Acceleration, organ for forward, 299 et seq. Accelerations, 204, 216, footnote, 225-226, 253. Accident, logical and historical, in science, 160, 168, 170, 213; in inventions and discoveries, 262 et seq. Accord, the pure triple, 46. Accumulators, electrical, 125 et seq.; 132, footnote. Acoustic color, 36. Acoustics, Sauveur on, 375 et seq. Action and reaction, importance of the principle of, 191. Adaptation, in organic and inorganic matter, 216, 229; in scientific thought, 214-235. Æsthetics, computation as a principle of, 34; researches in, 89, footnote; repetition, a principle of, 91. Africa, 186, 234, 237. Agreeable effects, due to repetition of sensations, 92, 97 et seq. Agriculture, transition to, 265. Air-gun, 135. Alcohol and water, mixture of oil and, in Plateau's experiments, 4. Algebra, economy of, 196. Alien thoughts in science, 196. All, the, 88. Amontons, 174, 346. Ampère, the word, 314. Ampère's swimmer, 207. Analogies, mechanical, 157, 160; generally, 236-258. Analogy, defined, 250. Analysis, 188. Analytical geometry, not necessary to physicians, 370, footnote. Anatomic structures, transparent stereoscopic views of, 74. Anatomy, character of research in, 255. Andrieu, Jules, 49, footnote. Animals, the psychical activity of, 190, 231; the language of, 238; their capacity for experience, 266 et seq. Animism, 186, 187, 243, 254. Anisotropic optical fields, 227. Apparatus for producing movements of rotation, 287 et seq. Arabesque, an inverted, 95. Arabian Nights, 219. Arago, 270. Aral, the Sea of, 239. Archæopteryx, 257. Archimedes, 4, 237. Arcimboldo, Giuseppe, 36. Area, principle of least superficial, 10 et seq. Ares, the bellowing of the wounded, 272. Aristotelians, 283. Aristotle, 348, 296. Art, development of, 28 et seq. Artillery, practical, 334-335. Artistic value of scientific descriptions, 254. Arts, practical, 108. Ascent, heights of, 143-151. Asia, 234. Assyrians, the art of, 79. Astronomer, measures celestial by terrestrial distances, 136. Astronomy, antecedent to psychology, 90; rigidity of its truths, 221. Atomic theories, 104. Atoms, 207. Attention, the rôle of, in sensuous perception, 35 et seq. Attraction, generally, 226; of liquid particles, 13-14; in electricity, 109 et seq. Aubert, 298. Audition. See _Ear_. Austrian gymnasiums, 370, footnote. Axioms, instinctive knowledge, 190. Babbage, on the economy of machinery, 196. Bach, 20. Backwards, prophesying, 253. Bacon, Lord, 48, 280. Baer, C. E. von, 235. Balance, electrical, 127, footnote; torsion, 109, 168. Balloon, a hydrogen, 199. Barbarism and civilisation, 335 et seq. Bass-clef, 101. Bass, fundamental, 44. Beats, 40-45, 377 et seq. Beautiful, our notions of, variable, 99.