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 Hers