II. The former of these is called the line of incidence; the latter the refracted line. III. The point of refraction is the common point of the line of incidence, and of the refracted line.
IV. The refracting superficies, which also is the separating superficies of the two mediums, is that in which is the point of refraction.
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.
In perpendicular motion there is no refraction.