Axioms
Ax. I.
The Angles of Reflexion and Refraction, lie in one and the same Plane with the Angle of Incidence.
Ax. II
The Angle of Reflexion is equal to the Angle of Incidence.
Ax. III
If the refracted Ray be returned directly back to the Point of Incidence, it shall be refracted into the Line before described by the incident Ray.
Ax. IV
Refraction out of the rarer Medium into the denser, is made towards the Perpendicular; that is, so that the Angle of Refraction be less than the Angle of Incidence.