y x v - d c - e - p o - b a - f - g q - r - 6. If the 3 lights, with two columnar shadow-casting bodies intersected between them, make six shadows with 9 intersections, what is the reason that the two first outer intersections do not double the darkness at q in their intersection, as is seen to happen at the other 7 intersections?

[n m] - S h - a - p o - b - e d - c - f L - g 5 K. Of simple shadows. The light n m is the cause that the columnar shadow-casting body o S makes the 2 shadows o f and o g, and the like do the lights of body h p in generating the shadows p L and p K; and for this reason the simple derived shadows b c are generated at those intersections b c and not at the intersections d e, because b c does not see either of the two lights, but rather sees those two lights — that is, one sees the shadow o g and the other sees o f, which are two compound shadows. Therefore the intersection of such compound shadows sees, or rather is seen by, the two lights, but the two lights do not see that intersection, because such a shadow would be destroyed. But each shadow sees a single light along its whole length, and each shadow joined together with the other will not make a simple shadow, because a simple shadow is one that sees no luminous body; and here, at such an intersection, each one by itself alone sees its own luminous body, and the one can never see the luminous body of the other. It follows that such a shadow remains without doubling its darkness and without being destroyed by seeing the two lights.