A TreAtffc ofKODlES. Chap. 13. 



given of reflection; fuppofeth the fuperficies which a ball light- 

 eth upon, to be a thin linen cloth, or fome other fuch matter as 

 will break cleanly by the force of the ball linking nnartly upon 

 it. And bccaufe that luperficies refifteth onely one way,therefore 

 he inferred that the velocity of the ball is leflened onely oneway 

 & not the other:fo that the velocity of its motion that way in 

 which it findcth no refinance, mull be (after the balls paflagc 

 throrgh the lincn^in a greater proportion to the velocity w ch it 

 hath the other way where it findeth refiftance,then it was before. 

 And therefore the b;ill will in lefle time arrive to its period on 

 the one fide then on the other : and confequently, it will lean 

 towards that fide, unto which the courfe wherein itfindeth no 

 oppofition doth carry ir. 



But how much he is miftaken upon the whole matter a little 



figure willfhew: Ictus therefore put a Rectangle Parallelo- 



A J) gram as before 



C 



o 



M 



A E, which I 

 double &mal-:c 

 the whole Pa- 

 rallelogram A 

 L, draw out 

 the line A B, 

 till it cometh 

 to L. Now we 



muft imagine that C E, is the cloth or paflablc fuperficies which 

 Monficur Des Cartes putteth ; and B L the line it would go in, 

 if there v^cre no refiftance. Next we muft (cek the pcrpendicr- 

 lar,nhich according to our explication, J s A C : for that falleth 

 from A the illuminant, perpendicularly upon C E ; although, 

 fome who defend Monfieiir Des Cartes, /cein to make another 

 line the perpendicular ; againfl the conception of all thofc that 

 write of Opticks. But, not to trouble our felvcs with terms ; the 

 queftion is, whether the ball that paflcth the cloth, rnuft (" alter 

 us psiTagc throrgh} deflect from the line B L, (which it woukl 

 have kept, had there been no refinance ) towards E ; or el:e 

 deflect fiom that line towards C. And both experience and rci- 

 fbn do affiire us, that it mufl turn towards C : but Monfieur 

 DCS Cartes faith towards E. 



Which to fhcw how itu contrary unto his ovvn ptincip!e;Jct 



us 



