Of the Moon: Mirrors, Rays and the Moon's Essence
Why the study of plane, concave and convex mirrors must come before the essence of the moon
Under the heading 'Of the Moon', Leonardo argues that to treat the essence of the moon one must first master the perspective of plane, concave and convex mirrors, the nature of the luminous ray, and how it bends passing through different media. He observes that solar rays striking sea waves appear as broad near the eye as at the far horizon, so that the reflected splendour is really pyramidal in figure though it looks parallel to us. A column of numbered propositions considers whether the moon holds a place among its elements like the earth, and whether it can be lighter than any element yet still solid and opaque.
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The essence of the moon requires optics first
To treat of the moon's essence, Leonardo says one must first describe the perspective of plane, concave and convex mirrors, define the luminous ray, and show how it bends through media of various natures. Only then can one judge where the reflected ray is strongest.
Solar rays on sea waves form a pyramidal figure
The solar rays striking the sea waves show themselves to the eye as broad at the angle of the eye as at the farthest waves on the horizon. Leonardo concludes the reflected solar splendour is therefore pyramidal in figure, not gaining breadth with distance, even though to our sight it appears parallel.
Three ray diagrams in the right margin
A stacked sequence of three figures fills the right margin: a triangle crossed by internal lines like a cone of rays, a plane surface reflecting rays into a triangular bundle, and a rounded body labelled with the word for the sun. They illustrate rays reflecting from plane and curved surfaces.
Propositions on lightness, opacity and the moon among its elements
Numbered propositions state that nothing very light is opaque and no lighter thing lies beneath a less light one, then ask whether the moon has a place among its elements as the earth does. If it neither falls to our centre nor descends, it must be lighter than any element — yet if lighter, why is it solid and not transparent?
Equal appearances scaled by distance and magnitude
Of things of various sizes that, set at various distances, show themselves equal, the proportion from distance to distance will be as that from magnitude to magnitude. This ties apparent equality to a fixed ratio of size and remoteness.