XXXI V NOTES. 



rayons d'une vitesse determined sont visibles, qu'eux seuls produisent daus 

 1'eeil la sensation de lumiere. Dans la theorie de 1'emission, le rouge, le 

 jauue, le vert, le bleu, le violet. solaires sont respectivement accompagnes de 

 rayons pareils, mais obscurs par defaut ou par exces de vitesse. A plus de 

 vitesse correspond une moindre refraction, comme moins de vitesse entraine 

 une refraction plus grande. Ainsi chaque rayon rouge visible est accompagne 

 de rayons obscurs de la meme nature, qui se refractent les uus plus, les autres 

 inoins que lui : ainsi il existe des rayons dans les stries noires de la portion 

 rouge du spectre; la meme chose doit etre admise des stries situees dans les 

 portions jaunes, vertes, bleues et violettes" Arago, in the Comp'tes rendus 

 de 1'Acad. des Sciences, T. xvi. 1843, p. 404. (Compare also T. viii. 1839, 

 p. 326; and Poisson, Traite de Me'canique, ed. 2, 1833, T. i. 168.) 

 According to the undulatory theory, the heavenly bodies send out waves of 

 infinitely different velocities of transverse vibration. 



( 145 ) p. 75. Wheatstone, in the Phil. Trans, of the Royal Society for 

 1834, p. 589 and 591. From the experiments described in this memoir, it 

 appears to follow that the human eye is capable of receiving impressions 

 from luminous phenomena, of which the duration is limited to one-millionth 

 part of a second (p. 591). On the hypothesis alluded to in the text, according 

 to which the Sun's light is analogous to the Earth's polar light, see Sir John 

 Herschel, Results of Astron. Observ. at the Cape of Good Hope, 1847, p. 

 351. The ingenious application of Wheatstone's revolving apparatus, im- 

 proved by Breguet, to a critical experiment in the decision between the emis- 

 sion and undulatory theories, as, according to the former, light should pass 

 quicker, and according to the latter slower, through water than through air, 

 has already been spoken of by Arago in the Comptes rendus, T. vii. 1838, p. 

 956. (Compare Comptes rendus pour 1850, T. xxx. p. 489 495 and 556.) 



( 146 ) p. 77. Steinheil, in Schumacher's Astr. Nachr. No. 679 (1849), S. 

 97 100 ; Walker, in the Proceedings of the American Philosophical Society, 

 Vol. v. p. 128 (compare older propositions of Pouillet, in the Comptes rendus, 

 T. xix. p. 1386). Still later ingenious experiments of Mitchel, Director of the 

 Observatory of Cincinnati (Gould's Astron. Journal, Dec. 1849, p. 3, on the 

 Velocity of the Electric Wave), and of Fizeau and Gounelle at Paris (April 

 1850), differ from Wheatstone's and Walker's results. Striking differences 

 between iron and copper, in respect to conduction, are shown by experi- 

 ments given in the Comptes rendus, T. xxx. p. 439. 



( M7 ) p. 77- See Poggendorff, in his Anualen, Bd. Ixxiii. 1848, S. 337; 

 and Pouillet, Comptes rendus, T. xxx. p. 501. 



