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 of Robert Hooke (b. 1635, d. 1703), one of the founders of the Royal Society, and at one time its Secretary.

Hooke, who was both an observer and a theorist, made two experimental discoveries which concern our present subject; but in both of these, as it appeared, he had been anticipated. The first was the observation of the iridescent colours which are seen when light falls on a thin layer of air between two glass plates or lenses, or on a thin film of any transparent substance. These are generally known as the "colours of thin plates," or "Newton's rings"; they had been previously observed by Boyle Hooke's second experimental discovery made after the date of the Micrographia, was that light in air is not propagated exactly in straight lines, but that there is some illumination within the geometrical shadow of an opaque body. This observation had been published in 1665 in a posthumous work of Francesco Maria Grimaldi (b. 1618, d. 1663), who had given to the phenomenon the name diffraction.

Hooke's theoretical investigations on light were of great importance, representing as they do the transition from the Cartesian system to the fully developed theory of undulations. He begins by attacking Descartes' proposition, that light is a tendency to motion rather than an actual motion. “There is," he observes, "no luminous Body but has the parts of it in motion more or less"; and this motion is "exceeding quick." Moreover, since some bodies (e.g. the diamond when rubbed or heated in the dark) shine for a considerable time without being wasted away, it follows that whatever is in motion is not permanently lost to the body, and therefore that the motion must be of a to-and-fro or vibratory character. The amplitude of the vibrations must be exceedingly small, since some luminous bodies eg, the diamond again) are very hard, and so cannot yield or bend to any sensible extent.