He delivers a Course of Optical Lectures at Cambridge—Is elected Fellow of the Royal Society—He communicates to them his Discoveries on the different Refrangibility and Nature of Light— Popular Account of them—They involve him in various Controversies— His Dispute with Pardies—Linus—Lucas—Dr. Hooke and Mr. Huygens— The Influence of these Disputes on the mind of Newton 47 CHAPTER V. Mistake of Newton in supposing that the Improvement of Refracting Telescopes was hopeless—Mr. Hall invents the Achromatic Telescope— Principles of the Achromatic Telescope explained—It is reinvented by Dollond, and improved by future Artists—Dr. Blair’s Aplanatic Telescope—Mistakes in Newton’s Analysis of the Spectrum—Modern Discoveries respecting the Structure of the Spectrum 63 CHAPTER VI. Colours of thin Plates first studied by Boyle and Hooke—Newton determines the Law of their Production—His Theory of Fits of easy Reflection and Transmission—Colours of thick Plates 75 CHAPTER VII. Newton’s Theory of the Colours of Natural Bodies explained— Objections to it stated—New Classification of Colours—Outline of a new Theory proposed 82 CHAPTER VIII. Newton’s Discoveries respecting the Inflection or Diffraction of Light—Previous Discoveries of Grimaldi and Dr. Hooke—Labours of succeeding Philosophers—Law of Interference of Dr. Young— Fresnel’s Discoveries—New Theory of Inflection on the Hypothesis of the Materiality of Light 98 CHAPTER IX. Miscellaneous Optical Researches of Newton—His Experiments on Refraction—His Conjecture respecting the Inflammability of the Diamond—His Law of Double Refraction—His Observations on the Polarization of Light—Newton’s Theory of Light—His “Optics” 106 CHAPTER X. Astronomical Discoveries of Newton—Necessity of combined Exertion to the completion of great Discoveries—Sketch of the History or Astronomy previous to the time of Newton—Copernicus, 1473–1543— Tycho Brahe, 1546–1601—Kepler, 1571–1631—Galileo, 1564–1642 110 CHAPTER XI.