1 1 8 Mifcellanea Curiofa. 



A Demonftration concerning the Mo- 

 tion of Light j communicated from 

 Paris. 



PHilofophers have been labour irig for many- 

 Years to decide by fome Experiment, whe- 

 ther the Action of Light be conveyed in an in- 

 ftant to diftant Places, or whether it requireth 

 time. M. Homer, of the fyyal Academy of Scien- 

 ces, hath deviled a way taken from the Obferva- 

 tions of the firft Satellit of Jupiter, by which he 

 demonftrates, that for thediftance of about 5 coo 

 Leagues, fuch as is very near the bignefs of the 

 Diameter of the Earth, Light needs not one Se- 

 cond of Time. 



Let (in Fig. 3. Plate 3.) A be the Sun, B Ju- 

 piter, C the firft Satellit of Jupiter, which enters 

 into the fhadow of Jupiter, to come out at D, 

 and let EFG H K^L be the Earth, piacecl at di- 

 vers diftances from Jupiter. 



Now fuppofe the Earth, being in L, towards 

 the fecond Quadrature of Jupiter, hath feen the 

 firft Satellit, at the time of its emerfion, or if- 

 foing out of the (hadow at D, and that about 

 4a£ Hours after (vfy after one Revolution of 

 this Satellit) the Earth being in K^, do fee it re- 

 turned in D : It is manifeft, that if the Light 

 require time to traverfe the Interval L the 

 Satellit will be feen returned later in D, than it 

 would have been if the Earth had remained in 

 £. So tha I the Revolution of the Satellit being 

 djus obferv'd by the pmerfions, will be retar- 



