685. Brounower accompanied Locke on his travels around the Dutch Republic and probably into Leeuwenhoek's front room to look at his curiosities.
from the journals in Life of John Locke p. 167
I saw, at Mr. Lewenhook's, several microscopical observations, which answer the desription he has given of them, &c. &c. The exceeding small and regular fibres of the crystalline humour are wonderful, if all the works of Nature were not so. Speakng of some of the small animals which Lewenhook mentuioned that he had discovered, there is a very long desription.
It was with much diffiulty I could perceive the tails he describes, if, at least, I did perceive any at all. The glasses we saw, he said, would magnify to a million of times, which I understood of cubicle augmentation, which is but 100 in length; but the best of all his glasses, and those by which he describes his spermatic animals, we did not see, nor, as I hear, does he show them to any one.
from Essay Concerning Human Understanding, Book II Chapter XXIII
For how much would that man exceed all others in knowledge, who had but the faculty so to alter the structure of his eyes, that one sense, as to make it capable of all the several degrees of vision which the assistance of glasses (casually at first lighted on) has taught us to conceive?
What wonders would he discover, who could so fit his eyes to all sorts of objects, as to see, when he pleased, the figure and motion of the minute particles in the blood, and other juices of animals, as distinctly as he does, at other times, the shape and motion of the animals themselves?
But to us, in our present state, unalterable organs so contrived, as to discover the figure and motion of the minute parts of bodies, whereon depend those sensible qualities we now observe in them, would perhaps be of no advantage. God has, no doubt, made them so, as is best for us in our present condition.
