Horse-chestnut bark, fluorescence of, 165

Huggins, Dr., his labours, 205

Huyghens advocates the conception of ether, 48, 58 ——his celebrated principle, 83

Huyghens on the double refraction of Iceland spar, 112

Iceland spar, 109 ——double refraction caused by, 110 ——this double refraction first treated by Erasmus Bartholinus, 112 ——character of the beams emergent from, 114 ——tested by tourmaline, 116 ——Knoblauch's demonstration of the double refraction of, 185

Ice-lens, combustion through, 167

Imagination, scope of the, 42 ——note by Maclaurin on this point, 43 note

Janssen, M., on the rose-coloured solar prominences, 204

Jupiter, Roemer's observations of the moons of, 20

Jupiter's distance from the sun, 20

Kepler, his investigations on the refraction of light, 14, 207

Kirchhoff, Professor, his explanation of Fraunhofer's lines, 193 ——his precursors, 201 ——his claims, 203

Knoblauch, his demonstration of the double refraction of heat of Iceland spar, 185

Lactantius, on the natural philosophers of his time, 13

Lamy, M., isolates thallium in ingots, 193

Lesley, Professor, his invitation, 2

Light familiar to the ancients, 5 ——generation of, 6, 7 ——spherical aberration of, 8 ——the rectilineal propagation of, and mode of producing it, 9 ——illustration showing that the angle of incidence is equal to the angle of reflection, 10, 11 ——sterility of the Middle Ages, 13 ——history of refraction, 14 ——demonstration of the fact of refraction, 14 ——partial and total reflection of, 16-20 ——velocity of, 20 ——Bradley's discovery of the aberration of light, 21, 22 ——principle of least time, 23 ——Descartes and the rainbow, 24 ——Newton's analysis of, 26, 27 ——synthesis of white light, 30 ——complementary colours, 31 ——yellow and blue lights produce white by their mixture, 31 ——what is the meaning of blackness? 32 ——analysis of the action of pigments upon, 33 ——absorption, 34 ——mixture of pigments contrasted with mixture of lights, 37 ——Wuensch on three simple colours in white light, 39 note ——Newton arrives at the emission theory, 45 ——Young's discovery of the undulatory theory, 49 ——illustrations of wave-motion, 58 ——interference of sound-waves, 58 ——velocity of, 60 ——principle of interference of waves of, 61 ——phenomena which first suggested the undulatory theory 62-69 ——soap-bubbles and their colours, 62-65 ——Newton's rings, 69-77 ——his espousal of the emission theory, and the results of this espousal, 77 ——transmitted light, 77 ——diffraction, 77, 89 ——origin of the notion of the attraction of gravitation, 92 ——polarity, how generated, 93 ——action of crystals upon, 98 ——refraction of, 106 ——elasticity and density, 108 ——double refraction, 109 ——chromatic phenomena produced by crystals in polarized, 121 ——the Nicol prism, 122 ——mechanism of, 125 ——vibrations, 125 ——composition and resolution of vibrations, 128 ——polarizer and analyzer, 127 ——recompounding the two systems of waves by the analyzer, 129 ——interference thus rendered possible, 131 ——chromatic phenomena produced by quartz, 139 ——magnetization, of, 141 ——rings surrounding the axes of crystals, 143 ——colour and polarization of sky, 149, 154 ——range of vision incommensurate with range of radiation, 159 ——effect of thallene on the spectrum, 162 ——fluorescence, 162 ——transparency, 167 ——the ultra-red rays, 170 ——part played in Nature by these rays, 175 ——conversion of heat-rays into light-rays, 176 ——identity of radiant heat and, 177 ——polarization of heat, 180 ——principles of spectrum analysis, 189 ——spectra of incandescent vapours, 190 ——Fraunhofer's lines, and Kirchhoff's explanation of them, 193 ——solar chemistry, 195-197 ——demonstration of analogy between sound and, 198, 199 ——Kirchhoff and his precursors, 201 ——rose-coloured solar prominences, 204 ——results obtained by various workers, 205 ——summary and conclusion, 206 ——polarized, the spectra of, 227 ——measurement of the waves of, 234

Lignum Nephriticum, fluorescence of, 164

Lloyd, Dr., on polarization of heat, 180, 209

Lockyer, Mr., on the rose-coloured solar prominences, 205

Lycopodium, diffraction effects caused by the spores of, 88

Magnetization of light, 141

Malus, his discovery respecting reflected light through Iceland spar, 115 ——discovers the polarization of light by reflection, 208

Masson, his essay on the bands of the induction spark, 202

Melloni, on the polarization of heat, 180

Metals, combustion of, 5, 6 ——spectrum analysis of, 190 ——spectrum bands proved by Bunsen and Kirchhoff to be characteristic of the vapour of, 192

Mill, John Stuart, his scepticism regarding the undulatory theory, 149

Miller, Dr., his drawings and descriptions of the spectra of various coloured flames, 201

Morton, Professor, his discovery of thallene, 162

Mother-of-pearl, colours of, 90

Nature, a savage's interpretation of, 4

Newton, Sir Isaac, his experiments on the composition of solar light, 26 ——his spectrum, 27 ——dispersion, 27 ——arrives at the emission theory of light, 45 ——his objection to the conception of an ether espoused and defended by Huyghens and Euler, 58 ——his optical career, 70 ——his rings, 69-77 ——his rings explained by the theory of 'fits,' 73 ——espouses the emission theory, 77 ——effects of this espousal, 77 ——his idea of gravitation, 92 ——his errors, 208

Nicol prism, the, 122

Ocean, colour of the, 35

OErsted, discovers the deflection of a magnetic needle by an electric current, 176

Optics, science of, 4

Pasteur referred to, 219

Physical theories, origin of, 41-44

Pigments, analysis of the action of, upon light, 33 ——mixture of, contrasted with mixture of lights, 37 ——Helmholtz reveals the cause of the green in the case of mixed blue and yellow pigments, 37 ——impurity of natural colours, 37

Pitch of sound, 59

Pluecker, his drawings of spectra, 202

Polariscope, stained glass in the, 130,131 ——unannealed glass in the, 136

Polarity, notion of, how generated, 93 ——atomic, 93-96 ——structural arrangements due to, 96 ——polarization of light, 112 ——tested by tourmaline, 116 ——and by reflection and refraction, 119 ——depolarization, 120

Polarization of light, 112 ——circular, 140 ——sky-light, 149, 157 ——of artificial sky, 156 ——of radiant heat, 180

Polarizer and analyzer, 127

Poles of a magnet, 93

Powell, Professor, on polarization of heat, 180

Prism, the Nicol, 122

Quartz, chromatic phenomena produced by, 139

Radiant heat, 172 ——diathermancy, or perviousness to radiant heat, 173 ——conversion of heat-rays into light rays, 174 ——formation of invisible heat-images, 179 ——polarization of, 180 ——double refraction, 182 ——magnetization of, 184

Rainbow, Descartes' explanation of the, 24

Refraction, demonstration of, 14

Refraction of light, 106 ——double, 109

Reflection, partial and total, 16-20

Respighi, results obtained by, 205

Ritter, his discovery of the ultraviolet rays of the sun, 159

Roemer, Olav, his observations of Jupiter's moons, 20 ——his determination of the velocity of light, 21

Rubidium, discovery of, 193

Rusting of iron, what it is, 5

Schwerd, his observations respecting diffraction, 87

Science, growth of, 176, 203

Scoresby, Dr., succeeds in exploding gunpowder by the sun's rays conveyed by large lenses of ice, 167

Secchi, results obtained by, 205

Seebeck, Thomas, discovers thermo-electricity, 176 ——discovers the polarization of light by tourmaline, 208

Selenite, experiments with thick and thin plates of, 124

Silver spectrum, analysis of, 190, 191

Sky-light, colour and polarization of, 149, 154 ——generation of artificial skies, 152

Snell, Willebrord, his discovery, 14 ——his law, 15, 24

Soap-bubbles and their colours, 63, 65

Sound, early notions of the ancients respecting, 51 ——interference of waves of, 58 ——pitch of, 59 ——analogies of light and, 56 ——demonstration of analogy between, and light, 198, 199

Sonorous vibrations, action of, 134

Spectrum analysis, principles of, 189

Spectra of incandescent vapours, 190 ——discontinuous, 191, 192 ——of polarized light, 227

Spectrum bands proved by Bunsen and Kirchhoff to be characteristic of the vapour, 192 ——its capacity as an agent of discovery, 193 ——analysis of the sun and stars, 193

Spottiswoode, Mr. William, 123, 227

Stewart, Professor Balfour, 202

Stokes, Professor, results of his examination of substances excited by the ultra-violet waves, 161 ——his discovery of fluorescence, 162 ——on fluorescence, 165 ——nearly anticipates Kirchhoff's discovery, 198, 202

Striated surfaces, colours of, 89

Sulphate of quinine first noticed and described by Sir John Herschel, 165

Sun, chemistry of the, 195

Sun, rose-coloured solar prominences, 204

Talbot, Mr., his experiments, 201

Tartaric acid, irregular crystallization of, and its effects, 131

Thallene, its effect on the spectrum, 162

Thallium, spectrum analysis of, 190, 191 ——discovery of, 193 ——isolated in ingots by M. Lamy, 193

Theory, relation of, to experience, 91

Thermo-electric pile, 176

Thermo-electricity, discovery of, 176

Tombeline, Mont, inverted image of, 19

Tourmaline, polarization of light by means of, 112

Transmitted light, reason for, 77

Transparency, remarks on, 167

Ultra-violet sun-rays, discovered by Ritter, 159 ——effects of, 160

Ultra-red rays of the solar spectrum, 171 ——part played by the, 173

Undulatory theory of light, bases of the, 47 ——Sir David Brewster's chief objection to the, 47

Undulatory theory of light, Young's foundation of the, 49 ——phenomena which first suggested the, 62, 69 ——Mr. Mill's scepticism regarding the, 143 ——a demonstrated verity in the hands of Young, 210

Vassenius describes the rose-coloured solar prominences in 1733, 204

Vitellio, his skill and conscientiousness, 14 ——his investigations respecting light, 207

Voltaic battery, use of, and its production of heat, 6, 7

Water, deportment of, considered and explained, 105, 106

Waves of water, 51 ——length of a wave, 52 ——interference of waves, 53-55

Wertheim, M., his instrument for the determination of strains and pressures by the colours of polarized light, 134

Wheatstone, Sir Charles, his analysis of the light of the electric spark, 202

Whirlpool Rapids, illustration of the principle of the interference of waves at the, 55

Willigen, Van der, his drawings of spectra, 202

Wollaston, Dr., first observes lines in solar spectrum, 193 ——discovers the rings of Iceland spar, 209

Woodbury, Mr., on the impurity of natural colours, 37

Wuensch, Christian Ernst, on the three simple colours in white lights, 39 note ——his experiments, 39 note

Young, Dr. Thomas, his discovery of Egyptian hieroglyphics, 49; ——and the undulatory theory of light, 49 ——Helmholtz's estimate of him, 50 ——ridiculed by Brougham in the 'Edinburgh Review,' 50 ——generalizes Grimaldi's observation on light, 56, 57 ——photographs the ultra-violet rings of Newton, 160

THE END