Luminosity, addition of one to another, 85-87 Luminosity of colour, 16 Luminosity of pigments, methods of determining, 81, 82 Luminosity of spectrum to normal-eyed and colour-blind persons, 76-78 Luminosity of sun at different altitudes, 69-71 Maxwell's colour-box, 152, 153 Maxwell's discs, 184-186 Measurement of amount of light reflected by different pigments, 88-92 Metals, light reflected from, 100 Mock suns, cause of change of colour in, 64 Molecular physics, 54 Molecular swings, 136, 137 Monochromatic light, 47 Negative images, 203 Normal vision, 77 Orange, finding luminosity of, 190 Percentages of skylight, sunlight, and gaslight, 110, 111 Phosphorescence, 32, 56 Pigments, absorption by, 57, 58 Plan of forming spectrum, 21 Polished and uneven surfaces, 13 Primary colours, definition of, 133-135 Prism, Iceland spar, 96 Prismatic spectrum into wave-lengths, conversion of, 28 Prisms, drawback to use of, 23 Prussian blue template, 107 " " light reflected from, 107 Purity of colour, 16 Rays, infra-red, 34 Rays, photography of dark, 34 Rays, ultra-violet, 34 Registering tint of pigments, 116 " " colours, 156 Retina, persistence of images on, 179 Ritter's rays, 32 Rood's colour scale, 26 Rotating sectors, 46 Scaling of spectrum, 49 Sectors, rotating, 46 Simple colours, how obtained, 112, 113 Slits placed in spectrum, 113 Soap-bubbles, 58, 59 Soap-films, 59 Spectrum, absorption of, 51, 52