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 Wallis, were the books first studied by Newton at Cambridge. He read the geometry of Descartes diligently, and looked into the subject of judicial astrology, which then engaged some attention. He read little of Euclid, and is said to have regretted, in a subsequent part of his life, that he had not studied the old mathematician more deeply.

The attention of Newton, while he was pursuing his studies at Cambridge, was attracted to a branch of natural philosophy hitherto little understood—namely, light. It was the opinion of the celebrated philosopher Descartes that light is caused by a certain motion or undulation of a very thin elastic medium, which he supposed pervaded space. Newton overturned this theory. Taking a piece of glass with angular sides, called a prism, he caused the sun to shine upon it through a small hole in the shutter of a darkened apartment. By this experiment he found that the light, in passing through the glass, was so refracted or broken, as to exhibit on the wall an image of seven different tints or colors; and after varying his experiments in a most ingenious way, he established the very interesting facts, that light is composed of rays resoluble into particles, that every ray of white light consists of three primary and differently colored rays (red, yellow, and blue), each of which three is more or less refrangible than the other. This remarkable discovery laid the foundation of the science of optics.

In 1665, the students of the university of Cambridge were suddenly dispersed by the breaking out of a pestilential disorder in the place. Newton retired for safety to his paternal estate: and though he lost for a time the advantages of public libraries and literary conversation, he rendered the years of his retreat a memorable era in his own existence, and in the history of science, by another of his great discoveries—that of the theory of gravitation, or the tendency of bodies towards the center of our globe. One day, while sitting in his garden, he happened to see an apple fall from a tree, and immediately began to consider the general laws which must regulate all falling bodies. Resuming the subject afterwards, he found that the same cause which made the apple fall to the ground, retained the moon and planets in their orbits, and regulated, with a simplicity and power truly wonderful, the motions of all the heavenly bodies. In this manner was discovered the principle of gravitation, by a knowledge of which the science of astronomy is rendered comparatively perfect.

On his return to Cambridge in 1667, he was elected Fellow of Trinity College; and two years afterwards, he was appointed professor of mathematics in the place of his friend Dr. Barrow, who resigned. His great discoveries in the science of optics formed for some time the principal subject of his lectures, and his new theory of light and colors was explained, with a clearness arising from perfect knowledge, to the satisfaction of a crowded and admiring audience. He was elected a Fellow of the Royal Society in 1671, and is reputed to have been compelled to apply for a dispensation from the usual payment of one shilling weekly, which is contributed by each member towards the expenses. He had at this period of life no income except what he derived from his college and professorship, the produce of his estate being absorbed in supporting his mother and her family. His personal wishes were so moderate, that he never could regret the want of money, except as much as it limited his purchases of books and scientific instruments, and restricted his power of relieving the dis