The Dutchman who thus first spied upon the world of infinitely small creatures was Anton van Leeuwenhoek (1632-1723), an untutored ex-shopkeeper and minor official of the picturesque city of Delft. (Some say that he was the janitor of the city hall.) He built his own microscopes — hundreds of them — and with them he observed anything that aroused his curiosity: the brain of a fly, the legs of a louse, sections of the crystalline lens of an ox's eye and the stinger of a bee.
Leeuwenhoek wrote almost 400 letters in all to the Royal Society describing his discoveries. These letters were long, rambling affairs, written in Dutch, the only language that he knew. They were translated into English and published in the philosophical transactions. In 1680 the Royal Society elected him to membership; he repaid the honor by bequeathing to it twenty-six of his microscopes.
Leeuwenhoek has been called the "father of bacteriology" — the study of bacteria — and the "founder of protozoology" — the study of protozoa, or one-celled animals. He first beheld bacteria in "scum scraped from his own teeth"; stagnant water provided him with great numbers of protozoa. He faithfully observed the size, shape and motions of these tiny organisms under a variety of conditions. He saw some tiny animals "put forth two little horns, continually moving"; others were "furnished with extremely thin feet. which moved very nimbly"; still others "swam gently along, moving as gnats do in the air," or else had a "serpentine motion.'"
In 1688 Leeuwenhoek turned his microscope on the tail of a tadpole and saw the tiny capillaries connecting the veins and the arteries. As we shall see, Malpighi had already discovered the capillaries; but Leeuwenhoek was not aware of the fact, and he reported with great gusto on his amazing "find." "A sight presented itself," he wrote, "more delightful than any mine eyes have ever beheld; for here I discovered more than fifty circulations of the blood in different places while the animal lay quiet in the water and I could bring it before my microscope at my wish. For I saw not only that in many places the blood was conveyed through exceedingly minute vessels from the middle of the tail toward the edges, but that each of the vessels had a curve or turning, and carried the blood back toward the middle of the tail, in order to be conveyed again to the heart . . . And thus it appears that an artery and a vein are one and the same vessel prolonged and extended."
When Leeuwenhoek died in 1723 at the advanced age of ninety-one, he had an amazingly large number of microscopic discoveries to his credit. Among other things, he had discovered spermatozoa — the male elements in human beings and lower animals. He had made out the red blood corpuscles; he had correctly noted that they are circular in the blood of man and mammals, but oval in the case of frogs and fishes. Leeuwenhoek contented himself throughout his long scientific career with being an observer; he took little or no interest in theories or general laws.
