(Munsell 1905), ch.1, pg. 7 Munsell, Albert H. (1905). A Color Notation. Boston: G. H. Ellis Co. Munsell's original description of his system. A Color Notation was published before he had established the irregular shape of a perceptual color solid, so it describes colors positioned in a sphere.
Centore, Paul (December 2012). "An open-source inversion algorithm for the Munsell renotation". Color Research & Application. 37 (6): 455–464. doi:10.1002/col.20715.