SHOWS HOW LIGHT FROM ANY SIDE CONVERGES TO ONE POINT.

Although the balls _a b c_ are lighted from one window,
nevertheless, if you follow the lines of their shadows you will see
they intersect at a point forming the angle _n_.

[Footnote: The diagram belonging to this passage is slightly
sketched on Pl. XXXII; a square with three balls below it. The first
three lines of the text belonging to it are written above the sketch
and the six others below it.]