Here it is shown how the meeting-point of the reflected rays does not pass the fourth part of the diameter of the spheres from which concave mirrors take their curvature, as is shown in the line a o.

If you wished to see the rays separated, let the mirror be taken in the shape of a half-ring which is flat on the inside; then make a mixture of white lead tempered with egg, and cover it with the brush two or three times; and when it is dry, scratch and lay bare the mirror with a wooden stylus so as not to scratch the mirror; and thus you will see the rays separated upon the plane a b c, as their causes are separated.

If this mirror is made in the shape of a half-ring, flat and polished within, and is wholly closed on the inside by the wall c d, save for a tiny aperture at d, then the sun will be seen mirrored in all its reboundings, as at c, e, f, g, h, i, K, L, n; and all the rest of the mirror will be dark.

Among concave mirrors of equal diameter, the one of greater concavity will make the meeting of its reflected rays more disunited and of less heat. This greater concavity can extend as far as the half of the sphere from which such concavity is caused, as is shown in c h d. From this you can know for yourself how the part n c e d o is of little worth, because the ray b o, which strikes it at the point o, bounces in eight places, that is from o to e, and from e to f, and to g, and to h, and to i, and to K, and to l, and to n; and it never intersects at the central cathetus m h. The second ray 7 e is likewise of little worth, since it goes rebounding over the surface of such a mirror in fewer places; nevertheless, although it does intersect at the central cathetus, that line is single, and therefore of little worth. Again, the ray 6 f is of little worth, because after the strike made at f it intersects the central cathetus in a single place, and strikes at K, and rebounds toward the sun. In the same way 5 g, after the strike it makes at g, rebounds at r. Therefore, of so great a concavity, only the part i g is less bad than any of those aforesaid, from g to d.