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"If two parallelograms, which have one angle of the one equal to one angle of the other, be equal to one another, the sides about the equal angles are reciprocally proportional, and conversely."

"Proposition I. If four quantities are proportional, the product of the extremes is equal to the product of the means, and conversely."

"Glasses (perspicua) can be so constructed that objects at a very great distance appear to be quite close at hand, and conversely. Thus we read the smallest letters from an incredible distance, number objects, however small, and make the stars appear as near as we wish. … Also objects can be made to appear so that the greatest seems the least, and conversely; what are high appear low and short, and conversely; and what is hidden appears manifest. … [translation of Roger Bacon's De Secretis Operibus Artis et Naturae.]"