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Tuesday, October 2, 2012

Anton van Leeuwenhoek


The Dutch naturalist and microscopist Anton van Leeuwenhoek (1632-1723), using simple microscopes of his own making, discovered bacteria, protozoa, spermatozoa, rotifers, Hydra and Volvox, and also parthenogenesis in aphids.
Anton van Leeuwenhoek was born on Oct. 24, 1632, at Delft. His schooling was informal, probably including some mathematics and physical sciences but no languages. At the age of 16 he was sent to Amsterdam to become an apprentice at a linendraper's shop where he remained for about 6 years.
In 1654 Van Leeuwenhoek returned to Delft and married Barbara de Mey, who was to bear him five children. He bought a house and shop and set up in business as a draper. He remained there for the rest of his life. His wife died in 1666 and in 1671 he remarried; his second wife bore him one child.
In 1660 Van Leeuwenhoek was appointed chamberlain to the sheriffs of Delft, an office which he held for 39 years. Little is known of his activities for the next 13 years; however, in his spare time he must have begun to grind lenses to make simple microscopes. As early as 1668 he took one of his microscopes on a visit to England and used it to examine chalk from the cliffs in Kent.
In 1673 Regnier de Graaf, a brilliant young physician of Delft, wrote a letter about Van Leeuwenhoek's work to Henry Oldenburg, Secretary of the Royal Society in London. This letter was published in Philosophical Transactions, and Oldenburg wrote to the author requesting further communications. Thus began a correspondence with the Royal Society which was to continue until Van Leeuwenhoek's death. All his observations were described in letters (at least 200), either to the Royal Society or to his friends, that were written in his own language, Nether-Dutch. He never wrote a scientific paper or a book. His letters are full of random observations, with little coherence, and were written in a conversational style. Despite the casual way in which he described his observations, he never confused the facts with his speculations, and so it is possible to identify easily many of the organisms he studied from his detailed descriptions.
The Royal Society elected him a fellow in 1680, an honor which pleased him although he never found time to visit London to sign the register. His discoveries soon made him famous, and many came to visit him in Delft. His enthusiasm for the study of nature never waned even in old age, and despite his infirmities he still continued to make observations and send letters to the Royal Society. After his death on Aug. 26, 1723, his daughter Maria sent a cabinet to the Royal Society which her father had prepared 22 years previously, containing 26 of his microscopes made from silver.
Van Leeuwenhoek's Microscopes
Apart from those microscopes sent to the Royal Society, Van Leeuwenhoek left 247 completely finished microscopes, most of which had an object mounted in front of the lens, and also 172 lenses mounted between metal plates. Properly speaking, the instruments were not microscopes at all but simple magnifying glasses. Each consisted of a single biconvex lens of remarkable clarity which was mounted between two metal plates. The lens was fixed, and the object to be examined was raised or lowered and rotated upon its axis by a coarse-threaded-screw. The lenses were of exceptional optical quality and had magnifying powers ranging from 50 to 200. The short (about 1 millimeter) focal lengths of the lenses would have necessitated placing the eye almost in contact with the lens, and it is not clear how Van Leeuwenhoek obtained the necessary illumination to achieve his remarkable results. He was always very secretive about his methods. Clifford Dobell suggested that he might have discovered some simple method of dark-ground illumination, whereas Barnett Cohen pointed out that the optical properties of spherical drops of fluid containing the objects under observation may have been used by Van Leeuwenhoek.
Microscopical Observations
Van Leeuwenhoek's curiosity was insatiable, and he examined everything he could with his microscopes, ranging from samples of about 200 biological species to mineral objects; he even attempted to observe the explosion of gunpowder.
Van Leeuwenhoek gave clearer descriptions of red blood cells than either of his contemporaries Marcello Malpighi and Jan Swammerdam. He first described them in 1674 and estimated their size to be, in modern terminology, 8.5 microns in diameter (the correct value is 7.7 microns). In 1682 he clearly described the nucleus within the red blood cells of fish, and in 1683 he noticed the sedimentation of erythrocytes from a suspension and their lysis upon addition of water. His description of the blood capillaries in the intestine in 1683 was accompanied by comments on a different type of capillary which contained "a white fluid, like milk"; he had discovered the lymphatic capillaries.
In 1677 Van Leeuwenhoek examined fresh semen, in which he observed living spermatozoa. His unique observations on microorganisms probably began in 1674, when he examined water from a lake near Delft. He gave the first description of the common green alga Spirogyra but also observed smaller organisms, which were probably free-living protozoa. Dobell believed that Van Leeuwenhoek saw Vorticella, Monas, Bodo caudatus, and Colpidium.
Van Leeuwenhoek also discovered parasitic protozoa, describing the flagellate Giardia in a sample of his feces, which also contained bacteria which can be identified as Spirochaeta. In a letter written in 1683 he describes and illustrates five different kinds of bacteria present in his own mouth: these can readily be identified as a motile bacillus, Selenomonas sputigena, a micrococcus, Leptothrix buccalis, and a spirochete. He continued to make observations on microorganisms until 1716, and while studying free-living protozoa, he also discovered other organisms such as Volvox, Hydra, and rotifers.
Van Leeuwenhoek's discovery of microorganisms has tended to overshadow his other work, which has not received full recognition. He was one of the first comparative anatomists, since he often followed a structure in several different species. As a pioneer of microdissection, he succeeded in obtaining results which are remarkable even by modern standards. Between 1680 and 1701 he carried out a series of microdissections, mainly on insects, and one of his most original discoveries was parthenogenesis in aphids. The parent aphids did not contain eggs, but young aphids just like the parent.
An Assessment
Van Leeuwenhoek's insatiable curiosity, coupled with remarkable tenacity and skill, makes him one of the most outstanding scientists of all time. In his own modest way he realized how rare his gifts were and also that other people's motives were not always those of a true student of nature. In a 1715 letter he noted: "Some go to make money out of science, or to get a reputation in the learned world. But in lens-grinding and discovering things hidden from our sight, these count for nought. And I am satisfied too that not one man in a thousand is capable of such study, because it needs much time ... and you must always keep thinking about these things if you are to get any results. And over and above all, most men are not curious to know: nay, some even make no bones about saying, What does it matter whether we know this or not""


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