88 THE POPULAR SCIENCE MONTHLY. 



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. Domi- 

 nic Cassini gave so much promise that at the age of twenty-five he 

 was appointed Professor of Astronomy at the University of Bo- 

 logna, 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 Romer, 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 instan- 

 taneous. Romer 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 al- 

 ways 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 meas- 

 ured. This discovery of Romer 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 stum- 

 bling-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 aperturp." 



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- 



