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Italian physicist, b. at Bologna, 2 April, 1618; d. in the same city, 28 Dec., 1663. He entered the Society of Jesus, 18 March, 1632; and, after the usual course of studies, spent twenty-five years as professor of belles-lettres in the colleges of the order. His tastes were, however, scientific, and he found time for study and research in physics and astronomy, to which he devoted himself almost entirely in his later years. He assisted P. Riccioli in his experiments (1640-1650) on falling bodies, and in his surveys, in 1645, to determine the length of an arc of the meridian. He was also a close observer of the moon's surface and constructed a map which was incorporated in Riccioli's "Almagestum Novum". He gave the names of illustrious philosophers and astronomers to the elevations and depressions on the moon to which Hevelius, before him, had applied the names borne by terrestrial seas and mountains.

Grimaldi's most important scientific work was done in optics, in which field he became a worthy predecessor of Newton and Huyghens. He made several discoveries of fundamental importance, but they were much in advance of the theory of the time, and their significance was not recognized until over a century later. The first of these is the phenomenon of diffraction. He allowed a beam of sunlight to pass through a small aperture in a screen, and noticed that it was diffused in the form of a cone. The shadow of a body placed in the path of the beam was larger than that required by the rectilinear propagation of light. Careful observation also showed that the shadow was surrounded by coloured fringes, similar ones being seen within the edges, especially in the case of narrow objects. He showed that the effect could not be due to reflection or refraction, and concluded that the light was bent out of its course in passing the edges of bodies. This phenomenon, to which he gave the name of diffraction, was also studied by Hooke and Newton; but the true explanation was only given by Fresnel on the basis of the wave theory. Grimaldi also discovered that when sunlight, entering a room through two small apertures, was allowed to fall on a screen, the region illuminated by the two beams was darker than when illuminated by either of them separately. He was thus led to enunciate the principle that an illuminated body may become darker by adding light to that which it already receives. This is, in reality, the well-known principle of interference afterwards so brilliantly employed by Young and Fresnel. It has been questioned whether the phenomenon observed by Grimaldi was really due to interference. He himself regarded it simply as a conclusive proof of the immaterial nature of light which he was then investigating. He was likewise the first to observe the dispersion of the sun's rays in passing through a prism. Grimaldi was conspicuous for his amiability, gentleness, and modesty. He was the author of "Physicomathesis de lumine, coloribus, et iride, aliisque annexis" (Bologna, 1665), published after his death.

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