CN you write MC/NG × CG as above, then the perpendicular velocity of any other emerging Ray CE which was AD/EF × CF, will be equal to the square Root of CD_q_ + (_MCq/NGq_ × CG_q_). And by squaring these Equals, and adding to them the Equals AD_q_ and MC_q_ - CD_q_, and dividing the Sums by the Equals CF_q_ + EF_q_ and CG_q_ + NG_q_, you will have _MCq/NGq_ equal to _ADq/EFq_. Whence AD, the Sine of Incidence, is to EF the Sine of Refraction, as MC to NG, that is, in a given _ratio_. And this Demonstration...