V. The _angle refracted_ is that, which the refracted line makes in the point of refraction with that line, which from the same point is drawn perpendicular to the separating superficies in a different medium. VI. The _angle of refraction_ is that which the refracted line makes with the line of incidence produced. VII. The _angle of inclination_ is that which the line of incidence makes with that line, which from the point of refraction is drawn perpendicular to the separating superficies. VIII. The _angle of incidence_ is the complement to a right angle of the angle of inclination. And so, (in fig. 1) the refraction is made in A B F. The refracted line is B F. The line of incidence is A B. The point of incidence and of refraction is B. The refracting or separating superficies is D B E. The line of incidence produced directly is A B C. The perpendicular to the separating superficies is B H. The angle of refraction is C B F. The angle refracted is H B F. The angle of inclination is A B G or H B C. The angle of incidence is A B D. IX. Moreover the _thinner medium_ is understood to be that in which there is less resistance to motion, or to the generation of motion; and the thicker that wherein there is greater resistance. X. And that medium in which there is equal resistance everywhere, is a _homogeneous medium_. All other mediums are _heterogeneous_. [Sidenote: In perpendicular motion there is no refraction.] If a body pass, or there be generation of motion from one medium to another of different density, in a line perpendicular to the separating superficies, there will be no refraction. For seeing on every side of the perpendicular all things in the mediums are supposed to be like and equal, if the motion itself be supposed to be perpendicular, the inclinations also will be equal, or rather none at all; and therefore there can be no cause from which refraction may be inferred to be on one side of the perpendicular, which will not conclude the same refraction to be on the other side. Which being so, refraction on one side will destroy refraction on the other side; and consequently either the refracted line will be everywhere, which is absurd, or there will be no refracted line at all; which was to be demonstrated. Coroll. It is manifest from hence, that the cause of refraction consisteth only in the obliquity of the line of incidence, whether the incident body penetrate both the mediums, or without penetrating, propagate motion by pressure only. [Sidenote: Things thrown out of a thinner into a thicker medium are so refracted that the angle refracted is greater than the angle of inclination.]