^{page 376 note *} Traité de la Peinture, quoted in Gehler's Wörterbuch, art. Atmosphäre.

^{page 376 note †} Farbenlehre, i. 59, quoted by Humboldt.

^{page 376 note §} Fabri's Dialogues (1669), of which I have found a copy in the Advocates' Library, contain many allusions to the imperfect transparency of the air, and the foreign particles mixed with it ; but I do not find his theory of the blue colour clearly stated.

^{page 376 note ∥} “On peut croire qu'il y a des couleurs primitives dans quelques corps, comme du bleu dans l'air. … Il semble qu'il y ait du verd dans l'eau.”—Mariotte, Œuvres, i. 299. Leide 1717.

^{page 376 note ‡} Eberhard in Rozier, i. 620.

^{page 377 note *} Optics, Book ii. Part iii. Prop. vii.

^{page 377 note §} Traité d'Optique, p. 365-368. He likewise explains the coloured shadows noticed by Buffon.

^{page 377 note ∥} Smith's Optics, vol. ii. Remarks, 378.

^{page 377 note †} Ibid. Prop. v. end.

^{page 377 note ‡} Book ii. Part iv. Obs. 13.

^{page 378 note *} Euler's Letters (translation), ii. 507.

^{page 378 note ‡} Page 81-89, &c. Edin. 1770.

^{page 378 note †} Nollet, Leçons de Physique, vi. 17. 1765.

^{page 379 note *} Opera Omnia, ii. p. ii. 82. Edit. 1768. “Cur vapores eleventur non spernenda qusæstio est, atque inter alia non malè concipiuntur in illis bullæ insensibiles ex pellicula aquae et aëre incluso constantes, quales sensus in liquoribus spumescentibus ostendit.”

^{page 379 note †} Theorie de l'Elevation des Vapeurs et des Exhalaisons, &c. Bordeaux, 1740. Quoted in Saussure's Hygrometrie, § 202, and in Kämtz, Lehrbuch der Meteorologie, iii. 48. The diameter he made and the thickness inch.

^{page 379 note ‡} Voyages dans les Alpes, iv. § 2083.

^{page 379 note ∥} Hygrometrie, § 372.

^{page 379 note **} Manchester Memoirs, 1st Serie9, ii. 214, &c.

^{page 379 note §} Hygrometrie, § 355.

^{page 379 note ¶} Rozier, Introduction, i. 618.

^{page 380 note *} See his Relation Historique, 8vo, ii. 116, &c.

^{page 380 note †} See his Nat. Phil. ii. 321. Compare pages 637, 638, 646, on Newton's Theory of the Colour of Bodies.

^{page 380 note ‡} Encyclopædia Britannica, art. Meteorology. The same theory is maintained in the article Physical Geography by Dr Traill, just published.

^{page 380 note ∥} On New Philosophical Instruments, p. 349.

^{page 380 note §} Peclet, Traité de Physique, ii. 307. Brussels edit.; Herschel on Light, art. 858, and Quetelet's Supplement to the French translation.

^{page 380 note ¶} Essay on Light, art. 1143.

^{page 380 note **} Vol. i. p. 4. &c. 1825.

^{page 381 note *} Gehler's Physikalisches Wörterbuch, vol. i. p. 6, Note.

^{page 381 note †} Schweigger's Journal, xxx. 81; and article Atmosphäre in Gehler.

^{page 381 note ‡} Edin. Journal of Science, v. 52.

^{page 381 note ∥} Lehrbuch der Chemie, Wöhler's edit. 1825, i. 346.

^{page 381 note §} Edin. Encyclopædia, art. Optics, p. 620. Compare articles Atmosphere and Cyanometer.

^{page 382 note *} Life of Newton, p. 78. 1831. Ed. Trans, xii. 538.

^{page 382 note †} Ed. Trans, xii. 544. Compare Encyc. Brit, new Edition, art. Optics, p. 510.

^{page 382 note ‡} Researches about Atmospheric Phenomena, 3d edit., 1823, p. 86. The continuation of the passage will be quoted further on.

^{page 383 note *} “In the splendour of a Neapolitan firmament, we may seek, in vain for that purple light so delightful to our boyish fancy.”—Tour in Italy.

^{page 383 note †} Encyclopædia Britannica, art. Meteorology.

^{page 383 note ‡} Bibliotheque Universelle(1830), tom. xliv. p. 337.—Translated in Taylor's Scientific Memoirs, vol. i.

^{page 383 note ∥} It is a curious circumstance, which I have never heard remarked, that Dr Priestley in a great measure anticipated the experiment of Nobili ; for, by successive electric discharges on the surface of many kinds of metal, he produced rings identical with those of Newton.—Priestley, Phil. Trans. 1778. These colours were no doubt produced by the heat developed in the same way as those mentioned in one part of Nobili's paper. The explanation of these colours, by supposing with the philosopher of Reggio (if I understand him aright), that they are produced by thin plates of adhering oxygen gas, is too evidently founded in error to require any notice.

^{page383 note §} Nobili quotes Amici's authority in confirmation of this novel assertion, and also for the alleged absence of green in the second order of colours. I think I can speak with much confidence as to the existence of blue of the first order in the depolarized tints of mica plates: but the attempt to shew (Bibl. Univ. xliv. p. 343 and 344, note), that there ought to be no blue, and that the first colour of Newton's scale should be white, seems to me a failure, arising from a degree of misconception of first principles which it is difficult to admit.

^{page 384 note *} In the translation of the paper in Taylor's Scientific Memoirs, i. 99, by an oversight, the maximum thickness of the cloudy vesicles is stated at the ten-millionth of an inch, instead of ten millionths of an inch, or a hundred times greater, as in the original. There is even a slight mistake in the latter; the tint he describes corresponding to plates of water, not of air, would require a thickness of seven millionths.

^{page 384 note †} Translated in the Edin. New Phil. Journal, vol. xv.

^{page 385 note *} Count Maistre explains the colour of the water by similar reasoning. He considers it blue for reflected, and yellowish-orange for transmitted light, and the green colour of the sea and some lakes he attributes to diffused particles which reflect a portion of the transmitted tint, and mingle with the blue. This is well confirmed by Davy's Observations, (Salmonia, 3d edit. p. 317). Arago has very ingeniously applied the same reasoning to the ocean, shewing that when calm it must be blue, but when ruffled, the waves acting the part of prisms, refract to the eye some of the transmitted light from the interior, and it then appears green, (Comptes Rendus, 23d July 1838.) Most authors have admitted the intrinsically blue or green colour of pure water, as Newton (Optics, b. i., part ii., prop, x.), Mariotte (already quoted), and Euler : Humboldt seems doubtful, (Voyage, 8vo, ii. 133).

^{page 385 note †} Encyc. Metropolitan, art. Meteorology, p. 163, &c.

^{page 385 note ‡} Annuaire 1832, p. 248. Whilst this Paper is passing through the press, I have seen a notice by M. Babinet (Comptes Rendus, 25th Feb. 1839), on the subject of the blue colour of the sun, which he considers as real, and endeavours to explain by the theory of mixed plates.

^{page 385 note ∥} Germ. “Glühen der Alpen.”

^{page 385 note §} Seventh Report of British Association. Transactions of Sections, p. 10.

^{page 386 note *} Lehrbuch der Meteorologie, iii. 58.

^{page 387 note *} Kamtz, Lebrbuch iii. 40.

^{page 388 note *} Ed. Trans, xii. 530.

^{page 388 note ‡} See Robison's Works, ii. 2, &c.

^{page 388 note †} Phil. Mag. 1833.

^{page 389 note *} See Young's article Chromatics, in Encyc. Brit., and Fraunhofer in Schumacher's Astronomische Abhandlungen. Drittes Heft. 1825.

^{page 389 note †} Relation Historique, 8vo, ii. 128.

^{page 389 note ∥} Diosemeia, 93. Quoted by Kämtz.

^{page 389 note †} New Spain (translation), ii. 326.

^{page 389 note §} Matt. xvi. 2, 3.

^{page 390 note *} For the reason why over water, see Davy's Paper, Phil. Trans. 1819.

^{page 390 note †} Quoted by Harvey in Encyc. Metrop. Meteorology, p. *166. The cause of the purple light mentioned here, probably arises from a mixture of the reflected blue of the pure sky (which is always present when purple is seen) with the yellow-orange, which condensing vapour first transmits. I do not think it at all necessary to affirm, however, that pure air has no transmitted colour of its own.

^{page 390 note ‡} Third edit. p. 87.