From Ingenious Pursuits: Building the Scientific Revolution by Lisa Jardine

I have had several gentlewomen in my house, who were keen on seeing the little eels in vinegar: but some of 'em were so disgusted at the spectacle, that they vowed they'd ne'er use vinegar again. But what if one should tell such people in the future that there are more animals living in the scum on the teeth in a man's mouth, than there are men in a whole kingdom?

A minor official in the city of Delft, Antoni van Leeuwenhoek had few of the prerequisites for scientific greatness. Speaking only Low Dutch (not even High Dutch), he had no formal scientific training. In fact, he had hardly any education at all. What he did have was a copy of Robert Hooke's Micrographia and a passion for all things tiny. Using his own homemade single-lens microscope — an instrument so small it could easily fit in the palm of your hand, with a thumb-sized wooden paddle and a lens the size of the head of a pin — Leeuwenhoek observed everything he could imagine. He eventually made more than 500 microscopes, and making diminutive, single-lens versions actually gave him better magnification than the larger, multi-lens microscopes in common use at the time. Leeuwenhoek's microscopes achieved a magnification up to 275 times, and they enabled him to see life where nobody thought living things existed. The scum on a man's teeth, for instance.

Leeuwenhoek was perhaps the first person to observe red blood cells in a living body — flowing through a tadpole tail. He saw the crystals that induced misery in gout sufferers. He studied dozens of insect species. He couldn't draw any better than he could write foreigners in their own language, but he collaborated with unnamed artists from Delft to produce exquisite illustrations, such as this drawing of a bee organ.

During the 20th century, the mentality emerged that Leeuwenhoek couldn't possibly have seen what he claimed to see. Modern microscopist and historian Brian Ford begged to differ. While contemporary exhibitions and documentaries showed dim, fuzzy images from microscopes of Leeuwenhoek's day, Ford found that, by adjusting lighting and focus carefully, he could produce images of exceptional detail. Ford could even see algae and protozoa, happily vindicating the 17th-century savant.

Leeuwenhoek's observations soon attracted the attention of the Dutch diplomat Sir Constantijn Huygens. In 1673, the English and Dutch were at war, but that didn't discourage Huygens from writing Robert Hooke, a member of the Royal Society of London, to introduce Leeuwenhoek. Like many of his fellow Englishmen, however, Hooke disdained the efforts of a foreigner, and refused to answer Huygens's letter. Undaunted, Huygens wrote another Royal Society member, Henry Oldenburg. Partly because he was responsible for the Royal Society's correspondence, and partly because he liked to stir up controversy — especially where Hooke was concerned — Oldenburg gladly answered Huygens and apologized effusively for Hooke's bad manners. It proved to be the beginning of a fruitful correspondence between Leeuwenhoek and the Royal Society, one that lasted nearly 50 years, in which Leeuwenhoek produced hundreds of papers. Even Hooke eventually came around. To his great credit, he devoted months to replicating Leeuwenhoek's observations, and even started learning Dutch to better understand Leeuwenhoek's writings.

From Seeing Further edited by Bill Bryson

When observing pepper, under the assumption that it had microscopic spikes to produce its effect on the tongue (he was wrong, it didn't), Leeuwenhoek made an accidental discovery: tiny organisms, known today as protozoa. (Shown here are rotifers: similarly small organisms, these with parasitic worms.) When the Royal Society was able to reproduce his experiment, Leeuwenhoek became an instant celebrity. Not surprisingly, the self-taught microscopist developed considerable self-confidence, enough to irritate at least one contemporary. Jan Swammerdam once remarked, "It is impossible to go into a discussion with Leeuwenhoek as he is biased and reasons in a very barbaric way, having no academic education."

Leeuwenhoek kept finding little animals everywhere, including "many very little living animacules, very prettily a-moving" in dental plaque. He described some microscopic "animals" as having "two tiny limbs near the head and two little fins at the rear of the body." His observations under the microscope fostered intense debate about the nature of plant and animal generation. Through it all, his doggedness was matched by his lack of squeamishness. He nearly blinded himself once when trying to observe a small gunpowder blast at close range. While studying the development of lice, he carried louse families in his stockings, allowing them to feast on his own flesh. He went long periods without changing his socks, to better accumulate inter-toe scum for later study. He expected comparable sacrifices from his wife, who had to keep insect eggs warm by carrying them in her bosom. Such measures disgusted another contemporary besides Swammerdam: Nicolaas Hartsoeker, an upstart initially impressed with the older man who later found him crass. Hartsoeker became Leeuwenhoek's less capable but better educated rival.

In one instance, however, the capable Leeuwenhoek failed to identify an animal much bigger than bacteria. At the request of the Royal Society's Robert Boyle, Leeuwenhoek examined samples of cochineal. A highly coveted dyestuff, cochineal was a New World product so closely guarded by the Spanish empire that Spain's rivals couldn't tell whether it was animal, vegetable or mineral. (It was an insect.) Prodded by speculation that cochineal was a berry, Leeuwenhoek concluded it was a berry — a surprising mistake for one so capable. Leeuwenhoek, however, was used to examining live specimens of entire bugs. The cochineal specimens were detritus with telltale signs of bug-ness — wings, legs and heads — removed. Upon later examination, he diagnosed accurately.

On another occasion, Leeuwenhoek wrote the Royal Society that describing a water sample loaded with such a great quantity of "animacules" that "all the water . . . seemed to be alive." It was unfortunate that the savants who marveled at the tiny organisms Leeuwenhoek found couldn't grasp their significance in causing disease. Two centuries — and thousands of deaths from dysentery, cholera, and typhoid — passed before anyone made the connection between winsome creatures swimming in drinking water and deadly illness.

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