Optica descriptio quadrati duplicis.
_Iam incipies frui compendio papyri complicatæ. Nam eam admovendo lineæ
plani, nullo negotio notare poteris puncta ~1~, ~2~, ~3~, ~4~, ~5~, ~6~,
linearum visualium, quæ ducentur ad ~O~ punctum perspectivæ. Exinde
complicatâ rursum chartulâ in crucem ad ~P~, notabuntur hæc puncta; ~7~,
coincidens cum puncto ~6~, nisi quadratum distet à linea plani; ~8~, ~9~,
~10~. Ductis autem rectis ex ~8~, ~9~, ~10~, ad punctum ~E~, ubi secant
visualem ~6~, ~7~, ~O~ fient parallelæ, eritque completa delineatio._
_In medio quadrati ~B~, aliud quadratum facilè describetur, ducendo
diagonales seu diametros ab angulo ad angulum, ut in figura._
The Fourth Figure.
_The Optical Delineation of a double Square._
Here you’ll find the Advantage of your folded Paper; for, applying it
to the Line of the Plan, you readily mark the Points 1, 2, 3, 4, 5, 6,
of the visual Lines, which must be drawn to the Point of Sight O. Then
folding the Paper cross-wise, as in P, you mark the Points 7, 8, 9,
10, placing the Point 7 on that of 6, unless you would have the Square
removed within the Line of the Plan. Then from 8, 9, 10, drawing Lines
to the Point of Distance E; where they intersect the Line 6, 7, O, draw
Parallels to the Line of the Plan; and your Work is done.
Within the Square B, you may easily inscribe another Square, by help of
the Diagonals; as may be seen in the Figure.
[Illustration: FIG. V.]