Continuous spectrum, 17 Contrast colours, 196-200 Diffraction gratings, 23 " spectra, 24 Dimness and brightness of spectrum, 29 Discs, spinning, 182 Dust, particles of, 62 Electric light, contrast colours in, 197 Electric light, crater of positive pole of, 20 Emerald green, light reflected from, 94 Equations, colour, 147 Essentials of spectrum, 22 Extraction of colour by white light, 126 Extraction of white light by colour, 131 Eye, sensitiveness of, 15 Fatigue of the retina, 202 Fluorescence, 31 Fundamental sensations, 140 Gamboge, matching, 189 Glass, light from sheet of, 14 Glass prisms, 21, 22 Glow-worm, 13 Green colour-blindness, 142 Heating effect of radiation, 38 Hue, 15 Images, after, 202 Images, persistence of, on retina, 179 Impurity of simple colours, 124 Indicator of sectors, 48 Infra-red rays, 32 " photography with, 34 Insensitiveness of the yellow spot to green, 118 Intensities of limelight, gaslight, and blue sky compared, 110, 121 Interference, 58, 59 Interference bands on soap film, 60 Invisible spectrum, methods for showing existence of, 32, 33 Kœnig's curves, 151 Kœnig's experiments, 140 Law of the scattering by fine particles, 66 Light from sun, imitation of, 63 Light, quality of, illumining object, 14 Light scattered, 62 Limelight, 19 Lines in solar spectrum, 26 Luminosity, 13