1867. Thomson & Tait, Nat. Phil., § 494. Any two confocal homogeneous solid ellipsoids of equal masses produce equal attraction through all space external to both.
1881. Maxwell, Electr. & Magn., I. 215. The general equation of a confocal system.
1882. Vines, Sachs’ Bot., 951. If the outline of the growing point is an ellipse, the periclinals will be confocal ellipses; the anticlinals will be confocal hyperbolas.
