158 DISSERTATION SECOND. [parti. 



Hence it occurred to him, that it was to the time which 

 light requires to traverse those distances that the whole 

 series of phenomena was to be ascribed. This explana- 

 tion was so simple and satisfactory, that it was readily 

 received. 



Though Roemer was the first who communicated this 

 explanation to the world, yet it seems certain that, it had 

 before occurred to Cassini, and that he was prevented 

 from making it known by a consideration which does him 

 great honour. The explanation which the motion of light 

 afforded, seemed not to be consistent with two circumstan- 

 ces involved in the phenomenon. If such was the cause 

 of the alternate acceleration and retardation above describ- 

 ed, why was it observed only in the eclipses of the first 

 satellite, and not in those of the other three ? This difficul- 

 ty appeared so great to Cassini, that he suppressed the 

 explanation which he would otherwise have given. 



The other difficulty occurred to Maraldi. Why did not 

 an equation or allowance of the same kind arise from the 

 position of Jupiter, with respect to his aphelion, for, all 

 other things being the same, his distance from the earth 

 must be greater, as he was nearer to that point of his orbit? 

 Both these difficulties have since been completely removed. 

 If the aforesaid inequality was not for some time observed 

 in any satellite but the first, it was only because the motions 

 of the first are the most regular, and were the soonest un- 

 derstood, but it now appears that the same equation belongs 

 to all the satellites. The solution of Maraldi's difficulty 

 is similar; for the quantity of what is called the equation 

 of the light, is now known to be affected by Jupiter's 

 place in his orbit. 



Thus, every thing conspires to prove the reality of the 

 motion of light, so singular on account of the immensity of 

 the velocity, and the smallness of the bodies to which it h 

 communicated. 



