Diagram 30 combines methods by Samuel Marolois, a mathematician and fortification designer, and Jean-François Niceron, a geometrical theorist.
For Marolois's technique, Turner directs his students to place the eye (E) at the center line (C) where it cuts the horizon line (H). Then draw the plan of the object (P) to be put into . Where perpendicular lines meet the base line (B), mark vanishing lines to the eye. Now draw transfer lines from the plan to the base line and connect to a point (V) at the side. The places at which all these lines intersect form the bottom of the cube.
Gallery label, August 2004