The Visible and Invisible Parts of the Spectrum--Methods for showing the Existence of the Invisible Portions--Phosphorescence--Photography of the Dark Rays--Thermo-Electric Currents p. 30 CHAPTER IV. Description of Colour Patch Apparatus--Rotating Sectors--Method of making a Scale for the Spectrum p. 41 CHAPTER V. Absorption of the Spectrum--Analysis of Colour--Vibrations of Rays--Absorption by Pigments--Phosphorescence--Interference p. 51 CHAPTER VI. Scattered Light--Sunset Colours--Law of the Scattering by Fine Particles--Sunset Clouds--Luminosities of Sunlight at different Altitudes of the Sun p. 62 CHAPTER VII. Luminosity of the Spectrum to Normal-eyed and Colour-blind Persons--Method of determining the Luminosity of Pigments--Addition of one Luminosity to another p. 76 CHAPTER VIII. Methods of Measuring the Intensity of the Different Colours of the Spectrum, reflected from Pigmented Surfaces--Templates for the Spectrum p. 88 CHAPTER IX. Colour Mixtures--Yellow Spot in the Eye--Comparison of Different Lights--Simple Colours by Mixing Simple Colours--Yellow and Blue from White p. 112 CHAPTER X. Extinction of Colour by White Light--Extinction of White Light by Colour p. 126 CHAPTER XI. Primary Colours--Molecular Swings--Colour Sensations--Sensations absent in the Colour-blind p. 133 CHAPTER XII. Formation of Colour Equations--Kœnig's Curves--Maxwell's Apparatus and Curves p. 147 CHAPTER XIII. Match of Compound Colours with Simple Colours--All Colours reduced to Numbers--Method of Matching a Colour with a Spectrum Colour and White Light p. 156 CHAPTER XIV. Complementary Colours--Complementary Pigment Colours--Measurement of Complementary Colours p. 167 CHAPTER XV. Persistence of Images on the Retina--The Use of Coloured Discs p. 179 CHAPTER XVI. Contrast Colours--Measurement of Contrast Colours--Fatigue of the Eye--After-Images p. 196 LIST OF ILLUSTRATIONS. FIG. PAGE Colour-patch apparatus _Frontispiece_ 1. Spectrum of sunlight 18 2. Carbon poles of an electric light 20 3. Curve for converting prismatic spectrum into wave-lengths 28 4. The thermopile 35 5. Heating effect of different sources of radiation 38 6. Colour-patch apparatus 42 7. Rotating sectors 45 8. Spectrum of Carbon Sodium and Lithium 48 9. Interference bands 60 10. Absorption of rays by the atmosphere 68 11. Luminosity curve of spectrum of the positive pole of the electric light 79 12. Rectangles of white and vermilion 82