Chap. 13. *sf Treaty efBODIE S. 



iSn | SCOme ^ ^ e0thcr fu P crficiw EGF.itmaketh the 

 'towards F- and therefore muft ft by the rule given 

 abov e be refraa d ^ k$ ^ rtjng ^ ^ jp 



perpendicular j and it will meet fomcwherc with the 

 .1 which experience flicweth us to be true. 



And taking a body of 

 concave furfaces we fhall 

 (according to this do- 

 ctrine of ours ) find the 

 caufes of refraction jtift 

 contrary; & accordingly 

 experience likewife flicw- 

 eth us the effects to be fb 

 too. And therefore fince 

 experience agreeth exa^K. 



1 O 



ly with our rules, we can- 

 not doubt but that the 

 principles upon which we 

 go arc well laid. 



But becaufe crooked g e 

 furfaces may have many A general! rule 



1 * ' * *1f *** I'r r\ ., .L^. 



irregularities; it will not 

 be amiflfc to give a rule 

 by which allofthemmay 

 be brought unto a cer- 

 tainty. And this it is,that 

 reflexions from crooked 



fuperficicfes are equall to the reflexions that arc made from 

 fuch plain fuperficicfes, as arctangents to the crooked ones in 

 that point from whence the reflexions are made. Which princi- 

 ple the matters ofOpticks do take out of a Mathematicall fup- 

 pofition of the unity of the reflecting point, in both thefurfaces; 

 the crooked and the plain: but we take it out of the infallibility 

 of the difference of fo lircle a part in the two different furfaces, 

 as ferveth to reflect a ray of light: for where the difference is fn- 

 fenfibJe in theciufcs, there likewise the difference is fb little 

 in the effe$s,is fenfe cannot judge of them: which is as much as 

 is requifae to our purpolc. Now feeing that in the Marhema- 

 ticall fuppofition, the point where the reflexion is made is in- 

 different 



