Moreover, it is easily seen that
is equal to the small arc on the sphere which measures the angle between the directions of the tangents at the beginning and at the end of the element
and is thus equal to
if
denotes the radius of curvature of the shortest line at this point. Thus we shall have
Suppose that an infinite number of shortest lines go out from a given point on the curved surface, and suppose that we distinguish these lines from one another by the angle that the first element of each of them makes with the first element of one of them which we take for the first. Let be that angle, or, more generally, a function of that angle, and the length of such a shortest line from the point to the point whose coordinates are Since to definite values of the variables there correspond definite points of the surface, the coordinates can be regarded as functions of We shall retain for the notation the same meaning as in the preceding article, this notation referring to any point whatever on any one of the shortest lines.
All the shortest lines that are of the same length will end on another line whose length, measured from an arbitrary initial point, we shall denote by Thus can be regarded as a function of the indeterminates and if denotes the point on the sphere corresponding to the direction of the element and also denote the coordinates of this point with reference to the centre of the sphere, we shall have
From these equations and from the equations
we have