J. Frans Gerritsen, «Entwicklung der Farbenlehre», Göttingen, 1984
mathani: It’s my second birthsday today. I posted 50 times in the last year (145 in the first). Your reception has been great, thanks. Two posts in particular gathered million+ notes together!? Here’s to another year. Also, I’m starting an alphabet series: A for Astroid Two circles with radii one-quarter and three-quarters the radius of the fixed circle are rolling inside. Points on their circumference both trace this four-cusp curve.