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