SIDELIGHTS ON RELATIVITY

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_E_, which, for facility of presentation, is shown in the drawing as
a bounded surface. Let _L_ be a disc on the spherical surface.  Now
let us imagine that at the point _N_ of the spherical surface,
diametrically opposite to _S_, there is a luminous point, throwing a
shadow _L'_ of the disc _L_ upon the plane _E_. Every point on the
sphere has its shadow on the plane. If the disc on the sphere _K_ is
moved, its shadow _L'_ on the plane _E_ also moves. When the disc
_L_ is at _S_, it almost exactly coincides with its shadow.  If it
moves on the spherical surface away from _S_ upwards, the disc
shadow _L'_ on the plane also moves away from _S_ on the plane
outwards, growing bigger and bigger. As the disc _L_ approaches the
luminous point _N_, the shadow moves off to infinity, and becomes
infinitely great.

Now we put the question, What are the laws of disposition of the
disc-shadows _L'_ on the plane _E_? Evidently they are exactly the
same as the laws of disposition of the discs _L_ on the spherical