LEUWENHOECK, ANTONY, a celebrated naturalist, was born at Delft in the year 1632, and died on the 26th of August 1723. The peculiar talent he displayed in cutting glasses for microscopes and spectacles at first procured him much reputation, owing to the superiority of the instruments he constructed; and he afterwards acquired still higher distinction as a naturalist and anatomist, by the variety of his researches on the internal structure of the different parts of the human body. His microscopical labours and observations were so numerous, that it would be impossible to give an exact detail of them; we shall, therefore, advert only to his principal researches. The antagonists of Harvey, author of the discovery of the circulation of the blood, objected to the doctrine of this great anatomist, that if the blood passed directly from the arteries to the veins, it could not nourish the parts it traversed. The question was undecided; and, in 1686, Leuwenhoeck com- Leuwenhoeck communicated to the Royal Society of London a memoir, in which, contrary to the opinion of Harvey, he maintained that the passage of the blood from the arteries to the veins was not immediate. Nevertheless, in 1690, having scrupulously re-examined the parts with his improved microscope, he discovered and demonstrated in the clearest manner the continuity of the arteries with the veins; and he even refused to admit any division between the capillary vessels, because, said he, it is impossible to determine where the arteries end and the veins begin. At this period, the chemical theory which prevailed in medicine assumed as certain the fermentation of the blood. Leuwenhoeck combated this hypothesis successfully, opposing to it his microscopic experiments, from which it results that there exist no bubbles of air in the blood-vessels, a phenomenon which ought to occur if the blood underwent fermentation. This experimenter having also directed his researches to the form of the globules of blood which Malpighi had already perceived, showed that these globules are oval and flattened, being composed of six small cones which float in the serum, and which, taken separately, do not reflect the red colour, but which, by their union, communicate to the blood the physical qualities it is known to possess. This discovery served as the basis of the theory of Boerhaave on inflammation. In order to support his system, Leuwenhoeck showed that the red capillary vessels proceed from other vessels, where the circulation of the blood takes place beyond the influence of the heart, and where this liquid appears white, because its globules are divided, in order to accommodate themselves to the tenacity of the canals through which it must pass. Ulterior experiments have not confirmed his ideas as to the physical composition of the blood; but his observations on the structure of the capillary vessels have been recognised as exact by the most enlightened anatomists. The brain and the nerves formed also subjects of research to Leuwenhoeck. He pretended that the cortical substance is entirely vascular; that the vessels which compose it are five hundred and twelve times smaller than the most attenuated capillary vessels; and that the globules composing the fluid contained in the cortical substance are thirty-six times smaller than those of which the red blood is formed. Lastly, he conceived that each of these globules is surrounded with a very fine reticulation of vessels and of fibres. But new experiments made in 1717 induced him to modify his ideas; and he then alleged that the brain is a fibrous structure, and that blood-vessels are intertwined amongst the fibres composing that organ. Science, however, has derived no advantage from these last researches, which are more fitted to perplex than to enlighten. Leuwenhoeck studied the structure of the crystalline lens, and described, with accuracy, the coats which compose this part of the organ of vision; adding very good figures to his description. Much has been said of the discovery of the animalcules which he perceived in the sperm. He described these minute bodies at great length, and conjectured that, having reached the uterus, they irritate that organ, attract the ovum, and communicate vitality to the embryo contained in it. Benjamin Martin has contested these observations, a detail of which may be found in the Natural History of Buffon. Leuwenhoeck employed his whole life, which was very long, in making anatomical observations and experiments; and he only wanted, to obtain more numerous results, that erudition and sagacity which are necessary to discern what is true from what is only apparent. It is thus that he often believed himself to have seen what did not exist, and that he persisted in his error. Amongst his paradoxes may be mentioned the opinion he supported, that the coat of the intestines, which the anatomists of his time called tunica villosa, is muscular. He also maintained that pulsation belonged to the veins, and not to the arteries.

The Czar Peter the Great was an admirer of Leuwenhoeck. This prince, passing by Delft in 1698, sent two of his attendants to request Leuwenhoeck to pay him a visit, and to bring some of his "admirable" microscopes; adding, that he would have gone to visit him at his residence, had it not been that he wished to escape the notice of the multitude. The naturalist, after having shown his instruments to the emperor, exhibited to him the curious phenomenon of the circulation of blood in the tail of an eel. Leuwenhoeck communicated to the Royal Society of London all his Memoirs, which are inserted in the Philosophical Transactions. They have also been printed (for the most part separately) in Dutch, at Delft and at Leyden. But a foreign hand has translated into Latin all the compositions of this celebrated naturalist, under the title of Arcana Naturae detecta, Delft, 1693–1699, in four vols. 4to; reprinted at Leyden in 1719, and with the author's correspondence in 1722.