If from two unequal quantities one takes unequal parts such that the inequality is of the same proportion as its whole, the remainder stays in the same proportion. All the angles of contingence of one and the same circle with the straight line are equal among themselves, and at equal distance from the angle they make triangles of equal base. This figure above here is the same as the one below; but I shall demonstrate the one below, it being less intricate.

Let a, b be the two given points upon the convex mirror, of which the one, whichever you wish, is the object represented to the mirror, the other the eye that sees the image of the object in that mirror. Now the site of that image upon the mirror is sought; and such a site is called the angle of incidence. Let then a, b be the eye and the object, from which let the line a d and the line b d be drawn, meeting at the centre of the mirror at d. Then from those two points draw two lines tangent to the circle, which will be a e, touching the circle at m, and the other b f, touching the circle at n; and where those lines intersect at the point c, there will be the angle of that incidence; and if that angle were high above the circle, lower it in the way that I have here taught you at the side, and so it will be right.