Karl Feuerbach (1800-1834) was a
geometer who discovered the
nine-point circle of a triangle.
One of the most striking theorems in
triangle geometry is
Feuerbach's theorem. He wrote in his 1822
The circle which passes through the feet of the altitudes of a
triangle touches all four of the circles which are tangent to
the three sides of the triangle; it is internally tangent to the
inscribed circle and externally tangent to each of the circles
which touch the sides of the triangle externally.