Relation of Theories to Experience Origin of the Notion of the Attraction of Gravitation Notion of Polarity, how generated Atomic Polarity Structural Arrangements due to Polarity Architecture of Crystals considered as an Introduction to their Action upon Light Notion of Atomic Polarity applied to Crystalline Structure Experimental Illustrations Crystallization of Water Expansion by Heat and by Cold Deportment of Water considered and explained Bearings of Crystallization on Optical Phenomena Refraction Double Refraction Polarization Action of Tourmaline Character of the Beams emergent from Iceland Spar Polarization by ordinary Refraction and Reflection Depolarization. LECTURE IV. Chromatic Phenomena produced by Crystals in Polarized Light The Nicol Prism Polarizer and Analyzer Action of Thick and Thin Plates of Selenite Colours dependent on Thickness Resolution of Polarized Beam into two others by the Selenite One of them more retarded than the other Recompounding of the two Systems of Waves by the Analyzer Interference thus rendered possible Consequent Production of Colours Action of Bodies mechanically strained or pressed Action of Sonorous Vibrations Action of Glass strained or pressed by Heat Circular Polarization Chromatic Phenomena produced by Quartz The Magnetization of Light Rings surrounding the Axes of Crystals Biaxal and Uniaxal Crystals Grasp of the Undulatory Theory The Colour and Polarization of Sky-light Generation of Artificial Skies. LECTURE V. Range of Vision not commensurate with Range of Radiation The Ultra-violet Rays Fluorescence The rendering of invisible Rays visible Vision not the only Sense appealed to by the Solar and Electric Beam Heat of Beam Combustion by Total Beam at the Foci of Mirrors and Lenses Combustion through Ice-lens Ignition of Diamond Search for the Rays here effective Sir William Herschel's Discovery of dark Solar Rays Invisible Rays the Basis of the Visible Detachment by a Ray-filter of the Invisible Rays from the Visible Combustion at Dark Foci Conversion of Heat-rays into Light-rays Calorescence Part played in Nature by Dark Rays Identity of Light and Radiant Heat Invisible Images Reflection, Refraction, Plane Polarization, Depolarization, Circular Polarization, Double Refraction, and Magnetization of Radiant Heat LECTURE VI.