Mapping of generalised circles by inversion w=Sz=1/z (primitive Möbius-transform). Straight lines are mapped to circles/lines.
<a href="//commons.wikimedia.org/wiki/User:Chrislb" title="User:Chrislb">Chrislb</a>
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created by <a href="//commons.wikimedia.org/wiki/User:Chrislb" title="User:Chrislb">Chrislb</a>