588 SPECIAL METHODS OF DISCOVERY. 



Cassini and Roemer were both led to infer that, as Jupiter 

 is nearer to the earth at one time than at another, the 

 eclipses might be seen some minutes earlier whenever the 

 rays of light from the moons had to travel a lesser distance 

 to reach the earth ; and, by further study and calculation, 

 Eoemer was enabled to make the grand discovery of the rate 

 of velocity of light. In 1676, a great number of observa- 

 tions of eclipses of Jupiter's satellites were accumulated, and 

 could be compared with Cassini's tables. Koemer, a Danish 

 astronomer, whom Picard had brought to Paris, perceived 

 that these eclipses happened constantly later than the 

 calculated time at one season of the year, and earlier at 

 another season a difference for which astronomy could 

 offer no account. The case was the same for all the satel- 

 lites ; if it had depended on a defect in the Tables of 

 Jupiter, it might have affected all, but the effect would 

 have had a reference to the velocities of the satellites. 

 The cause, then, was something extraneous to Jupiter. 

 Koemer conceived the happy idea of comparing the error 

 with the earth's distance from Jupiter, and it was found 

 that the eclipses happened later in proportion as Jupiter 

 was farther off. Thus we see the eclipse later, as it is 

 more remote ; and thus light, the messenger which brings 

 us intelligence of the occurrence, travels over its course in 

 a measurable time. By this evidence, light appeared to 

 take about eleven minutes in travelling the diameter of 

 the earth's orbit. 



' This discovery, like so many others, once made, 

 appears easy and inevitable ; yet Dominic Cassini had 

 entertained the idea for a moment, and had rejected it : 

 and Fontanelle had congratulated himself publicly on 

 having narrowly escaped this seductive error. The objec- 

 tions to the admission of the truth arose principally from 

 the inaccuracy of observation, and from the persuasion 



