in plates, 67
----of striated surfaces, 89, 90

Comet of 1680, Newton's estimate of the temperature of, 168

Crookes, Mr., his discovery of thallium, 193

Crystals, action of, upon light, 98
----built by polar force, 98
----illustrations of crystallization, 99
----architecture of, considered as an introduction to their action upon
light, 98
----bearings of crystallization upon optical phenomena, 106

Crystals, rings surrounding the axes of, uniaxal and biaxal, 145

Cuvier on ardour for knowledge, 220

De Tocqueville, writings of, 215, 222, 223

Descartes, his explanation of the rainbow, 24, 25
----his ideas respecting the transmission of light, 43
----his notion of light, 207

Diamond, ignition of a, in oxygen, 169

Diathermancy, 173

Diffraction of light, phenomena of, 78
----bands, 78, 79
----explanation of, 80
----colours produced by, 89

Dollond, his experiments on achromatism, 28

Draper, Dr., his investigation on heat, 172

Drummond light, spectrum of, 195

Earth, daily orbit of, 74

Electric beam, heat of the, 168

Electricity, discoveries in, 217, 218

Emission theory of light, bases of the, 45
----Newton espouses the theory, and the results of this espousal, 77

Ether, Huyghens and Euler advocate and defend the conception of an, 48, 58
----objected to by Newton, 58

Euler espouses and defends the conception of an ether, 48, 58

Eusebius on the natural philosophers of his time, 13

Expansion by cold, 104

Experiment, uses of, 3

Eye, the, its imperfections, grown for ages towards perfection, 8
----imperfect achromatism of the, 29, _note_

Faraday, Michael, his discovery of magneto-electricity, 218

'Fits,' theory of, 73
----its explanation of Newton's rings, 74
----overthrow of the theory, 77

Fizeau determines the velocity of light, 22

Fluorescence, Stokes's discovery of, 161
----the name, 174

Forbes, Professor, polarizes and depolarizes heat, 180

Foucault, determines the velocity of light, 22
----his experiments on absorption, 197, 198

Fraunhofer, his theoretical calculations respecting diffraction, 87
----his lines, 193
------their explanation by Kirchhoff, 193

Fresnel, and Dr. Young, 50
----his theoretical calculations respecting diffraction, 87
----his mathematical abilities and immortal name, 210

Goethe on fluorescence, 165

Gravitation, origin of the notion of the attraction of, 92
----strength of the theory of, 148

Grimaldi, his disco