Newton's great life work--the achievement which insured to his name a place among the immortals--was suggested to him by accident. As the story goes, while he was walking one day in a garden, he saw an apple fall from a tree. He speculated upon the reasons for its falling, and ultimately concluded that the same force which causes an apple to fall from a tree holds the heavenly bodies in their places. Further investigation brought him to the unfolding of this general law of gravitation: "Every body in nature attracts every other body with a force directly as its mass, and inversely as the square of its distance." This law is the greatest law of nature. It is the central fact of the physical universe, the cement of the material world, the mighty, mystic shepherdess of space, that keeps the planets from wandering off alone. It is this awful, silent power reaching out from the enormous mass of the sun, that lashes the planets in their furious race, and yet holds them tightly reined in their orbits. Newton was one of the greatest mathematicians, scientists, and thinkers in the history of the world. He died at Kensington, England, on March 20, 1727, and was buried in Westminster Abbey, with the illustrious dead of Great Britain. [Illustration: SIR ISAAC NEWTON] The operation of this law of gravitation pointed the way to the discovery of the planet Neptune, which is considered the greatest triumph of mathematical astronomy since the days of Newton. Prior to the discovery of Neptune, Uranus was the outermost known planet of the solar system. It was noticed that Uranus was being pulled out of its proper path. It was being tugged away by some strange force beyond the edge of the known planetary system. As the result of a skilful and laborious investigation, Leverrier, a young French astronomer, wrote in substance to an assistant in the observatory at Berlin: "Direct your telescope to a point on the ecliptic in the constellation of Aquarius in longitude 326°, and you will find within a degree of that place a new planet, looking like a star of the ninth magnitude, and having a perceptible disk." Leverrier did not know of the existence of such a planet. He calculated its existence, location, and mass from the fact that some such body must be there, to account for the disturbance caused to Uranus. The telescope in the Berlin Observatory was directed to the place designated by Leverrier, and on the night of September 23, 1846, in exact accordance with his prediction and within half an hour after the astronomers had begun looking, Neptune was discovered within less than one degree from the exact spot where Leverrier had calculated it must be. Such are the triumphs of the human mind. Such are the failures of nature to hide her secrets from the inquiry of man, even behind untold millions of miles.