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SIR ISAAC NEWTON 347 tainly pursued his experiments to a late period of his life in the hope of effecting some valua- ble transmutations. In 1669 he became Luca- sian professor of mathematics at Cambridge, and during that and the next year spent con- siderable time in writing notes for a Latin translation of Kinckhuysen's algebra. In 1669- '72 he delivered a course of lectures on optics in the university, and from time to time com- municated to the royal society the results of his researches on light and colors. The new doctrine of the compound nature of light in- volved Newton in a long and acrimonious con- troversy both at home and abroad. Newton considered light to consist of material parti- cles. Hooke believed it to result from a se- ries of undulations of an elastic medium per- vading all bodies. With this theory, which Huygens maintained in common with Hooke, Newton's alleged discovery seemed incompati- ble, and was accordingly strenuously resisted. On the other side Newton himself rejected Huygens's beautiful law of double refraction in Iceland spar, "founded on the finest ex- perimental analysis of the phenomena," be- cause it was presented as a corollary of the un- dulatory theory. Hooke in the one case, and Newton in the other, failed alike to see, or re- fused to admit, that the principle in question was true or false independently of what light is, or how it is propagated. It is remarkable that Newton should have missed, in the course of his optical experiments and the controversy which followed, the discovery of the different dispersive powers of different bodies. The opinion that all bodies produce spectra of equal length under the same angle of refraction, though "unsupported by experiments," ob- serves Brewster, " and not even sustained by any theoretical views, seems to have been im- pressed upon his mind with all the force of an axiom ; and when, under the influence of this blind conviction, he pronounced the improve- ment of the refracting telescope to be despe- rate, he checked for a long time the progress of this branch of science, and furnished to future philosophers a lesson which cannot be too deeply studied." .From 1671 to 1676 his optical researches and the disputes in which they involved him seem to have occupied most of his time. He wrote to Leibnitz, Dec. 9, 1675 : "I was so persecuted 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." In January, 1672, he was elected fellow of the^ royal society ; but he offered his resig- nation in March, 1673, on the ground of being unable to attend the meetings. An interesting document in Newton's handwriting, entitled " A Scheme for establishing the Royal Soci- ety," has been brought to light by Sir David Brewster. His desire, it seems, was to con- vert the royal society into an institution like the academy of sciences in Paris. On Dec. 9, 1675, he sent to the society his " Hypothesis explaining the Properties of Light " (reprinted in the "Philosophical Magazine," September, 1846), and his " Explanation of the Colors of Thin Plates and of Natural Bodies," which Brewster says "is perhaps the loftiest of all his speculations." The phenomena of colors he ascribes to a supposed property of light, which he calls "fits of easy reflection and transmis- sion." The theory is that every particle of light, from its first discharge from a luminous body, possesses, at equally distant intervals, dispositions to be reflected from and transmit- ted through the surfaces of bodies upon which it is incident. This appears to have been his last communication to the society on optical subjects. In 1704 he published his great work on optics ; much of it was written as early as 1675, and most of the remainder about 1687. In 1679 Newton's attention was recalled to the subject of universal gravitation by a letter from Hooke, who declared that " if gravity decreased according to the reciprocal of the square of the distance, the path of a projectile would be an ellipse having the centre of the earth in the focus." Newton had hitherto con- fined his researches to bodies revolving in cir- cular orbits. He now demonstrated the mathe- matical necessity of the three laws of Kepler as applied to the motion of a body projected in free space, and acted upon continually by a force directed toward the focus and varying inversely as the square of the distance. New- ton could not consider the law of gravitation established so long as the serious discrepancy found in his calculations upon the moon re- mained unaccounted for. In 1680, hearing of a new measurement of a degree of the merid- ian by Picard, the French astronomer, which differed materially from the commonly received estimate, he went over the calculation again on the basis of the new measure, and the result was in exact agreement with observation. He perceived that the earth, by its axial rotation and the mutual attraction of the particles of matter composing its mass, must be flattened at the poles, and he determined the amount of this flattening, though according to an incorrect law for the variation of the earth's density. He showed that the spheroidal figure of the earth, combined with its diurnal motion, would cause the weights of bodies at the surface to vary in different latitudes ; and this result of pure theory explained a singular fact first no- ticed by the French astronomer Richer, who in 1672 had found that a clock regulated to mean time of Paris lost 2m. 28s. daily at Cay- enne, within 5 of the equator. This led him to an explanation of the precession of the equi- noxes. Kepler and others before him had spoken of an attraction of the waters of the earth by the moon. Newton explained the tides. He saw that the masses of the planetary bodies could be determined by observing the effects of their mutual attraction, and that from this cause their several motions would be disturbed. Thus he was conducted to an