47 1 2 3 Zehe (1980), Secondary sources ↑ Huygens, C. (1944), Société Hollaise des Sciences, eds., "Discours de la Cause de la Pesanteur (1690)", Oeuvres complètes de Christiaan Huygens, Den Haag, 21: 443–488 CS1 maint: Uses editors parameter (link) 1 2 Van Lunteren (2002), Secondary sources 1 2 Newton, I. (1846), Newton's Principia : the mathematical principles of natural philosophy (1687), New York: Daniel Adee ↑ I. Newton, letters quoted in detail in The Metaphysical Foundations of Modern Physical Science by Edwin Arthur Burtt, Double day Anchor Books. ↑ Newton, 1692, 3rd letter to Bentley ↑ Isaac Newton (1726).