Principles of Spectrum Analysis Prismatic Analysis of the Light of Incandescent Vapours Discontinuous Spectra Spectrum Bands proved by Bunsen and Kirchhoff to be characteristic of the Vapour Discovery of Rubidium, Cæsium, and Thallium Relation of Emission to Absorption The Lines of Fraunhofer Their Explanation by Kirchhoff Solar Chemistry involved in this Explanation Foucault's Experiment Principles of Absorption Analogy of Sound and Light Experimental Demonstration of this Analogy Recent Applications of the Spectroscope Summary and Conclusion APPENDIX. On the Spectra of Polarized Light Measurement of the Waves of Light INDEX ON LIGHT LECTURE I. INTRODUCTORY USES OF EXPERIMENT EARLY SCIENTIFIC NOTIONS SCIENCES OF OBSERVATION KNOWLEDGE OF THE ANCIENTS REGARDING LIGHT DEFECTS OF THE EYE OUR INSTRUMENTS RECTILINEAL PROPAGATION OF LIGHT LAW OF INCIDENCE AND REFLECTION STERILITY OF THE MIDDLE AGES REFRACTION DISCOVERY OF SNELL PARTIAL AND TOTAL REFLECTION VELOCITY OF LIGHT ROEMER, BRADLEY, FOUCAULT, AND FIZEAU PRINCIPLE OF LEAST ACTION DESCARTES AND THE RAINBOW NEWTON'S EXPERIMENTS ON THE COMPOSITION OF SOLAR LIGHT HIS MISTAKE AS REGARDS ACHROMATISM SYNTHESIS OF WHITE LIGHT YELLOW AND BLUE LIGHTS PRODUCE WHITE BY THEIR MIXTURE COLOURS OF NATURAL BODIES ABSORPTION MIXTURE OF PIGMENTS CONTRASTED WITH MIXTURE OF LIGHTS. § 1. _Introduction_. Some twelve years ago I published, in England, a little book entitled the 'Glaciers of the Alps,' and, a couple of years subsequently, a second book, entitled 'Heat a Mode of Motion.' These volumes were followed by others, written with equal plainness, and with a similar aim, that aim being to develop and deepen sympathy between science and the world outside of science. I agreed with thoughtful men[1] who deemed it good for neither world to be isolated from the other, or unsympathetic towards the other, and, to lessen this isolation, at least in one department of science, I swerved, for a time, from those original researches which have been the real pursuit and pleasure of my life.