For foure angles of a Tetraedrum are equall to three angles of the Octoedrum: And therefore 12. are equall to
nine. Therefore nine angles of an octaedrum doe countervaile eight solid right angles.
And
9 If eight triangles, equilaters and equall be joyned together by their edges; they shall comprehend an octaedreum.
This construction is easie, as it is manifest in the example following: Where thou seest as it were two equilater and equall triangles of a double pentaedrum to cut one another.
10 If a right line of each side perpendicular to the center of a quadrate and equall to the halfe diagony be tied together with the angles, it shall comprehend an octaedrum, 14. d xiij.
For the perpendicular yu, and su, with the semidiagoni's, ua, uo, ui, ue, shall be made equall by the [2 e vij], the eight sides ya, ye, yo, yi, se, si, sa, so; And also eight triangles.
Therefore