Perspective of a rhombus and the reading of light and shade
A parallel seen edge-on as a two-angled figure, and where a shaded body's form is clearest
Three notes: two on perspective and one on painting. It continues an argument that a parallel figure seen opposite its middle appears as a two-angled body with curved sides, though in truth it has four angles (two in the middle, above and below, and two at the opposite extremes), its obtuse angle widening as the eye recedes and narrowing as it approaches. A marginal note on the rhombus figure adds that its two obtuse angles are unequal in proportion to the eye's nearness to each. A closing note 'of painting' observes that the outlines of shadowed bodies are least clear within their lights and shadows and sharpest in the parts between light and shade; an elongated lozenge is drawn in the upper margin.
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A parallel seen edge-on appears two-angled, but is truly four-angled
The obtuse angle in question is not the greatest possible, for it widens as the eye moves farther away and narrows as it comes nearer. When the eye faces the middle of any parallel, the figure looks like a two-angled body with curved sides; in truth it has four angles, two in the middle, above and below, and two at the opposite extremes.
Rhombus figure: unequal obtuse angles
In the two-angled (rhombus) figure the two obtuse angles are unequal. One exceeds the other in proportion as the eye stands nearer to it than to the other. An elongated lozenge drawn in the margin illustrates the case.
Where a shaded body's outline is clearest
The boundaries and shape of shadowed bodies are poorly known within their shadows and within their lights alike. It is in the parts set between light and shadow that the body's form is seen most distinctly. This transitional zone gives the eye its first and clearest degree of knowledge.