Euthyphron, questioned by Socrates, 1. Evolute, the word, 342. Evolution, theory of, as applied to ideas, 216 et seq. Ewald, 298, 304. Excluded perpetual motion, logical root of the principle of, 182. Exner, S., 302, 305. Experience, communication of, 191; our ready, 199; the principle of energy derived from, 179; the wellspring of all knowledge of nature, 181; incongruence between thought and, 206. Experimental research, function of, 181. Explanation, nature of, 194, 237, 362. Eye, cannot analyse colors, 20; researches in the theory of the, 18 et seq.; loss of, as affecting vision, 98. Eyes, purpose of, 66 et seq.; their structure symmetrical not identical, 96. Face, human, inverted, 95. Facts and ideas, necessary to science, 231. Facts, description of, 108; agreement of, 180; relations of, 180; how represented, 206; reflected in imagination, 220 et seq.; the result of constructions, 253; a continuum of, 256 et seq.; equations for obtaining, 180. Falling bodies, 204, 215; Galileo on the law of, 143 et seq., 284. Falling, cats, 303, footnote. Falstaff, 309. Familiar intermediate links of thought, 198. Faraday, 191, 217, 237; his conception of electricity, 114, 271. Fechner, theory of Corti's fibres, 19 et seq. Feeling, cannot be explained by motions of atoms, 208 et seq. Fetishism, 186, 243, 254; in our physical concepts, 187. Fibres of Corti, 17 et seq. Fick, his theory of diffusion, 249. Figures, symmetry of, 92 et seq. Figures of liquid equilibrium, 4 et seq. Fire, use of, 264. Fishes, 306. Fixed note, determining of a, 377. Fizeau, his determination of the velocity of light, 55 et seq. Flats, reversed into sharps, 101. Flouren's experiments, 272, 290. Flower-girl, the baskets of a, 95. Fluids, electrical, 112 et seq. Force, electric, 110, 119, 168; unit of 111; living, 137, 149, 184; generally 253. See the related headings. Forces, will compared to, 254. Foreseeing events, 220 et seq. Formal conceptions, rôle of, 183. Formal need of a clear view of facts, 183, 246; how far it corresponds to nature, 184. Formative forces of liquids, 4. Forms of liquids, 3 et seq. Forward movement, sensation of, 300. Forwards, prophesying, 253. Foucault, 57, 70, 296. Foucault and Toepler, method of, for detecting optical faults, 313 et seq., 320. Foundation of scientific thought, primitive acts of knowledge, the, 190. Fourier, on processes of heat, 249, 278. Fox, a, 234. Franklin's pane, 116. Frary, 338, footnote. Fraunhofer, 271. Freezing-point, lowered by pressure, 162. Fresnel, 271. Fritsch, 321. Frogs, larvæ of, not subject to vertigo, 298. Froude, 333. Frustra, misuse of the word, 345. Future, science of the, 213.