γ, δ in four harmonic points, since any plane through the ray gives four
harmonic lines of intersection.




*35.* These results may be put together as follows:

_Given any two assemblages of points, rays, or planes, perspectively
related to each other, four harmonic elements of one must correspond to
four elements of the other which are likewise harmonic._

If, now, two forms are perspectively related to a third, any four harmonic
elements of one must correspond to four harmonic elements in the other. We
take this as our definition of projective correspondence, and say:




*36. Definition of projectivity.* _Two fundamental forms are protectively
related to each other when a one-to-one correspondence exists between the
elements of the two and when four harmonic elements of one correspond to
four harmonic elements of the other._

[Figure 6]

FIG. 6