15 - Leeuwenhoek and the “Little Animals”


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15. Leeuwenhoek and the “Little Animals”

Medicine, on its march through millennia, many times has been the beneficiary of men who, uninhibited by the mores of formal education, have allowed their curiosities free rein and have followed leads to basic discoveries that had eluded professional practitioners for centuries. By unorthodox methods, frequently scorned as unscientific, it has been their God-given privilege to come upon revelations that have been boons to all mankind through advancement of the healing arts. Such a man was Antony van Leeuwenhoek.

Leeuwenhoek was not the inventor of the microscope, although it often has been credited to him erroneously. But there is no question that he deserves to be called the father of microscopy, and credited as the man who laid foundations for the sciences of bacteriology and protozoology. With tiny lenses laboriously ground by his own secret methods, Leeuwenhoek became the first man to observe and to report on fascinating, multifarious microscope forms of life with which, though unbeknown to him, man had coexisted for untold centuries.

Antony van Leeuwenhoek was born at Delft, Holland, and October 24, 1632. His father was a basket maker, and died when Antony was five years, he was sent to a linen draper’s shop in Amsterdam to learn the business.

After six years in Amsterdam, Leeuwenhoek returned to his native town. Shortly thereafter, Leeuwenhoek married, bought a house and shop, and set himself up in business as a draper and haberdasher.

In addition to his drapery business, Leeuwenhoek enjoyed considerable civic success. He held a municipal office comparable to that of alderman; he was official wine-gauger; and he was licensed as a surveyor.

There seems to be no record of how or when Leeuwenhoek became interested in grinding lenses and using them to investigate objects too small to be seen by the naked eye. Noris it clear how he learned the art of lens-grinding. Throughout his life he kept his technical methods secret. Within the confines of his “closet,” as he called his workroom, he turned out hundreds of tiny lenses, and mounted them laboriously but crudely between two thin sheets of silver or brass with small openings masking all but the central are of the lens. Most of his instruments were but two to three inches in height, an inch or less in width, and, except for thumbscrews, less than a half inch in depth. Solid specimens were mounted before the lenses on needle points, adjustable with thumbscrews both for height and for distance. Other microscopes were designed to hold small glass vials, or capillary tubes, to bring liquids within extremely short focal ranges. All of Leeuwenhoek’s microscopes were made with single lenses; he used no compound lenses. By variations of grinding, he secured lenses of various magnifications. Most of them had magnification factors of not more than 160 diameters, although one in the collection at Utrecht University Museum is reported to magnify 275 diameters The first report of Leeuwenhoek’s activity in this new and fascinating field came in 1673, when his good friend and fellow-townsman, the famed physician Reinier de Graaf, wrote to the secretary of the Royal Society of London, describing Leeuwenhoek’s work and enclosing a letter from the microscopist. This letter was the first of no less than 375 communications written by Leeuwenhoek to the society over a 50-year period. Evidently Fellows of the Royal Society liked the reports on Leeuwenhoek’s observations, for the secretary, Henry Oldenburg, encouraged him to continue his correspondence.

Leeuwenhoek’s letter forms a unique contribution to the literature of science and particularly of medicine. They were nearly all written in Dutch, the only language he knew; and in a simple, native, conversational style- sometimes frank and earthy. A native honesty ran through them, however; while Leeuwenhoek’s untutored and unscientific interpretations of what he was most careful to distinguish between descriptions of things he actually saw, and those he conjectured or “imagined.” He was so intent on telling what he had seen or thought that he had no time to worry about grammar or niceties of literary composition. He worked entirely by himself, receiving no help from contemporary microscopists. He disliked and resented interference, and he distrusted the knowledge- and sometimes the purpose- of persons who went to see him or offered him advice.

In 1680, Leeuwenhoek was elected a Fellow of the Royal Society- a distinct honor and recognition for one living outside England. In 1699, the Academie des Sciences of Paris elected him a correspondent. His observation excited great interest, and were widely read- but that was all. Nobody seriously attempted to reappear or to extend them. As the seventeenth century closed, Leeuwenhoek was the only earnest microscopist then living in the world.

Leeuwenhoek’s specimen came from his everyday surroundings. No material or substance escaped the insatiable curiosity of the draper of Delft: rain water, scum from the surface of ponds, infusions of peppercorns, unborn mussels, animal and human tissues, scrapings, excreta of all sorts, and all kinds of mineral and vegetable matter, all came before his lenses.

Notable among Leeuwenhoek’s peculiar habits was this practice: when he found a specimen particularly to his linking, he left it attached to the microscope and made another instrument. Over the years he accumulated many of these. He placed these in pairs in small lacquered boxes, 12 to 24 boxes in a case. One such case he willed to the Royal Society. Leeuwenhoek’s peculiar habits was this practice; when he found a specimen particularly to his liking, he left it attached to the microscope and made another instrument. Over the years he accumulated many of these. He placed these in pairs in small lacquered boxes, 12 to 24 boxes in a case. One such case he willed to the Royal Society.

Leeuwenhoek was the first man to discover and to describe protozoa and bacteria (which he called ‘animalcules”). He used “a grain of sand” (roughly a cube of about 1/30 of an inch) as his standard of comparison, and described sizes of his “animalcules” by estimating that it would take 1,000, 100,000 of them to equal the bulk of “a grain of sand”. Some of his estimates of identifiable species show a remarkable relation to modern micron measurements. He described with no little wonder the tremendously rapid multiplication of his “animalcules” when samples were allowed to remain standing for a few days; and he was aware of the relative purity of fresh rain water or snow water. Leeuwenhoek made drawings representative of various protozoa, bacilli, cocci, and spirochetes. He studied animal parts, such as blood, bones, eyes, hair, and muscles; he was the first to note striations in muscle fibers; and he is known, particularly, for his study and descriptions of spermatozoa. In his world of microscopic biologic forms, he saw: cellular division, birth, life and death of his “animalcules”; parthenogenesis of aphids: and budding of hydra. He even tried to observe the explosion of gunpowder under the microscope- an experiment that almost cost him his eyesight.

In one of his letters, after reporting on his observations on microscopic forms both in wine vinegar and in scrapings from between his teeth, Leeuwenhoek revealed a wry bit of sophisticated humor:

“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 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? Especially in those who don’t ever clean their teeth?...”

Leeuwenhoek also reported, in 1686, and frequently demonstrated for visitors, the capillary circulation of blood by placing a very small eel or fish in a glass tube and focusing on the transparent tail. Malpihi had preceded him, in 1661, in this observation, but it is doubtful that Leeuwenhoek knew of Malpighi’s work. Leeuwenhoek also concluded that vessels leading away from the heart were arteries and those toward the heart were veins, bearing out, by ocular observation, facts about which Harvey could only conjecture.

As soon as his discoveries became famous, Leeuwenhoek was visited by all manner of people who wanted to look through his lenses. The list of names of celebrities who called on him is long and impressive: included were travelers, writers, physicians, noted scientists, statesmen, kings, queens, an emperor of Germany, and Czar Peter the Great of Russia. Leeuwenhoek naturally felt flattered, but frankly confessed in one of his letters that he was bored by such interruptions and preferred to be left in peace to carry on his work. Also, he showed such callers only certain of his specimens, refusing to reveal others; and at no time would he disclose his methods of lens-grinding, or sell one of his microscopes.

Naturally, publication of observations of such a multitude of previously unknown things gave rise to arracks from unbelievers, from jealous contemporaries, and from those irked by Leeuwenhoek’s lack of formal training. Of such criticisms, Leeuwenhoek wrote: “I am well aware that these my writings will not be accepted by some… they’re still saying… I’m conjurer, and that I show people what don’t exist… I well known there are whole Universities that don’t believe there are living creatures in the male seed: but such things don’t worry me; I know I’m in the right.’

Though handicapped by failing eyesight and by other rigors of old age, Leeuwenhoek continued his observations and his letters until the end of his life. He dictated the last record within 36 hours of his death. Leeuwenhoek died peacefully on august 26, 1723, in his ninety-first year, and his body was buried in Old church in Delft.

Neither physician nor surgeon; actually, in formal terms not even a scientist, Leeuwenhoek was recognized by his contemporaries as well as by modern students as one of the most painstaking observers of all time, and the first to report on the great and wonderful world of microbes. Though Leeuwenhoek drew no conclusions regarding the relationship of his “animalcules” to causation of disease or to contagion, other persons soon connected the “little animals” with earlier philosophical speculations regarding the existence of living germs of diseases. Though theoretical implications of Leeuwenhoek’s observations were recognized, no one made real or practical use of the knowledge during the next 150 years. Yet Leeuwenhoek’s work laid the foundation for Pasteur’s pioneering and for the almost explosive growth of bacteriology and of protozoology in the latter half of the nineteenth century. The revolution in medical thinking and in medical practice which these developments brought about has resulted in prolonging lives of countless millions of people throughout the world during the past 100 years.


Antony van Leeuwenhoek, draper of seventeenth-century Delft, Holland, in his spare time retired to his “closet” to observe the wonders of the microscopic world through tiny lenses he laboriously ground and mounted. He was first to report having seen “animalcules”- protozoa and bacteria- and confirmed by direct observation circulation of the blood. Though 200 years were to elapse before practical application of his discoveries contributed to medicine, his work laid the foundation for modern medicine’s tremendous century-long on-slaught against diseases caused by bacteria and other microbiologic forms of life.