Edison at once busied himself with inventing. He improved and invented various machines used in the stock markets, and in 1872 perfected his system of duplex telegraphy. Two years later he brought out the wonderful quadruplex system, by which four messages may be sent over the same wire at the same time. This system saved millions of dollars and dispensed with thousands of miles of poles and wires. He started a large factory at Newark, New Jersey, employing some three hundred men. Sometimes he was working on as many as forty-five improvements and original inventions at once. In 1876 he stopped manufacturing and turned all his attention to inventing. In that year he established a laboratory at Menlo Park, New Jersey, twenty-five miles from New York City. When this laboratory was outgrown, he founded a new one at Orange, New Jersey, the largest laboratory ever established by one man for scientific research and invention. It comprises one building 250 feet long and three stories high, and four smaller buildings, each one hundred feet long and one story high. The principal building contains a library of thirty thousand reference books, a lecture room, and an exhibition room, where a remarkable collection of instruments of almost every kind is to be seen. When Edison began working to produce an incandescent electric light for illuminating residences and small rooms, most of the scientists of England said that such a light could not be produced. For nine years he worked on this invention. The chief problem was to find, for the horseshoe thread or filament used to give off the light, a material that should glow with sufficient intensity and yet not be consumed by the great heat necessary to produce the light. In his search for this material he tried all kinds of rags and textiles steeped in various chemicals, different kinds of paper, wood, inner and outer bark, cornstalks, etc. Finally he sent one of his assistants to the East, and in Japan a kind of bamboo was found answering the requirements. Perseverance won, and the incandescent electric light became a reality about 1880. [Illustration: AN INCANDESCENT LIGHT] Thomas Edison is one of the most systematic of workers, and nearly all his inventions have been the result of intelligent and methodical labor directed toward a definite aim. He reads carefully what other investigators have found out, so as not to waste time in going over fruitless ground. He also keeps copious note books of his own operations, so that there may be no loss of time and energy. His invention of the phonograph, however, was accidental. While he was working to improve the telephone, the idea of the phonograph suddenly came into his mind. A little while afterward the first phonograph, crude but successful, was finished. At first this instrument was regarded as a toy, but later the invention was sold for a million dollars.