Mathematics, however wonderful, was only a tool for calculating quantities; it did not profess to understand or describe reality. When Newton turned from the tool to the ultimate quest, he addressed himself first to the mystery of light. His first lectures at Cambridge were on light, color, and vision; characteristically he did not publish his Opticks till thirty-five years later, 1704. He had no itch to print.

In 1666 he bought a prism at Stourbridge Fair, and began optical experiments. From 1668 onward he made a succession of telescopes. Hoping to avoid some defects persisting in the refracting telescope, he made with his own hands a reflecting telescope, on theorems set forth by Mersenne (1639) and James Gregory (1662), and presented it to the Royal Society at its request in 1671. On January 11, 1672, he was elected to membership in the Society.

Even before making telescopes he had reached (1666) one of his basic discoveries—that white light, or sunlight, is not simple or homogeneous, but is a compound of red, orange, yellow, green, blue, indigo, and violet. When he passed a small ray of sunlight through a transparent prism he found that the apparently monochrome light divided into all these colors of the rainbow; that each component color emerged from the prism at its own specific angle or degree or refraction; and that the colors arranged themselves in a row of bands, forming a continuous spectrum, with red at one end and violet at the other. Later investigators showed that various substances, when made luminous by burning, give different spectra; by comparing these spectra with the one made by a given star, it became possible to analyze in some degree the star’s chemical constituents. Still more delicate observations of a star’s spectrum indicated its approximate rate of motion toward or from the earth; and from these calculations the distance of the star was theoretically deduced. Newton’s revelation of the composition of light, and its refraction in the spectrum, has therefore had almost cosmic consequences in astronomy.

Hardly foreseeing these results, but feeling (as he wrote to Oldenburg) that he had made “the oddest if not the most considerable detection which hath hitherto been made in the operations of nature,” 18 Newton sent to the Royal Society early in 1672 a paper entitled “New Theory about Light and Colors.” It was read to the members on February 8, and aroused a controversy that crossed the Channel to the Continent. Hooke had described in his Micrographia (1664) an experiment similar to Newton’s with the prism; he had not deduced from it a successful theory of color, but he felt slighted by Newton’s ignoring his priority, and he joined with other members of the Society in criticizing Newton’s conclusions. The dispute lingered on for three years. “I am so persecuted,” wrote the thinskinned Newton, “with discussions arising out of my theory of light that I blamed my own imprudence for parting with so substantial a blessing as my quiet to run after a shadow.” 19 For a time he was inclined to “resolutely bid adieu to philosophy eternally except what I do for my own satisfaction.” 20

Another point of controversy with Hooke concerned the medium through which light is transmitted. Hooke had adopted Huygens’ theory that light traveled on the waves of an “ether.” Newton argued that such a theory could not explain why light traveled in straight lines. He proposed instead a “corpuscular theory”: light is due to the emission, by a luminous body, of innumerable tiny particles traveling in straight lines through empty space with a speed of 190,000 miles per second. He rejected ether as a medium of light, but later accepted it as a medium of gravitational force.*

Newton gathered his discussions of light into the Opticks of 1704. Significantly, it was written in English (the Principia was in Latin), and was addressed “to Readers of quick Wit and Understanding not yet versed in Opticks.” At the end of the book he listed thirty-one queries for further consideration. Query I suggested prophetically: “Do not bodies act upon light at a distance, and by their action bend its rays, and is not this action strongest at the least distance?” And Query XXX: “Why may not Nature change bodies into light, and light into bodies?”

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