Leonardo Da Vinci by Kathleen Krull

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