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A leaf with a concave surface seen from the under side and
up-side-down will sometimes show itself as half in shade, and half
transparent. Thus, if _o p_ is the leaf and the light _m_ and the
eye _n_, this will see _o_ in shadow because the light does not fall
upon it between equal angles, neither on the upper nor the under
side, and _p_ is lighted on the upper side and the light is
transmitted to its under side. [Footnote: See Pl. XXVIII, No. 2, the
upper sketch on the page. In the original they are drawn in red
chalk.]