In other words, the calculation of 1' and P introduces no differentials higher than the third order but that of R' requires the fourth order of differentials. In the language of modern geometry, the former can be determined by the consideration of four consecutive points of the curve, or by that of two consecutive osculating circles; but the latter requires the consideration of two consecutive osculating spheres, and therefore of five consecutive points of the curve (supposed to be one of double curvature). Other investigations, in the present and immediately following Series (398.