In Chapter II of his book *Non-Euclidean Geometry* (1961; first published in Polish, 1956), Stefan Kulczycki defines a mapping of the hyperbolic plane onto the interior of a disk. Its construction begins by specifying a center *O* and an acute angle *α*. Given those, the image *P’* of an arbitrary point *P* is found as follows: drop a perpendicular *PQ* from *P* to a radial line making the angle *α* with *OP*, then mark off the distance *OP’* = *OQ* on *OP*. Thus *P'* lies on *OP* between *O* and *P*. Using Hjelmslev's theorem in absolute geometry (1907), one shows that the mapping preserves straightness.

Where did this mapping, together with its property of preserving straightness, first appear?