_If Rays of any sort pass perpendicularly into several Mediums, the
Intervals of the Fits of easy Reflexion and Transmission in any one
Medium, are to those Intervals in any other, as the Sine of Incidence to
the Sine of Refraction, when the Rays pass out of the first of those two
Mediums into the second._
This is manifest by the 10th Observation.