350 LECTURE XXXVIII. 



plement to Herschel, p. 432. Becquerel on the Constitution of the Solar Spectrum, 

 Scientific Memoirs, iii. 537. 



Atmospheric Refraction. Ordinary. Cassini, Novissimae Motuum Solis Epne- 

 merides a Malvasia supputatae, 1G61. Hist, et Mem. i. 103, 1700, p. 39, H. 112 ; 

 1714, p. 33, H.61; 1742, p. 203, H. 72; 1743, p. 249, H. 140. Halley, Ph. 

 Tr. 1721, p. 169, with Newton's table. Lacaille, Hist, et Mem. 1755, p. 547. H. 

 111. Lambert, Route de la Lumiere par les Airs, A la Haye, 1758. Lagrange, 

 Hist.etMem.de Berl. 1772, p. 259. Maskelyne, Ph. Tr. 1777, p. 722. Her- 

 schel, ibid. 1785, p. 88. Oriani, Ephem. Mediol. 1788. Hennert, Hind. Arch.ii. 

 1, 129. Kramp, Analyse des Refractions Astronom. et Terr. 4to, Strasb. 1799. 

 Humboldt's Voy. i. 134. Bessel, Fundamenta Astronomiae, fol. Regiom. 1818, 

 pp.28, 43. Konigsb. Beobacht. vii. 38; viii. 22. Svanberg, Nov. Act. Upsal, 

 ix. 89. Plana, Recherches Analytiques, 4to, Turin, 1823. Ivory, Ph. Tr. ,1823, 

 p. 409. T. Young, ibid. 1824, p. 159. Forster, Ph. Mag. 1824, p. 192. 



Extraordinary. Mariotte on the Rainbow, Hist, et Mem. i. 189. Halley on 

 do. Ph. Tr. 1698, p. 193 ; 1700, p. 714. Weidler de Parheliis, 4tq, Wittemb. 

 1738. Biisch, Tractatus duo Optici, Hamb. 1788. Huddart on Horizontal Re- 

 fractions, Ph. Tr. 1797, p. 29. Latham, ib. 1798, p. 357. Monge on the Mirage 

 in Egypt, Ann. de Ch. xxix. 207. Vince on Horizontal Refraction, Ph. Tr. 1799, 

 p. 13. Biot, Mem. del'Inst. i. 266. Brandes, Beobachtungen uber die Strahlen- 

 brechung, Oldenb. 1807. Frauenhofer, Theorie der Hofe, &c. Schumacher's 

 Ast. Abh. iii. 33. Arago, Bullet. Univ. 1825. 



LECTURE XXXVIII. 



ON VISION. 



THE medium of communication, by which we become acquainted with 

 all the objects that we have been lately considering, is the eye ; an organ 

 that exhibits to an attentive observer, an arrangement of various sub- 

 stances, so correctly and delicately adapted to the purposes of the sense of 

 vision, that we cannot help admiring, at every step, the wisdom by which 

 each part is adjusted to the rest, and made to conspire in effects, so remote 

 from what the mere external appearance promises, that we have only been 

 able to understand, by means of a laborious investigation, the nature and 

 operations of this wonderful structure, while its whole mechanism still 

 remains far beyond all rivalship of human art. 



The eye is an irregular spheroid, not very widely differing from a 

 sphere ; it is principally composed of transparent substances, of various 

 refractive densities, calculated to collect the rays of light, which diverge 

 from each point of an object, to a focus on its posterior surface, which is 

 capable of transmitting to the mind the impression of the colour and 

 intensity of the light, together with a distinction of the situation of the 

 focal point, as determined by the angular place of the object. (Plate 

 XXX. Fig. 436.) 



The first refraction happens at the surface of the cornea, or that trans- 

 parent coat which projects forwards from the ball of the eye : but the 

 cornea, being very nearly of equable thickness, has little effect by its own 

 refractive power, and serves only to give a proper form to the aqueous 



