Observatory, and it is a remarkable fact that for four successive generations, covering a period of one hundred and twenty-two years, a Cassini was the director of the Paris Observatory. Dominic Cassini gave so much promise that at the age of twenty-five he was appointed Professor of Astronomy at the University of Bologna, and his reputation had already become so great that when Louis XIV, through his ambassador, requested Pope Clement IX and the Senate of Bologna to permit him to go to Paris, they yielded only for the limited term of six years. But, once in Paris, Louis XIV knew how to keep him.
The young Academy of Sciences received a great impetus through the labors of such men as Römer, Huygens, Cassini, Picard, and Mariotte. Cassini completed the unfinished work of Huygens's observations, and Huygens could not have elaborated his doctrines of the undulatory theory of light had not Romer just previously proved the velocity of light.
It was universally believed and taught that light was instantaneous. Römer observed that the eclipses of the satellites of Jupiter were earlier or later than the calculated time according to the time of year. He discovered that the premature eclipses always occurred when the earth was in its orbit nearest to Jupiter, and the delayed eclipses when farthest away; that the difference in time was about eleven minutes, which he correctly assumed as the time it took light to traverse the orbit of the earth. The velocity of light was thus mathematically established and measured. This discovery of Römer was made use of by Huygens in his development of the undulatory theory of light.
Hooke had, indeed, in his Micrographia, suggested some such an explanation of light; but to Huygens justly belongs the great honor of giving his theory a scientific basis. He taught that light was propagated in waves spherically, after the manner of sound. The adherents of the emission theory of light argued that light only moved in straight lines, but not around a corner as sound does, and as light should do if it moved in like manner.
The point was well taken, and for a long while was the stumbling-block in the way of the undulatory theory. Huygens met this objection, and time has proved its correctness. He says light will not be diffused beyond the rectilinear space when it passes through an aperture, "for, although the partial waves produced by the particles compressed in the aperture do diffuse themselves beyond the rectilinear space, these waves do not concur anywhere except in front of the aperture."
The adaptation of the pendulum to clocks by Huygens was of inestimable value to astronomy by furnishing a standard measure of time. His method of grinding lenses so improved the defining power of telescopes that he was enabled to discover the true na-
