LEUWENHOECK, Anthony Van, a famous naturalist, was born at Delft, in Holland, in 1632. At first he was well known as a grinder of optical glasses; and though destitute of a liberal education, was induced to use the excellent microscopes which his skill had made, in examining the textures and fluids of animal life. His success in anatomy and physiology was soon so marked, that an account of some of his discoveries was published through the influence of Dr Grew in the Philosophical Transactions of the Royal Society of London, of which he became a Fellow in 1680. His numerous contributions to the records of that society are comprised between Nos. 94 and 380. In 1697 he was elected a corresponding member of the Academy of Sciences at Paris; and in the following year, his fame had become so far-spread, that Peter the Great, when passing through Delft, sent for him and requested to see his "admirable microscopes," and the wonderful secrets which they revealed. The naturalist showed him the circulation of the blood in the tail of an eel. Leuwenhoeck died at his native place on the 26th of August 1723.

One of his most important investigations was that by which he discovered, in 1690, that the arteries and veins were continuous, an opinion, however, which he had contradicted in a memoir communicated to the Royal Society of London in 1686. The theory of the fermentation of the blood then universally held, he refuted by proving experimentally, that there were no bubbles of air in the blood, and therefore there could be no fermentation. He experimented, but with little success, on the form of the globules of the blood, and likewise on the brain and nerves, which he showed in 1717 to be of fibrous texture, with blood-vessels interspersed between the fibres; a view which has since been discovered to be very nearly correct. Another object of his study was the crystalline lens, the different coats of which he described with accuracy, illustrating his descriptions with figures. He also examined the spermatic amulets, and asserted that he had discovered them; a claim which was contested with Ludwig, Hahn, and Hartsecker. Others of his investigations ended in mistakes, owing partly to his deficient erudition, and partly to a certain native stubbornness, which led him to consider a confession of error to be degrading.

Leuwenhoeck contributed 26 papers to the Memoirs of the Academy of Sciences. A collection of his works, printed (for the most part separately) in Dutch at Delft and at Leyden, were translated into Latin under the title of Arcana Nature deleta, Delft, 1695–1699. The best collection of his works, translated into Latin, consists of his Correspondence, 2 vols. 4to, Leyden and Delft, 1719; and Arcana, 2 vols. 4to, printed at Leyden in 1722.