Leonardo Da Vinci*
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For eyeballs (notoriously squishy and hard to cut), he had the brainstorm of coating them with egg white first. Then he boiled them, to make them firmer, like hard-boiled eggs.
He didn’t discard the bones and, in fact, was the first person to describe the human skeleton correctly. To discover how the parts of the body worked together, he would take a skeleton, insert copper wires where the muscles would go, and study how the contraction or relaxation of the wires caused different movements.
He once befriended a one-hundred-year-old man in a hospital, chatting for hours about his unusual longevity. Then the man died, and Leonardo immediately dissected his body to find out the cause of death. In describing the shriveled “artery that feeds the heart,” he may have written the first description of arteriosclerosis (the hardening of the arteries) in history. A short time after dissecting the old man, he dissected the body of a two-year-old child. He noted all the differences between the healthy young organs and those he found in the body of the old man.
More than a century ahead of his time, Leonardo theorized that the heart was a thick muscle that pumped blood. The heart, according to Galen, was not even a muscle at all, but some unique tissue unlike anything else in the body. And in Leonardo’s day, medical schools were teaching that blood came from two places—the liver as well as the heart.
Scientists then, and for centuries before, believed the valve to the heart to be a passive one. To test this, Leonardo created a glass model of the human aortic valve, inserting it into a cow’s heart filled with water. He then poured water into the valve with bits of paper mixed in so that he could follow the movement of this new water. With this experiment, he demonstrated the correct motions of the valve opening and closing: he proved the valve was active.
Leonardo was the first person known to make a drawing of a baby inside a womb, although it wasn’t entirely accurate. (The sacklike placenta was the wrong shape; it looked more like a cow placenta.) He was also one of the first to state that the mother and father have equal influence on an embryo—the belief of the day was that all characteristics of a baby came from the father.
Leonardo distilled his anatomy research into some 1,500 three-dimensional, multilayered drawings—again, he not only wanted to see but to record what he saw. The results were the first attempts at accurate depiction of human organs, muscles, and bones in history. His drawings have influenced medical textbooks to this day. There are cutaways and cross sections to show layers of an organ from various angles, as well as see-through images and sketches that portray as much motion as possible. His goal was to show the parts of the body in three dimensions. Five hundred years later, the drawings appear perfectly at home on the Internet.
Anatomy led to studies about vision and eyes, and Leonardo tried to break new ground in optics, although his knowledge was often primitive. In his day, many accepted Plato’s belief that we see because our eyes project rays of light onto objects, then the rays are reflected back to the eye in an image. But Leonardo questioned Plato: how could this be true? If it were, wouldn’t we see objects closer to us before we see ones farther away? But we don’t. Our eyes take in a scene all at once.
Leonardo observed that when a knife stuck in a table was made to vibrate, it gave the illusion of two knives. For Leonardo, this was more evidence that the eye receives images. It also told him that the eye finds it hard to distinguish images in quick succession.
As another test, Leonardo put a glass of water on a windowsill so that sunlight struck it. He observed that sunlight penetrated the glass and separated into different colors. His conclusion: the colors—the changes in the light—were a result of the water in the glass, not of what the eye was “projecting.” In this and other experiments, he was influenced by the eleventh-century Arab philosopher Alhazen, who wrote a collection of essays on optics called Opticae Thesaurus.
Having mastered linear perspective as an artist also helped Leonardo develop his theories about vision. The principles of perspective set out by his old friend Alberti (who in turn stood on the shoulders of Brunelleschi) contained their own optical theories. But no artist was exploring vision as thoroughly as Leonardo was.
He kept revising his ideas until he came up with his own, simpler theory about light. From watching ripples made by stones tossed in a river, he leaped to the theory that light traveled in waves, and many believe he was the first person to realize this.
He was the first to write about the difference between peripheral (on the edge) and central vision. Also, he understood that a pair of eyes gathers information stereoscopically; the image seen by the left eye blends with the same image as seen by the right eye, allowing for depth perception. He discovered the reasons for farsightedness, and the principle behind the contact lens. He accurately listed the conditions under which the pupil of the eye changes in size. And he created a variety of optical devices, including what some believe was an early form of the telescope.
Leonardo had no great wealth to finance a laboratory; indeed, he brought the humblest of tools to his experiments. To test theories in optics and other fields, he used buckets, funnels, the eye of a needle, the ends of candles, metal boxes, sheets of paper pierced with holes, and the strings of a lute.
As with anatomy and optics, his notebook studies about water were breathtaking in their ambition. Pages had titles like “How to deal with rivers,” “Of the flow and ebb,” “Of what is water,” and “Of the sea, which to many fools appears to be higher than the earth which forms its shore.”
He studied all aspects of hydraulics (how to control water and use its power). He devised a scheme to divert rivers into canals and to reroute the Arno River, and invented various ingenious machines, among them drawings for an underwater vehicle resembling a submarine. He would spend hours on the banks of a river with his ear to a submerged tube, learning about how sound travels in water. Perhaps at some point he fell in—he even wrote swimming instructions and what to do if you were caught in a whirlpool.
Ripples and waves—how did they move? Leonardo dropped different-shaped objects into a bucket of water and saw that the ripples always formed in a circular pattern. He dropped in two objects at the same time to record the effect of merging ripples. His powers of observation were so keen that what he could see with the naked eye requires high-speed photography to record.
Watching waves—and depicting them in beautiful, curling drawings—led him into areas such as meteorology and geology. He seemed to have understood the principle of erosion, describing the way waves carry sand away and the way water “gnaws at mountains.” He learned the effect of the moon on the tides, speculated about continent formation, and analyzed the nature of fossil shells found on moun taintops. One would think that, with his artistic eye, he would have been most interested in the beauty of shells and fossils—their forms and patterns. But he was after something else: to understand why they were there at all.
Leonardo grasped the principle that flooding water deposits layer upon layer of sediments (soil and sand), which turn into rock. At the same time, rivers erode rocks and carry their sediments to the sea, in a continuous cycle. He wrote, “The stratified stones of the mountains are all layers of clay, deposited one above the other by the various floods of the rivers.”
In Leonardo’s day, there were two theories about why fossils and shells were found in rocks on the tops of mountains. Some people believed the shells were carried there by the biblical Flood; others thought that these shells had grown in the rocks.
Leonardo pooh-poohed both hypotheses—such opinions “cannot exist in a brain of much reason.” From his direct observation of shells and fossilized seaweed during walks in the Italian Alps, he came up with a third theory, one that is closer to the modern one. Shell fossils were once living organisms that had been buried at a time before the mountains were formed: “Where there is now raised land, there was once ocean.” To Leonardo, as to modern paleon tologists, fossils indicated that the history of the earth extended far beyond
human records (such as the Bible)—“things are much more ancient than letters.” Such theories would have offended a strict religious sensibility, as would his scrutiny of Bible passages for lack of scientific logic.
One day Leonardo wrote, “The sun does not move,” in large letters on a page all by itself. We don’t know exactly what he meant by this. He wasn’t sure—he contradicted himself elsewhere in the notebooks. But was he beginning to question Ptolemy’s ancient and still-popular view that the sun moved around the earth?
The notebooks covered a wealth of miscellaneous offerings—whatever interested Leonardo’s butterfly mind. He tried to come up with a formula for making a synthetic material, something like plastic. It combined saffron, poppy dust, and whole lilies boiled together with eggs and glue.
Numerous themes, however, recur over and over: for example, the manipulation of nature through technology. The pages detailed all sorts of machines he designed with gears, cogwheels, screws, and pulleys. He invented a bicycle that would have really worked. He borrowed freely from what others were doing at the time (as he did in all fields), but never without questioning the work or trying to improve it. Machines of all sorts fascinated him. In fact, Leonardo viewed the human body as a machine—the ultimate machine—capable of being understood by looking at its different parts.
Leonardo wanted to find new sources of energy. In an era when the main source of power was muscle (of men and horses), he looked at new ways of using water, wind, and steam. He constructed a device to measure the volume of steam coming off a certain quantity of boiling water. Some think he anticipated the invention of the steam engine hundreds of years later; at the very least, he understood the concept of steam as power. He also proposed using solar energy, trapped by mirrors he invented, to help out the textile industry.
And, of course, there was mastery of the air, his favorite and most obsessive dream. His notebooks played endlessly with this theme. He drew parachutes, gliders made from silk and reeds, wings with all combinations of strings and pulleys, and even a sort of helicopter with a whirling spiral of fabric above it. He puzzled about the best shape—should the flying object be a butterfly with four wings, a canoe with attachments, or more like a windmill’s sails? Whatever the shape, his flying devices anticipated sophisticated principles of aerodynamics, the branch of physics having to do with motion of the air.
His ideas were the results of years of observing birds and sketching them. “The bird is an instrument operating through mathematical laws,” he believed—laws that could be figured out and applied to human flight. “As much pressure is exerted by the object against the air as by the air against the body,” he wrote—a startling observation not fully developed until Isaac Newton in the seventeenth century. He observed bats and flies equally, and was a great admirer of the aerial techniques of bees. He borrowed ideas from everyone before him who had ever contemplated flying.
Historians agree that his contraptions were probably not technically workable, but they disagree as to whether he actually made or tested any flying machine himself. If he did, we have no record of it.
Leonardo wasn’t always right. For example, he was intrigued by the study of physiognomy, the “science” of evaluating a person’s character by his or her facial features. Like everyone in his time, he believed that a person’s inner value was reflected on the outside. That is, good-looking people were virtuous, and ugly ones were bad. (Physiognomy was all the rage until completely debunked well after Leonardo’s day.)
He believed in a sixth sense, “common sense,” that ruled the other five senses. It was located, Leonardo said, in the center of the brain, “between sensation and memory.” Like others of his day, he speculated (wrongly) about which sections of the brain related to distinct skills, and thought that nerves from the brain led directly to all body parts.
He was the first person to depict correctly the relationship of the small and large intestines, but in general he failed to grasp the digestive process. He was clueless about peristalsis (rhythmic contractions of the esophagus that propel food along), believing instead that food moved because of intestinal gas. He thought the purpose of the appendix was to relieve gas pressure. (Actually, it has no known purpose.)
His drawings of the reproductive system, based on Galen, were imaginative, but more inaccurate than accurate. His knowledge of women’s anatomy lagged behind his knowledge of men’s. Doctors at the time believed that a woman’s uterus had seven chambers; Leonardo accepted this at first, though he soon realized it was false. But he never challenged the ancient belief that during pregnancy, a woman’s menstrual blood travels up the blood vessels to the nipples to become the mother’s breast milk.
Sometimes his theories were more poetry than science: “Tears come from the heart and not from the brain,” he once wrote. He believed that children who were born out of love and desire would become intelligent and beautiful, while “unworthy” children would result from relationships of reluctance or scorn.
Partly because he was so far ahead of his time, his descriptions of experiments and theories were sometimes confusing. The proper scientific vocabulary simply didn’t exist yet.
His desire to link things could lead him astray; he tried to make connections or parallels that didn’t exist. Leonardo believed that, just as the heart inside our body pumps blood, an “underground” heart was the source of rivers, instead of the water cycle we know today.
He was always stretching to formulate all-encompassing principles—“everything travels in waves,” “every natural phenomenon is produced by the shortest possible route,” “motion is the principle of all life.” Sometimes thinking big like this caused Leonardo to see patterns not always there. In his most famous drawing, the anatomically correct Vitruvian Man, he showed how the human body could be both a square and a circle. These shapes, he theorized, formed the basis of everything in the world. In this case, his theory was incorrect—another example of his seeing too much interconnection.
Sadly, he never gained mastery over mathematics, especially algebra. He even occasionally made basic mistakes in his arithmetic. He’d add up a list of numbers in his notebooks—and come up with the wrong total.
But in whatever he was investigating, Leonardo accepted nothing at face value. His theories were based on observation, documentation, and proof: “There is nothing more deceptive than to rely on your own opinion, without any other proof.”
In the thousands upon thousands of pages in the notebooks, he was thinking like a scientist.
CHAPTER TEN
“I Have Wasted My Hours”
FROM 1500, WHEN Leonardo turned orty-eight, until just before his death in 1519, he was essentially homeless. Without even a country to ground him, he lived at times from day to day. A steady, sympathetic patron was once again proving elusive.
Traveling about with his small household made up of Salai and Luca Pacioli, he tried, within his limited means, to act the part of a refined aristocrat. He had the best horses. His servants were always well dressed, and he himself wore brocade and other fine fabrics.
In his trunks were precious cargo, forty books and his secret notebooks, except when he thought he might be in personal danger. Then he would leave them in a monastery, with someone he knew, for safe-keeping.
At least one observer noticed that he had grown “weary of the paintbrush.” He often turned commissions over to his assistants. The reason to accept art commissions was to finance his experiments. He spent his days observing, measuring, dissecting, questioning, weighing, and analyzing—and cataloging it all in his notebooks.
In Florence, after the overthrow of the pleasure-loving, free-thinking Medici in 1494, the most powerful person was a teacher of religion named Girolamo Savonarola. In 1496, he staged a mass burning of what he considered immoral books and works of art. Luckily, only a few trusted friends knew about Leonardo’s notebooks, or even of his interest in science. But many of his friends, labeled decadent, suffered under Savonarola.
Wherever he traveled, he drew gorgeous maps, depicting geography with more detail and accuracy than any previous cartographers. But after nine months, Borgia’s atrocities may have proved too upsetting to Leonardo, who quit his post.
In 1504, he was invited to depict a Florentine battle victory for the city’s town hall. His archrival, the twenty-nine-year-old Michelangelo, was invited to paint another battle scene at the same time on another wall in the same room.
The two geniuses had never gotten along. Michelangelo showed no interest in science, which to Leonardo meant his art was inferior. Michelangelo had once publicly insulted the older artist for his habit of leaving things unfinished. Leonardo, for possibly the first time in his life, had no instant come-back. He just blushed.
Leonardo put three years into his battle scene. He struggled to convey all the horrors of war. But while he was experimenting, trying to achieve the most brilliant colors possible, the paint on the wall ran and . . . well, he never actually finished the painting.
Leonardo also worked on several portraits during these years. The only one that survives is one he never titled. We call it the Mona Lisa. Leonardo seems to have had a special affection for the picture, for he never sold it, taking it with him on all of his subsequent travels.