L the x axis be directed along the line joining the two centers. The origin of coordinates is placed at the midpoint between the centers. The unit of distance is taken as half the distance between the centers and the unit of mass as the sum of the masses of both centers. Furthermore, let the mass of the center c2 be ra, and, therefore, that of the other center is 1 — m. Also, let n be the distance from the center Ci to a point M and r2 the distance from c2 to the same point. Consequently we have ry> = (x I) + y + z* r2 = (x + I) + y + z (1)