On shadow. / Proposition. / The primary light and the derivative reflected lights, surrounding dense and spherical bodies, will cause the boundaries of that body's primary shadow to be so much more distinct and defined on the part near the lights, as the primary light is brighter than the derivative. / Comment. / That is called primary light which first illuminates the shadowy bodies; and derivative is the name for that which rebounds from those bodies onto the parts that are remote from the primary light.

K, t p q, m h n, o, a b c d / Rays doubled by intersection in the lights and in the shadows will likewise be of double brightness [and] darkness. The shadowed part of this upper body will be brighter at m h n than at t q p, because in that part the two derivative reflected lights intersect — that is, a b and d c — as appears in the triangle m n o; whereas at t m it sees only d c. / Demonstration. / Let K be the primary light, which illuminates the shadowed body at t p and the places a b, c d. From a b, c d the derivative light departs and rebounds onto the opposite body at m n. / And the whole part of the body at h will be more luminous than at q, because it is seen by a doubled light, namely a b and d c; whereas q is seen only by a single light, and therefore will be dark. / Proposition.

r u, o t, n, a b c d e f / That part of the primary shadow will be the more luminous which can more equally see the middles of the derivative lights. / Comment. / It can plainly be known that the part of shadowy bodies seen by a greater quantity of light is the more luminous, and especially when that part is illuminated by two lights, as is seen in reflected lights, which place between themselves the derivative shadow made among them by the opposing dense bodies. / Demonstration. / N will be the part of the body more luminous than any other in it, because it is equally seen by the two greatest powers of the lights set against it. That is, b is the greatest power of the light a c, and likewise e is the greatest of d f; and both see the body at said n. d c and likewise a f, being the extremes, are the lesser powers, and these see the body at r o and at u t; that place being seen by less light, the part they illuminate will be darker. The whole triangle o t n will be seen by double lights of varying qualities of brightness. / Proposition.

a b c, d e f / Every luminous body, with the whole of itself and with a part, illuminates the part and the whole of the object turned to face it. / Definition. / This proposition is quite evident, namely that it cannot be denied that wherever the whole pupil of the eye looks, in that place it does not look with every part of itself, and the place seen by that pupil does the like toward the pupil. / Demonstration. / Let a c be the luminous body, d f the illuminated object which, although composed of infinite points, we shall prove with only three, namely d, e, f. Now see that e is seen by the part b of the luminous body and by the whole a c, as is shown by the lines a e and c e and by the central line b e; and again at the point d it sees the whole a c and the center b. And you will find the same at f, and so it happens for all the parts of the object d f.