Constructions on angles, triangles and a circle
Segments a-c-b, an inscribed right triangle, and angle figures keyed to propositions
This recto gathers several plane constructions keyed to numbered propositions. A segment is divided at c (a-c-b); a circle carries two perpendicular radii forming an isosceles right triangle on a chord; and isosceles triangles with their heights are noted as 'proved by the fourth' and 'by the fourth'. A second group studies the angles made where segments cross — marked 'thirteenth', 'fourteenth' and 'line one' — including a point where four segments meet. Some numerals were written in the ordinary, non-mirrored sense.
On this page
Divided segment a-c-b
A line segment runs from a to b with an intermediate point marked c, isolating the two parts of the division.
Right triangle inscribed in a circle, 'by the fourth'
Within a circle two perpendicular radii close on a chord to form an isosceles right triangle whose height is drawn; the note 'by the fourth' cites the proposition that justifies it.
Angles at crossings: 'thirteenth' and 'fourteenth'
Where one segment meets another at an angle the figure is marked 'thirteenth', repeated below with a perpendicular added at the intersection. A curved pen-stroke above is headed 'fourteenth'. Both numbers were written the ordinary way round.
Second column of angle views, 'line one'
A vertical segment and four views of the angles made by intersecting lines fill the second column; a perpendicular crossing at the foot is captioned 'line one'.