t 



ADDITIONS TO THE NEWTONIAN THEORY. 263 



most part, a work of labour only. The progress of 

 the other portions of our knowledge respecting light 

 is more striking. In 1676, a great number of obser- 

 vations of eclipses of Jupiter's satellites were accu- 

 mulated, and could be compared with Cassini's 

 Tables. Romer, 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 errour was the same for all 

 the satellites ; 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 velo- 

 cities of the satellites. The cause, then, was some- 

 thing extraneous to Jupiter. Romer had the happy 

 thought of comparing the errour with the earth's 

 distance from Jupiter, and it was found that the 

 eclipses happened later in proportion as Jupiter 

 was further off 5 . 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 describing 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 



5 Bailly, ii. 17- fi Ib. ii. 410. 



