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STEVE EMBER: At that time, a deadly plague was spreading across England. To escape the disease, Newton returned to the family farm. He did more thinking than farming. In doing so, he found the answers to some of the greatest mysteries of science.

Newton used his great skill in mathematics to form a better understanding of the world and the universe. He used methods he had learned as a boy in making things. He experimented. Then he studied the results and used what he had learned to design new experiments.

Newton's work led him to create a new method in mathematics for measuring areas curved in shape. He also used it to find how much material was contained in solid objects. The method he created became known as integral calculus.

SHIRLEY GRIFFITH: One day, sitting in the garden, Newton watched an apple fall from a tree. He began to wonder if the same force that pulled the apple down also kept the moon circling the Earth. Newton believed it was. And he believed it could be measured.

He called the force "gravity." He began to examine it carefully.

He decided that the strength of the force keeping a planet in orbit around the sun depended on two things. One was the amount of mass in the planet and the sun. The other was how far apart they were.

STEVE EMBER: Newton was able to find the exact relationship between distance and gravity. He multiplied the mass of one space object by the mass of the other. Then he divided that number by the square of their distance apart. The result was the strength of the gravity force that tied them to each other.

Newton proved his idea by measuring how much gravity force would be needed to keep the moon orbiting the Earth. Then he measured the mass of the Earth and the moon, and the distance between them. He found that his measurement of the gravity force produced was not the same as the force needed. But the numbers were close.

Newton did not tell anyone about his discovery. He put it aside to work on other ideas. Later, with correct measurements of the size of the Earth, he found that the numbers were exactly the same.

SHIRLEY GRIFFITH: Newton spent time studying light and colors. He used a three-sided piece of glass called a prism.

He sent a beam of sunlight through the prism. It fell on a white surface. The prism separated the beam of sunlight into the colors of a rainbow. Newton believed that all these colors -- mixed together in light -- produced the color white. He proved this by letting the beam of rainbow-colored light pass through another prism. This changed the colored light back to white light.

STEVE EMBER: Newton's study of light led him to learn why faraway objects seen through a telescope do not seem sharp and clear. The curved glass lenses at each end of the telescope acted like prisms. They produced a circle of colored light around an object. This created an unclear picture.

A few years later, Newton built a different kind of telescope. It used a curved mirror to make faraway objects seem larger.

Light reflected from the surface of the mirror, instead of passing through a curved glass lens. Newton's reflecting telescope produced much clearer pictures than the old kind of telescope.