A nephroid is an epycloid that can be generated by rolling a circle on the outside of a circle with doubled radius. It was called by Richard Proctor (1878) because its shape looks like a kidney (see https://mathworld.wolfram.com/Nephroid.html).

A nephroid of Freeth (called from Thomas Jacob Freeth, 1879) is a strophoid of the circle and has nothing to do with Proctor's nephroid (from what I undertand) and doesn't not look like a kidney, nor Proctor's nephroid (see https://mathworld.wolfram.com/FreethsNephroid.html)

So: why is Freeth's nephroid called a nephroid ?

Note: I could not find the original paper of Freeth where he may have explained the name.

A nephroid is an epycloid that can be generated by rolling a circle on the outside of a circle with doubled radius. It was called by Richard Proctor (1878) because its shape looks like a kidney (see https://mathworld.wolfram.com/Nephroid.html).

A nephroid of Freeth (called from Thomas Jacob Freeth, 1879) is a strophoid of the circle and has nothing to do with Proctor's nephroid (from what I undertand) and doesn't look like a kidney, nor Proctor's nephroid (see https://mathworld.wolfram.com/FreethsNephroid.html)

So: why is Freeth's nephroid called a nephroid ?

Note: I could not find the original paper of Freeth where he may have explained the name.