direction from to is the same as that from to negative, if the contrary of one of these conditions hold; positive again, if the contrary of both conditions be true. In other words, the surface is considered positive if we go around the circumference of the figure in the same sense as negative, if we go in the contrary sense.
If we consider now a finite part of the line from to and denote by the values of the angles at the two extremities, then we have
the sign of the area being taken as explained.
Now let us assume further that, from the origin upon the curved surface, infinitely many other shortest lines go out, and denote by that indefinite angle which the first element, moving counter-clockwise, makes with the first element of the first line; and through the other extremities of the different curved lines let a curved line be drawn, concerning which, first of all, we leave it undecided whether it be a shortest line or not. If we suppose also that those indefinite values, which for the first line were be denoted by for each of these lines, then is capable of being represented in the same manner on the auxiliary sphere by the space Since evidently the space
If the bounding line is also a shortest line, and, when prolonged, makes with the angles if, further, denote the same at the points that did at in the line then we have
but
therefore
The angles of the triangle evidently are