The 100 Most Influential Scientists of All Time (11 page)

In 1677 he described for the first time the spermatozoa from insects, dogs, and man, though Stephen Hamm
probably was a codiscoverer. Leeuwenhoek studied the structure of the optic lens, striations in muscles, the mouthparts of insects, and the fine structure of plants and discovered parthenogenesis in aphids. In 1680 he noticed that yeasts consist of minute globular particles. He extended Marcello Malpighi's demonstration in 1660 of the blood capillaries by giving (in 1684) the first accurate description of red blood cells. In his observations on rotifers in 1702, Leeuwenhoek remarked that “in all falling rain, carried from gutters into water-butts, animalcules are to be found; and that in all kinds of water, standing in the open air, animalcules can turn up. For these animalcules can be carried over by the wind, along with the bits of dust floating in the air.”

A friend of Leeuwenhoek put him in touch with the Royal Society of England, to which, from 1673 until 1723, he communicated by means of informal letters most of his discoveries and to which he was elected a fellow in 1680. His discoveries were for the most part made public in the society's
Philosophical Transactions
. The first representation of bacteria is to be found in a drawing by Leeuwenhoek in that publication in 1683.

His researches on the life histories of various low forms of animal life were in opposition to the doctrine that they could be produced spontaneously or bred from corruption. Thus, he showed that the weevils of granaries (in his time commonly supposed to be bred from wheat as well as in it) are really grubs hatched from eggs deposited by winged insects. His letter on the flea, in which he not only described its structure but traced out the whole history of its metamorphosis, is of great interest, not so much for the exactness of his observations as for an illustration of his opposition to the spontaneous generation of many lower organisms, such as “this minute and despised creature.” Some theorists asserted that the flea was produced
from sand, others from dust or the like, but Leeuwenhoek proved that it bred in the regular way of winged insects.

Leeuwenhoek also carefully studied the history of the ant and was the first to show that what had been commonly reputed to be ants' eggs were really their pupae, containing the perfect insect nearly ready for emergence, and that the true eggs were much smaller and gave origin to maggots, or larvae. He argued that the sea mussel and other shellfish were not generated out of sand found at the seashore or mud in the beds of rivers at low water but from spawn, by the regular course of generation. He maintained the same to be true of the freshwater mussel, whose embryos he examined so carefully that he was able to observe how they were consumed by “animalcules,” many of which, according to his description, must have included ciliates in conjugation, flagellates, and the
Vorticella
. Similarly, he investigated the generation of eels, which were at that time supposed to be produced from dew without the ordinary process of generation.

Leeuwenhoek's methods of microscopy, which he kept secret, remain something of a mystery. During his lifetime he ground more than 400 lenses, most of which were very small—some no larger than a pinhead—and usually mounted them between two thin brass plates, riveted together. A large sample of these lenses, bequeathed to the Royal Society, were found to have magnifying powers of between 50 and, at the most, 300 times. In order to observe phenomena as small as bacteria, Leeuwenhoek must have employed some form of oblique illumination, or other technique, for enhancing the effectiveness of the lens, but this method he would not reveal. Leeuwenhoek continued his work almost to the end of his long life of 90 years.

Leeuwenhoek's contributions to the
Philosophical Transactions
amounted to 375 and those to the
Memoirs of the Paris Academy of Sciences
to 27. Two collections of his
works appeared during his life, one in Dutch (1685–1718) and the other in Latin (1715–22); a selection was translated by S. Hoole,
The Select Works of A. van Leeuwenhoek
(1798–1807).

(b. July 18, 1635, Freshwater, Isle of Wight, Eng.—d. March 3, 1703, London)

E
nglish physicist Robert Hooke discovered the law of elasticity, known as Hooke's law. He also conducted research in a remarkable variety of fields.

In 1655 Hooke was employed by Robert Boyle to construct the Boylean air pump. Five years later, Hooke discovered his law of elasticity, which states that the stretching of a solid body (e.g., metal, wood) is proportional to the force applied to it. The law laid the basis for studies of stress and strain and for understanding of elastic materials. He applied these studies in his designs for the balance springs of watches. In 1662 he was appointed curator of experiments to the Royal Society of London and was elected a fellow the following year.

One of the first men to build a Gregorian reflecting telescope, Hooke discovered the fifth star in the Trapezium, an asterism in the constellation Orion, in 1664 and first suggested that Jupiter rotates on its axis. His detailed sketches of Mars were used in the 19th century to determine that planet's rate of rotation. In 1665 he was appointed professor of geometry in Gresham College. In
Micrographia
(1665; “Small Drawings”) he included his studies and illustrations of the crystal structure of snowflakes, discussed the possibility of manufacturing artificial fibres by a process similar to the spinning of the silkworm, and first used the word cell to name the microscopic honeycomb cavities in cork. His studies of microscopic fossils
led him to become one of the first proponents of a theory of evolution.

Hooke suggested that the force of gravity could be measured by utilizing the motion of a pendulum (1666) and attempted to show that the Earth and Moon follow an elliptical path around the Sun. In 1672 he discovered the phenomenon of diffraction (the bending of light rays around corners); to explain it, he offered the wave theory of light. He stated the inverse square law to describe planetary motions in 1678, a law that Newton later used in modified form. Hooke complained that he was not given sufficient credit for the law and became involved in bitter controversy with Newton. Hooke was the first man to state in general that all matter expands when heated and that air is made up of particles separated from each other by relatively large distances.

(b. Nov. 29, 1627, Black Notley, Essex, Eng.—d. Jan. 17, 1705, Black Notley)

J
ohn Ray (spelled Wray until 1670) was a leading 17th-century English naturalist and botanist who contributed significantly to progress in taxonomy. His enduring legacy to botany was the establishment of species as the ultimate unit of taxonomy.

Ray was the son of the village blacksmith in Black Notley and attended the grammar school in nearby Braintree. In 1644, with the aid of a fund that had been left in trust to support needy scholars at the University of Cambridge, he enrolled at one of the colleges there, St. Catherine's Hall, and moved to Trinity College in 1646. Ray had come to
Cambridge at the right time for one with his talents, for he found a circle of friends with whom he pursued anatomical and chemical studies. He also progressed well in the curriculum, taking his bachelor's degree in 1648 and being elected to a fellowship at Trinity the following year; during the next 13 years he lived quietly in his collegiate cloister.

Ray's string of fortunate circumstances ended with the Restoration. Although he was never an excited partisan, he was thoroughly Puritan in spirit and refused to take the oath that was prescribed by the Act of Uniformity. In 1662 he lost his fellowship. Prosperous friends supported him during the subsequent 43 years while he pursued his career as a naturalist, which began with the publication of his first work in 1660, a catalog of plants growing around Cambridge. After he had exhausted the Cambridge area as a subject for his studies, Ray began to explore the rest of Britain. An expedition in 1662 to Wales and Cornwall with the naturalist Francis Willughby was a turning point in his life. Willughby and Ray agreed to undertake a study of the complete natural history of living things, with Ray responsible for the plant kingdom and Willughby the animal.

The first fruit of the agreement, a tour of the European continent lasting from 1663 to 1666, greatly extended Ray's first-hand knowledge of flora and fauna. Back in England, the two friends set to work on their appointed task. In 1670 Ray produced a
Catalogus Plantarum Angliae
(“Catalog of English Plants”). Then in 1672 Willughby suddenly died, and Ray took up the completion of Willughby's portion of their project. In 1676 Ray published
F. Willughbeii … Ornithologia (The Ornithology of F. Willughby
…) under Willughby's name, even though Ray had contributed at least as much as Willughby. Ray also completed
F. Willughbeii … de Historia Piscium
(1685; “History of Fish”), with the Royal Society, of which Ray was a fellow, financing its publication.

Ray had never interrupted his research in botany. In 1682 he had published a
Methodus Plantarum Nova
(revised in 1703 as the
Methodus Plantarum Emendata
…), his contribution to classification, which insisted on the taxonomic importance of the distinction between monocotyledons and dicotyledons, plants whose seeds germinate with one leaf and those with two, respectively. Ray's enduring legacy to botany was the establishment of species as the ultimate unit of taxonomy. On the basis of the
Methodus
, he constructed his masterwork, the
Historia Plantarum
, three huge volumes that appeared between 1686 and 1704. After the first two volumes, he was urged to compose a complete system of nature. To this end he compiled brief synopses of British and European plants, a
Synopsis Methodica Avium et Piscium
(published posthumously, 1713; “Synopsis of Birds and Fish”), and a
Synopsis Methodica Animalium Quadrupedum et Serpentini Generis
(1693; “Synopsis of Quadrupeds”). Much of his final decade was spent on a pioneering investigation of insects, published posthumously as
Historia Insectorum
.

In all this work, Ray contributed to the ordering of taxonomy. Instead of a single feature, he attempted to base his systems of classification on all the structural characteristics, including internal anatomy. By insisting on the importance of lungs and cardiac structure, he effectively established the class of mammals, and he divided insects according to the presence or absence of metamorphoses. Although a truly natural system of taxonomy could not be realized before the age of Darwin, Ray's system approached that goal more than the frankly artificial systems of his contemporaries. He was one of the great predecessors who made possible Carolus Linnaeus's contributions in the following century.

Nor was this the sum of his work. In the 1690s Ray also published three volumes on religion.
The Wisdom of God Manifested in the Works of the Creation
(1691), an essay in natural religion that called on the full range of his biological learning, was his most popular and influential book. It argued that the correlation of form and function in organic nature demonstrates the necessity of an omniscient creator. This argument from design, common to most of the leading scientists of the 17th century, implied a static view of nature that was distinctly different from the evolutionary ideas of the early and mid-19th century. Still working on his
Historia Insectorum
, John Ray died at the age of 77.

(b. Dec. 25, 1642 [Jan. 4, 1643, New Style], Woolsthorpe,
Lincolnshire, Eng.—d. March 20 [March 31], 1727, London)

E
nglish physicist and mathematician Sir Isaac Newton was the culminating figure of the scientific revolution of the 17th century. In optics, his discovery of the composition of white light integrated the phenomena of colours into the science of light and laid the foundation for modern physical optics. In mechanics, his three laws of motion, the basic principles of modern physics, resulted in the formulation of the law of universal gravitation. In mathematics, he was the original discoverer of the infinitesimal calculus. Newton's
Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy
), 1687, was one of the most important single works in the history of modern science.

Newton was elected to a fellowship in Trinity College in 1667, and from 1670 to 1672 he delivered a series of
lectures and developed them into the essay “Of Colours,” which was later revised to become Book One of his
Opticks
. Newton held that light consists of material corpuscles in motion. The corpuscular conception of light was always a speculative theory on the periphery of his optics, however. The core of Newton's contribution had to do with colours. He realized that light is not simple and homogeneous—it is instead complex and heterogeneous and the phenomena of colours arise from the analysis of the heterogeneous mixture into its simple components.

The ultimate source of Newton's conviction that light is corpuscular was his recognition that individual rays of light have immutable properties. He held that individual rays (that is, particles of given size) excite sensations of individual colours when they strike the retina of the eye. He also concluded that rays refract at distinct angles—hence, the prismatic spectrum, a beam of heterogeneous rays, i.e., alike incident on one face of a prism, separated or analyzed by the refraction into its component parts—and that phenomena such as the rainbow are produced by refractive analysis. Because he believed that chromatic aberration could never be eliminated from lenses, Newton turned to reflecting telescopes; he constructed the first ever built. The heterogeneity of light has been the foundation of physical optics since his time.