Projectiles, the effects of the impact of, 310, 327-328; seen with the naked eye, 311, 317; measuring the velocity of, 332; photography of, 309-337. Prony's brake, 132. Proof, nature of, 284. Prophesying events, 220 et seq. Psalms, quotation from the, 89. Pseudoscope, Wheatstone's, 96. Psychology, preceded by astronomy, 90; how reached, 91 et seq.; helps physical science, 104; its method the same as that of physics, 207 et seq. Pully arrangement, illustrating principle of least superficial area, 12-13. Purkinje, 284, 285, 291, 299. Purposes, the acts of nature compared to, 14-15; nature pursues no, 66. Puzzle-lock, a, 26. Puzzles, 277. Pyramid of oil, 6. Pythagoras, his discovery of the laws of harmony, 32, 259. Quality of tones, 36. Quantitative investigation, the goal of, 180. Quantity of electricity, 111, 118, 119, 167-170, 173; of heat, 166, 167-171, 174, 177, 244; of motion, 184. Quests made of the inquirer, not by him, 30. Quételet, 15, footnote. Rabelais, 283. Raindrop, form of, 3. Rameau, 34. Reaction and action, principle of, 191. Reactions, disclosure of the connexion of, 270 et seq. Realgymnasien, 365. Realschulen, 365, 373. Reason, stands above the senses, 105. Reflex action, 210. Reflexion, produces symmetrical reversion of objects, 93 et seq. Refraction, 29, 193, 194, 208, 230, 231. Reger, 328. Reliefs, photographs of, 68. Repetition, its rôle in æsthetics, 89, footnote, 91 et seq., 97, 98 et seq., 390. Reproduction of facts in thought, 189, 193, 198, 253. Repulsion, electric, 109 et seq., 168. Research, function of experimental 181; the aim of, 205. Resemblances between facts, 255. Resin, solution of, 7. Resistance, laws of, for bodies travelling in air and fluids, 333 et seq. Resonance, corporeal, 392. Response of sonorous bodies, 25. Retina, the corresponding spots of 98; nerves of compared to fingers of a hand, 96 et seq. Reversible processes, 161 et seq., 175, 176, 181, 182. Rhine, the, 94. Richard the Third, 77. Riddles, 277. Riders, 379. Riegler, 319. Riess, experiment with the thermo-electrometer, 133 et seq., 169. Rigid connexions, 142. Rind of a fruit, 190. Rings of oil, illustrating formation of rings of Saturn, 5. Ritter, 291, 299. Rods of Corti, 19. Rolph, W. H., 216. Roman Church, Latin introduced with the, 340 et seq. Romans, their provinciality and narrow-mindedness, 270. Romeo and Juliet, 87. Römer, Olaf, 51 et seq. Roots, the nature of, in language, 252. Rosetti, his experiment on the work required to develop electricity, 131. Rotating bodies, 285. Rotation, apparatus of, in physics, 59 et seq.; sensations of, 288 et seq. Rousseau, 336. Rubber pyramid, illustrating the principle of least superficial area, 10-11. Ruysdael, 279. Sachs, Hans, 106.