ANTHONY VAN LEEUWENHOEK, or LEUWENHOEK (1632-1723), a microscopist of remarkable scientific ability, was born at Delft, in Holland, in 1632. He does not seem to have had the advantage of a liberal education, but was probably brought up as a glass-grinder, early acquiring a reputation for the excellent lenses with which he furnished the microscopists who were then turning their attention to the minute structure of organized bodies. He appears soon to have found that single lenses of very short focus were preferable for this purpose to the compound microscopes then in use; and it is clear from the dis-coveries he made with these that they must have been of very excellent quality.1 These discoveries were for the most part originally given to the world in the Philosophical Transactions of the Royal Society, to the notice of which learned body he was first introduced by De Graaf in 1673. He was elected a fellow in 1680, and was chosen in 1697 a corresponding member of the Academy of Sciences in Paris. He died at his native place in 1723; and Sir Martin Folkes, then vice-president of the Royal Society, says in the eulogium. he pronounced:"We have seen so many and those of his most surprising discoveries, so per-fectly confirmed by great numbers of the most curious and judicious observers, that there can surely be no reason to distrust his accuracy in those others which have not yet been so frequently or so carefully examined."
His capital discovery was undoubtedly that of the capillary circulation of the blood, first announced in 1690, which afforded the link still wanting for the completion of the doctrine of Harvey, by showing that the blood passes from the arteries into the veins through a network of extremely minute vessels, the thin walls of which allow the fluid plasma to transude into the tissues it traverses, so as to serve for their nutrition. He first sought to discern this in the comb of a young cock, then in the ear of a white rabbit, and then in the membrane of a bats wing; but, though in the last he was able to follow an artery to its ultimate subdivision, he found that, as soon as "it became so small as only to admit one or two globules to pass through it at a time, he then lost sight of it," partly in consequence of "the membrane of the wing being covered with a kind of scale" (epidermis). His first success was obtained with the tail of a newly-hatched tadpole, in which, he says, "I could distinctly perceive the whole circuit of the blood, in its passage to the extremities of the vessels, and in its return towards the heart,"its move-ment being made apparent by that of the globules carried along, in its current. These corpuscles, which had been previously discovered by Malpighi, were correctly described by Leeuwenhoek as flattened circular disks in man, and as oval disks in tadpoles. He afterwards observed the capillary circulation in the tail-fins of small fishes, and recognized the ellipticity of the corpuscles in that class also. He even made out the capillary circulation in the broad thin extremities of the two smallest or hind feet of small crabs about an inch in diameter, and correctly remarked that the corpuscles of their blood were colourless and far fewer than those of fishes or tadpoles, "the globules in red blood being (I am well assured) twenty-five times more in number than those, in the same space, in the blood of a crab." To us it seems not a little surprising that his assertions in regard to the capillary circulation were deemed incredible by some of his scientific contem-poraries. It is recorded, however, that Peter the Great, when passing through Delft in 1698, requested Leeuwen-hoek to pay him a visit, and to bring his microscope with him, and that the czar was particularly impressed by the spectacle of the circulation in the tail of a small eel.
Among Leeuwenhoeks discoveries in the minute, anatomy of man and the higher animals may be specially mentioned the tubules of teeth, the fibrous structure of the crystalline lens, the solidity of the human hair (which had been previously represented as tubular), the structure of the epidermis, and the parallel tubules of the medullary substance of the brain,which last, however, he supposed to be vessels conveying fluid substance from the highly vascular cortical layers, for the support and nourishment of the spinal marrow and nerves. He was also an in-dependent discoverer of the spermatozoa, although anticipated by a few months by Ludwig Hamm, a student at Leyden.
As might be expected, he made many observations on the anatomy of insects; and among the most interesting of these are his discovery of the composite structure of the eyes (which he recognized also in the eyes of the shrimp), the scales on the wings not only of moths but of the gnat, and the annular (really spiral) structure in the walls of the "vessels" (tracheae) of their wings. He also proved that cochineal, which had been supposed to be "the fruit of some tree," is really the dried body of an insect, which he not unnaturally supposed to be allied to the ladybird. He likewise gave a very good account of the spinnerets and poison-claws of spiders, and of the comb-like appendages to their feet. He made, a special study, also, of the anatomy of the flea,besides following out its reproduction with great care, as will presently appear.
In examing the stomachs of shrimps, be found in them some minute shells, of which he figured a specimen so exactly that it can be at once recognized as a Nonionina,probably the first recent foraminifer that had been distinctly noticed. But one of his most interesting observations is that which he made upon a small Balanus attached to a mussel-shell; for he not only gives a good figure of the animal, but describes the way in which it retreats into its shell, and closes its orifice by two shelly valves. His figure most distinctly shows its articulate character, which has only in modern times caused its removal from the molluscous to the annulose sub-kingdom.
Not less admirable were his observations on the structure of plants. He made very careful sections of stems of the oak, elm, beech, willow, fir, and other trees, in different directions, of which he gave careful figures and descriptions, specially noting the horizontal arrangement of the cells in the "medullary rays," and the peculiar "pitting" of the woody fibre of the fir, as well as the absence of large vessels in the. latter. He also examined the structure of various germinating seeds, and gave accurate descriptions of the relation of the embryo to the cotyledons.
Although, when he adventured into physiological speculation, Leeuwenhoeks ideas (like those of the best physiologists of his time) were often very crude, his reasonings upon the facts actually observed by him are often remarkably cogent and sagacious. Thus, to estimate the insensible perspiration, he placed his hand within a dry glass jar, closed the space between its neck and his wrist by stuffing his handkerchief into it, and carefully collected and weighed the moisture which accumulated in its interior during a given time; and by a computation based on the ratio of the surface of the hand to that of the entire body he concluded that about 28 oz. of fluid are daily lost by transpiration, which is not far from the truth. So, again, he triumphantly refuted the chemical theories which then reigned in medicine, and which assumed that the blood under-goes a fermentation like that of wine or beer, by the statement that he had never seen in the blood-vessels the bubbles of gas which must be generated in them if this doctrine were correct. In one important point, however, he allowed his imagination to supplement the necessary imperfection of his observations, maintaining that each blood-disk is made up of six coherent particles, an idea probably suggested by the crenated appearance which the blood--disks often present.
It is to Leeuwenhoek that we owe the refutation of the then current biological doctrine that animals of high organization can be "Produced spontaneously, or bred from corruption." This doctrine had been previously attacked by Redi, who showed that
the putrefaction of meat will not engender maggots, if the access of blow-flies be prevented. But even Redi, while upholding the doctrine " Omne vivuin ex vivo," believed that the insects found within the galls of plants, and the parasitic worms by which the
human body is sometimes infested, are generated by a peculiar modification of the living vegetable or animal substance. It was Leeuwenhoek who first explicitly took up the position that every living organism reproduces its like, no type originating in any
other way than by the ordinary generative process of its kind: "Omne vivum ex ovo." This he established by careful and prolonged observation, in a great variety of cases in which "spon-taneous generation" had been reputed to take place; and he further continually adduced the great complexity of organization which his nicroscopic researches had revealed in what had been pre-viously regarded as creatures of the lowest grade, as an argument against the doctrine that they are "bred from corruption." Thus he followed out the whole history of oak-galls, and showed that they are a product of a peculiar vegetable growth, excited by the insertion of an egg by the winged insect, and supplying the maggot, when hatched, with food. So, again, he showed that the weevils of granaries, then commonly supposed to be bred from wheat, as well as in it, are nothing else than grubs hatched from eggs deposited by winged insects; and he practically applied this conclusion, by recommending that granaries thus infested should be repeatedly fumigated with sulphur at the time when these insects come forth, so as to kill them before they deposit eggs. His chapter on the flea, in which be not only describes its struc-ture, but traces out the whole history of its metamorphoses from its first emergence from the egg, is full of interest,not so much for the exactness of his observations, as for its incidental revela-tion of the extraordinary ignorance then prevalent in regard to the origin and propagation of "this minute and despised creature," which some asserted to be produced from sand, others from dust, others from the dung of pigeons, and others from urine, but which he, showed to be "endowed with as great perfection in its kind as any large animal," and proved to breed in the regular way of winged insects. He even noted the fact that the pupa of the flea is sometimes attacked and fed upon by a mite,an observation which suggested the well-known lines of Swift.
Although Bonnet is usually credited with the discovery of the viviparous propagation of the Aphides, this had been really made by Leetiwenhoek half a century previously. For, his attention having been drawn to the blighting of the young shoots of fruit--trees, which was commonly attributed to the ants found upon them, he was the first to find the Aphides that really do the mischief; and, upon searching, after his wont, into the history of their generation, he observed the young within the bodies of their parents. He carefully studied also the history of the ant, and was the first to show that what had been commonly reputed to be "ants egg" are really their pupae, containing the perfect insect nearly ready for emersion, whilst the true eggs are far smaller, and give origin to "maggots " or larvae.
Of the sea-mussel, again, and other shell-fish, he argued (in reply to a then recent defence of Aristotles doctrine by Bonami, a learned Jesuit of Italy) that they are not generated out of the mud or sand which is found on the sea-shore or the beds of rivers at low water, but from spawn, by the regular course of generation. "For my part," he says, "I hold it equally impossible for a small shell-fish to be produced without generation as for a whale to have its origin ia the mud."1 And he maintained the same to be true of the fresh-water mussel (Unio), whose ova he examined so carefully that he saw in them the rotation of the embryo, a phenomenon supposed to have been first discovered long afterwards. "This uncommonly to pleasing spectacle," he says," was enjoyed by myself, my daughter, and the engraver for three whole hours, and we thought it one of the most delightful that could be exhibited. "Not only was he the first discoverer of the rotifers, but he showed "how wonderfully nature has provided for the preservation of their species," by their tolerance of the drying-up of the water they inhabit, and the resist-ance afforded to the evaporation of the fluids of their bodies by the impermeability of the casing in which they then become enclosed. "We can now easily conceive," he says, "that in all rain-water which is collected from gutters in cisterns, and in all waters exposed to the air, animalcules may be found; for they may be carried thither by the particles of dust blown about by the winds. Although Baker is usually credited with the first careful study of the "wheel-animalcule," yet he really added very little to the account long previously given of it by Leeuwenhoek. In the same spirit he investigated the generation of eels, which were at that time supposed, not only by the ignorant vulgar, but by "respectable and learned men," to be produced from dew (!) without the ordi-nary process of generation. He was rewarded by the discovery of their viviparous propagation,his careful and prolonged observa-tions on which point, though since called in question, have never been refuted. The spectacle of the minute eels lying together in a transparent liquid within the body of the female gave him, he says, great pleasure,affording a complete answer to those who said behind his back, "Since Air Leeuwenhoek is endeavouring to establish the regular generation of all animals, let him show us in what manner eels are bred."
Altogether it does not seem too much to affirm that Leeuwenhoek is well entitled to be considered, not only as "the father of scientific microscopy," but as having contributed more than any other naturalist to the overthrow of the doctrine of "spontaneous generation," and as having set a most admirable example of scientific inethod in the prosecution of biological research.
Leeuwenhoeks contributions to"the Philosophical Transactions amounted to one hundred and twelve. He, also published twenty six papers in the Memoirs of the Paris Academy of Sciences. Two collections of his works appeared during his life, one in Dutch, and the other in Latin,the most complete edition having been published at Leyden shortly before his death in 4 vols. (1719-22). A selection from this, translated into English by S. Hoole, was published in London (1798-81), 2 vol,. 4to. (W. B. C.)
FOOTNOTE (page 410)
1 It is much to be regretted that a cabinet which he bequeathed to the Royal Society of London, containing twenty-six of these single microscopes, each mounted with a suitable object, and accompanied by a magnified drawing of it, the whole being the work of his own hands,is no longer in its possession. Baker, in his Treatise on the Microscope, affirms, from personal and careful examination, that (con-trary to the statements of some writers who represented Leeuwenhoek as having worked with globules of glass) "every one of the twenty-six microscopes is a double-convex lens, and not a sphere or globule"; and he states that their magnifying powers range from 40 to 160 diameters.
FOOTNOTE (page 412)
1 Leenwenhoeks argument in this instance was partly based on false premises. For he imagined the Lepraliae with which mussel-shells are often encrusted to be the eggs of the mussels, and the contained Polyzoa, whose sixteen tentacles he figures, to be the young mussels.
The above article was written by William Benjamin Carpenter, C.B., M.D., LL.D., F.R.S.; Fullerian Professor of Physiology at the Royal Institution; Professor of Medical Jurisprudence at University College, 1849; Examiner and Registrar of University of London, 1856; editor of the British and Foreign Medico-Chirurgical Review and a Popular Cyclopaedia of Science; author of Principles of General and Comparative Physiology and The Microscope and its Revelations.