[12] Also the behaviour of light during refraction proved that a form of Least Action known as Fermat’s principle of minimum time was involved.
[13] In this discussion we are always assuming that the observer is standing right above the coin; hence we are not considering the variations in apparent shape due to a slanting line of sight. This latter problem is of a totally different nature.
[14] The reason why Riemann refers specifically to the infinitely small is because he took it as proved that for ordinary extensions experiment had shown space to be Euclidean, and because for exceedingly great extensions he did not consider that measurements would be feasible.
[15] This metrical ether must not be confused with the classical ether of optics and electromagnetics.
[16] In all fairness to Einstein, however, it should be noted that he does not appear to have been influenced directly by Riemann.
[17] A fact by no means evident a priori.
[18] Calling
the dimensionality of the non-Euclidean space the number of dimensions of the generating Euclidean space would be
