The 100 Most Influential Scientists of All Time (6 page)

Leonardo's fascination with anatomical studies reveals a prevailing artistic interest of the time. In his own 1435 treatise
Della pittura
(“On Painting”), theorist Leon Battista Alberti urged painters to construct the human figure as it exists in nature, supported by the skeleton and musculature, and only then clothed in skin. The date of Leonardo's initial involvement with anatomical study is not known nor can it be determined exactly when Leonardo began to perform dissections, but it might have been several years after he first moved to Milan, at the time a centre of medical investigation. His study of anatomy, originally pursued for his training as an artist, had grown by the 1490s into an independent area of research. As his sharp eye uncovered the structure of the human body, Leonardo became fascinated by the
figura istrumentale dell' omo
(“man's instrumental figure”), and he sought to comprehend its physical working as a creation of nature. Over the following two decades, he did practical work in anatomy on the dissection table in Milan, then at hospitals in Florence and Rome, and in Pavia, where he collaborated with the physician-anatomist Marcantonio della Torre. By his own count Leonardo dissected 30 corpses in his lifetime.

Leonardo's early anatomical studies dealt chiefly with the skeleton and muscles. Yet even at the outset, he combined anatomical with physiological research. From observing the static structure of the body, Leonardo proceeded to study the role of individual parts of the body in mechanical activity. This led him finally to the study of the internal organs; among them he probed most deeply into the brain, heart, and lungs as the “motors” of the senses
and of life. His findings from these studies were recorded in the famous anatomical drawings, which are among the most significant achievements of Renaissance science. The drawings are based on a connection between natural and abstract representation. He represented parts of the body in transparent layers that afford an “insight” into the organ by using sections in perspective, reproducing muscles as “strings,” indicating hidden parts by dotted lines, and devising a hatching system. The genuine value of these
dimostrazione
lay in their ability to synthesize a multiplicity of individual experiences at the dissecting table and make the data immediately and accurately visible. As Leonardo proudly emphasized, these drawings were superior to descriptive words. The wealth of Leonardo's anatomical studies that have survived forged the basic principles of modern scientific illustration. It is worth noting, however, that during his lifetime, Leonardo's medical investigations remained private. He did not consider himself a professional in the field of anatomy, and he neither taught nor published his findings.

Although he kept his anatomical studies to himself, Leonardo did publish some of his observations on human proportion. Working with the mathematician Luca Pacioli, he considered the proportional theories of Vitruvius, the 1st-century BCE Roman architect, as presented in his treatise
De architectura
(
On Architecture
). Imposing the principles of geometry on the configuration of the human body, Leonardo demonstrated that the ideal proportion of the human figure corresponds with the forms of the circle and the square. In his illustration of this theory, the so-called Vitruvian Man, Leonardo demonstrated that when a man places his feet firmly on the ground and stretches out his arms, he can be contained within the four lines of a square, but when in a spreadeagle position, he can be inscribed in a circle.

Leonardo envisaged the great picture chart of the human body he had produced through his anatomical drawings and Vitruvian Man as a
cosmografia del minor mondo
(“cosmography of the microcosm”). He believed the workings of the human body to be an analogy, in microcosm, for the workings of the universe. Leonardo wrote: “Man has been called by the ancients a lesser world, and indeed the name is well applied; because, as man is composed of earth, water, air, and fire … this body of the earth is similar.” He compared the human skeleton to rocks (“supports of the earth”) and the expansion of the lungs in breathing to the ebb and flow of the oceans.

According to Leonardo's observations, the study of mechanics, with which he became quite familiar as an architect and engineer, also reflected the workings of nature. Throughout his life Leonardo was an inventive builder. He thoroughly understood the principles of mechanics of his time and contributed in many ways to advancing them. His two Madrid notebooks deal extensively with his theory of mechanics; the first was written in the 1490s, and the second was written between 1503 and 1505. Their importance lay less in their description of specific machines or work tools than in their use of demonstration models to explain the basic mechanical principles and functions employed in building machinery. As in his anatomical drawings, Leonardo developed definite principles of graphic representation—stylization, patterns, and diagrams—that offer a precise demonstration of the object in question.

Leonardo was especially intrigued by problems of friction and resistance, and with each of the mechanical elements he presented—such as screw threads, gears, hydraulic jacks, swiveling devices, and transmission
gears—drawings took precedence over the written word. Throughout his career he also was intrigued by the mechanical potential of motion. This led him to design a machine with a differential transmission, a moving fortress that resembles a modern tank, and a flying machine. His “helical airscrew” (
c
. 1487) almost seems a prototype for the modern helicopter, but, like the other vehicles Leonardo designed, it presented a singular problem: it lacked an adequate source of power to provide propulsion and lift.

Wherever Leonardo probed the phenomena of nature, he recognized the existence of primal mechanical forces that govern the shape and function of the universe. This is seen in his studies of the flight of birds, in which his youthful idea of the feasibility of a flying apparatus took shape and that led to exhaustive research into the element of air; in his studies of water, the
vetturale della natura
(“conveyor of nature”), in which he was as much concerned with the physical properties of water as with its laws of motion and currents; in his research on the laws of growth of plants and trees, as well as the geologic structure of earth and hill formations; and finally in his observation of air currents, which evoked the image of the flame of a candle or the picture of a wisp of cloud and smoke. In his drawings based on the numerous experiments he undertook, Leonardo found a stylized form of representation that was uniquely his own, especially in his studies of whirlpools. He managed to break down a phenomenon into its component parts—the traces of water or eddies of the whirlpool—yet at the same time preserve the total picture, creating both an analytic and a synthetic vision.

In an era that often compared the process of divine creation to the activity of an artist, Leonardo reversed the analogy,
using art as his own means to approximate the mysteries of creation, asserting that, through the science of painting, “the mind of the painter is transformed into a copy of the divine mind, since it operates freely in creating many kinds of animals, plants, fruits, landscapes, countrysides, ruins, and awe-inspiring places.” With this sense of the artist's high calling, Leonardo approached the vast realm of nature to probe its secrets. His utopian idea of transmitting in encyclopaedic form the knowledge thus won was still bound up with medieval Scholastic conceptions; however, the results of his research were among the first great achievements of the forthcoming age's thinking because they were based to an unprecedented degree on the principle of experience.

(b. Feb. 19, 1473, Toruń, Pol.—d. May 24, 1543, Frauenburg, East Prussia [now Frombork, Pol.])

P
olish astronomerNicolaus Copernicus (Polish: Mikołaj Kopernik) proposed that the planets have the Sun as the fixed point to which their motions are to be referred; that the Earth is a planet which, besides orbiting the Sun annually, also turns once daily on its own axis; and that very slow, long-term changes in the direction of this axis account for the precession of the equinoxes. This representation of the heavens is usually called the heliocentric, or “Sun-centred,” system—derived from the Greek
helios
, meaning “Sun.”

Copernicus's theory had important consequences for later thinkers of the scientific revolution, including such major figures as Galileo, Kepler, Descartes, and Newton. Copernicus probably hit upon his main idea sometime between 1508 and 1514, and during those years he wrote a manuscript usually called the
Commentariolus
(“Little Commentary”). However, the book that contains the final
version of his theory,
De revolutionibus orbium coelestium libri vi
(“Six Books Concerning the Revolutions of the Heavenly Orbs”), did not appear in print until 1543, the year of his death.

In Copernicus's period, astrology and astronomy were considered subdivisions of a common subject called the “science of the stars,” whose main aim was to provide a description of the arrangement of the heavens as well as the theoretical tools and tables of motions that would permit accurate construction of horoscopes and annual prognostications. At this time the terms
astrologer, astronomer
, and
mathematician
were virtually interchangeable; they generally denoted anyone who studied the heavens using mathematical techniques. Furthermore, practitioners of astrology were in disagreement about everything, from the divisions of the zodiac to the minutest observations to the order of the planets; there was also a long-standing disagreement concerning the status of the planetary models.

From antiquity, astronomical modeling was governed by the premise that the planets move with uniform angular motion on fixed radii at a constant distance from their centres of motion. Two types of models derived from this premise. The first, represented by that of Aristotle, held that the planets are carried around the centre of the universe embedded in unchangeable, material, invisible spheres at fixed distances. Since all planets have the same centre of motion, the universe is made of nested, concentric spheres with no gaps between them. As a predictive model, this account was of limited value. Among other things, it had the distinct disadvantage that it could not account for variations in the apparent brightness of the planets since the distances from the centre were always the same.

A second tradition, deriving from Claudius Ptolemy, solved this problem by postulating three mechanisms: uniformly revolving, off-centre circles called eccentrics; epicycles, little circles whose centres moved uniformly on the circumference of circles of larger radius (deferents); and equants. The equant, however, broke with the main assumption of ancient astronomy because it separated the condition of uniform motion from that of constant distance from the centre. A planet viewed from a specific point at the centre of its orbit would appear to move sometimes faster, sometimes slower. As seen from the Earth and removed a certain distance from the specific centre point, the planet would also appear to move nonuniformly. Only from the equant, an imaginary point at a calculated distance from the Earth, would the planet appear to move uniformly. A planet-bearing sphere revolving around an equant point will wobble; situate one sphere within another, and the two will collide, disrupting the heavenly order. In the 13th century a group of Persian astronomers at Marāgheh discovered that, by combining two uniformly revolving epicycles to generate an oscillating point that would account for variations in distance, they could devise a model that produced the equalized motion without referring to an equant point. This insight was the starting point for Copernicus's attempt to resolve the conflict raised by wobbling physical spheres.

In the
Commentariolus
, Copernicus postulated that, if the Sun is assumed to be at rest and if the Earth is assumed to be in motion, then the remaining planets fall into an orderly relationship whereby their sidereal periods increase from the Sun as follows: Mercury (88 days), Venus (225 days), Earth (1 year), Mars (1.9 years), Jupiter (12 years), and Saturn (30 years). This theory did resolve the disagreement
about the ordering of the planets but, in turn, raised new problems. To accept the theory's premises, one had to abandon much of Aristotelian natural philosophy and develop a new explanation for why heavy bodies fall to a moving Earth. It was also necessary to explain how a transient body like the Earth, filled with meteorological phenomena, pestilence, and wars, could be part of a perfect and imperishable heaven. In addition, Copernicus was working with many observations that he had inherited from antiquity and whose trustworthiness he could not verify. In constructing a theory for the precession of the equinoxes, for example, he was trying to build a model based upon very small, long-term effects. Also, his theory for Mercury was left with serious incoherencies.