The axioms of descriptive geometry, by A.N. Whitehead.

29, 30]

THE DEDEKIND PROPERTY

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proper projective points corresponds step by step with the geometry of
the original descriptive space. Thus the geometry of descriptive space
can always be investigated by considering it as a convex region in a
projective space. This simply amounts to considering the associated
proper projective points and adding thereto the improper projective
points. A particular case arises when the Euclidean axiom (cf. ~ 10,
above) is assumed. The improper projective points then lie on a single
improper projective plane. Thus in Euclidean Geometry when the
'plane at infinity' is considered, the associated projective geometry has
been introduced, and this plane is the single improper projective plane.

w.

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