This kidney formed curve ^{1)} is the epicycloid with two
cusps. The rolling circle is half
the size as the rolled circle. The English mathematician *R.A. Proctor* (1837-1888)
gave the curve its name, in 1878 in his book 'The geometry of cycloids'.
Some other properties of the curve are:
- the curve is the catacaustic of the circle, with the source at infinity
*Christiaan Huygens*, who was the first to describe the curve in 1678, found this (also in 1678), and he published this findings in 1690 in his essay
'Traité de la lumière'. One had to wait for 1838 that *Airy* proved the fact theoretically.
- the curve is the catacaustic of the cardioid, with the cusp as source.
It was *Jakob Bernoulli* who showed this in 1692.
- the nephroid is the evolute of Cayley's sextic
- the nephroid is the envelope of a
set of circles with centers on a circle and tangent to a diameter
The nephroid has a length of 24, and its area is 12π.
The curve is a sextic curve^{2)}.
The nephroid is said to be the perfect multi-seater dining-table.
The curve can be linearly stretched to an **elongated nephroid**.
**notes**
1) Nephros (Greek) = kidney
2) With Cartesian equation: (x^{2} + y^{2 }- 4)^{3} = 108 y ^{2} |