The Leeuwenhoek Lecture, 1963 The size of small organisms

Leeuwenhoek’s letters are good reading. They show us a man who was busily inquisitive about everything, with firm opinions and an emphatic way of expressing them. When he found that Hooke had printed part of one of his letters in a book, he commented pointedly that people had asked him to write a book himself but that he preferred not to (2, 361). When told that people in Paris could not see the globules he described in many materials, he said he was absolutely unconcerned and advised them to come to Holland and be shown (1, 279). He saw the comic aspects of some of his work; the editors explain that in Dutch his reference to a recent letter on lice as ‘my last lousy observations’ has the same connotations as in English (2, 195). Nevertheless, he was conditioned by his time. He called in two Lutheran pastors to attest the truth of his observations (2, 257) and was at pains to emphasize that his studies on human sperm were made ‘zonder eenige zondige bezoedeling van mij zelf’. Of more direct relevance to the theme of this lecture is his mechanical approach to the interpretation of nature. Repeatedly he tries to explain the pungency of pepper or ginger by postulating pointed particles in them that pricked the tongue, and he suggested that in purgatives the spikes are long enough to penetrate the layer of mucus on the gut wall. He assumed that these particles were too small to be resolved by his microscopes and so was delighted when he saw (2, 243) what he took to be narrow rods in eel blood; for he attributed to these the blood’s ability to irritate the eye. He clearly postulated a micro-anatomy in what he saw and attributed physiological significance to it. It is fortunate that he was not a well-read and educated man. Had he been so he would have known the prolix confusion of Athanasius Kircher’s microscopical observations and deductions and might have become a victim of what Charles Singer aptly calls the ‘ vermicular obsession ’ that, by the end of the seventeenth century, made people see worms in most diseased and many healthy organisms. Leeuwenhoek did not know what he was supposed to be seeing and was content to record what he saw.