 It is also noteworthy to mention that in his "Mémoire sur quelques phénomènes de la vision" Monge proposed an early implicit explanation of the color constancy phenomenon based on several known observations.Leonhard Euler, in his 1760 paper on curvature in the "Berlin Memoirs", had considered, not the normals of the surface, but the normals of the plane sections through a particular normal, so that the question of the intersection of successive normals of the surface had never presented itself to him.Monge's paper gives the ordinary differential equation of the curves of curvature, and establishes the general theory in a very satisfactory manner; the application to the interesting particular case of the ellipsoid was first made by him in a later paper in 1795.