Jacob Steiner's Vorlesungen über synthetische Geometrie, B. G. Teubner, Leipzig 1867 (from Google Books: (German) Part II follows Part I) Part II, pg. 96
Korn, G. A., & Korn, T. M., Mathematical Handbook for Scientists and Engineers: Definitions, Theorems, and Formulas for Reference and Review (Mineola, NY: Dover Publications, 1961), p. 42.
Berger, M., Geometry Revealed: A Jacob's Ladder to Modern Higher Geometry (Berlin/Heidelberg: Springer, 2010), p. 127.