THE models are six in number. They are intended to show a line (1) by its projections, (2) by its traces; the inclination of an oblique plane (3) to the vertical plane, (4) to the horizontal plane; ...

(1) CHRISTIAN VON STAUDT'S “Beiträge zur Geometrie der Lage” was published so long ago as 1857; about the year 1871 Felix Klein wrote a series of papers emphasising the fact that it is possible to ...