and since we always choose the point so that
then for the shortest line
or and must coincide. Therefore
and we have here, instead of curved lines upon the auxiliary sphere, only to consider. Every element of the second line is therefore to be regarded as lying in the great circle And the positive or negative value of refers to the concavity or the convexity of the curve in the direction of the normal.
We shall now investigate the spherical angle upon the auxiliary sphere, which the great circle going from toward makes with that one going from toward one of the fixed points e.g., toward In order to have something definite here, we shall consider the sense from to the same as that in which and lie. If we call this angle then it follows from the theorem of Art. 7 that
or, since and
we have
Furthermore,
or
and