534 Prof. V. Karapetoff on Divergence and 



three mutually perpendicular directions which are defined 

 by the line of force AB through : — 



(1) The tangent t to the line of force ; 



(2) The principal normal n to the same curve ; 



(3) The binormal b of the same curve. 



Fig. 3. 



B 



\tD 



>b 



Fig. 4. 



These three axes determine the following three planes 



(1) The osculating plane, tn, in which fig. 2 is drawn : 



(2) The rectifying plane, tb, in which fig. 3 is drawn ; 



(3) The normal plane, nb, in which fig. 4 is drawn. 



