n, 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