Christiaan Huygens was the Dutch physicist who formulated a principle that forms a key part of the theory of waves. Huygens' interests spanned all aspects of mathematics, optics, astronomy and mechanical philosophy. He invented the pendulum clock in 1657, based on Galileo's suggestions, about the theory of pendulums and harmonically driven systems. In 1655, he discovered the rings and fourth satellite of Saturn using a refracting telescope that he and his brother had built. His greatest achievement was the formulation of his wave theory of light, which he published in his Treatise on Light (1690). This theory dealt with refraction and reflection, and also predicted (as Newtonian theory did not) that light should travel more slowly in a dense medium than in a less dense one. Huygens was, with Newton and Galileo, one of the greatest physicists of the seventeenth century.
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