Gauss, Lagrange. Its importance in the construction of maps
is obvious. Gauss was the first to represent a surface upon another with a view of more easily arriving at its properties. Plücker, Chasles, Cayley, thus represented on a plane the
geometry of quadric surfaces; Clebsch and Cremona, that of
cubic surfaces. Other surfaces have been studied in the same way by recent writers, particularly M. Nöther of Erlangen,
Armenante, Felix Klein, Korndörfer, Caporali, H. G. Zeuthen
of Copenhagen. A fundamental question which has as yet received only a partial answer is this: What surfaces can be represented by a correspondence upon a given surface? This and the analogous question for curves was studied by Clebsch. Higher correspondences between surfaces have been investigated by Cayley and Nöther. The theory of surfaces has been studied also by Joseph Alfred Serret (1819–1885), professor
at the Sorbonne in Paris, Jean Gaston Darboux of Paris,
John Casey of Dublin (died 1891), W. R. W. Roberts of Dublin,
H. Schröter (1829–1892) of Breslau. Surfaces of the
fourth order were investigated by Kummer, and Fresnel's
wave-surface, studied by Hamilton, is a particular case of
Kummer's quartic surface, with sixteen canonical points and sixteen singular tangent planes.56
The infinitesimal calculus was first applied to the determination of the measure of curvature of surfaces by Lagrange,
Euler, and Meusnier (1754–1793) of Paris. Then followed the
researches of Monge and Dupin, but they were eclipsed by
the work of Gauss, who disposed of this difficult subject in a
way that opened new vistas to geometricians. His treatment is embodied in the Disquisitiones generales circa superficies curvas (1827) and Untersuchungen über gegenstände der höheren Geodäsie of 1843 and 1846. He defined the measure of curvature at a point to be the reciprocal of the product of the two principal radii of curvature at that point. From this flows the theorem of Johann August Grunert (1797–1872;
professor in Greifswald), that the arithmetical mean of the radii of curvature of all normal sections through a point is the radius of a sphere which has the same measure of curvature as has the surface at that point. Gauss's deduction of the formula of curvature was simplified through the use of determinants by Heinrich Richard Baltzer (1818–1887) of Giessen.69