338 LECTURE XXXVI. 



in this manner for judging of the magnitude of the celestial bodies. The 

 divided object glass micrometer affords another mode of measurement.:* 

 the object glass being divided into two semicircular portions, one of which 

 slides on the other ; each portion acts as a separate lens, and two images 

 of every part of the object being formed, the angular distance of any two 

 points is determined by bringing their images together, and measuring the 

 displacement of the moveable portion of the object glass which is required 

 for procuring the coincidence. Sometimes also a similar purpose is 

 answered by inserting a divided glass in the eye piece, which acts nearly 

 on the same principle, and which seems to be somewhat less liable to flrror. 

 In a reflecting telescope of Cassegrain's construction, Mr. Ramsden t has 

 also produced the same effect by dividing the convex speculum, and 

 causing a part of it to turn round an axis. All these arrangements parti- 

 cularly deserve the attention of those who are employed in practical astro- 

 nomy and in geography, since the advancement of these sciences much 

 depends on the accuracy of the telescopic and microscopic measures, which 

 are performed by means of optical instruments. (Plate XXVIII. Fig. 

 416, 417.) 



LECT. XXXVI. ADDITIONAL AUTHORITIES. 



Photometry. Marie, Nouvelle Decouverte en Lumiere, 1701. Mairan, Hist, 

 et Mem. 1721. Celsius, Nouvelle Idee sur la Mesure de la Lumiere, ibid. 1735, 

 H. 5. Euler, Hist, et Mem. de Berlin, 1750, p. 280. Karsten, Photometric, 

 Greifswald, 1777. Fontana, Mem. della Soc. Ital. i. 111. Fossombroni, Sull' 

 Intensita del Lume, fol. Arezzo, 1781. Langsdorf, Grundlehren der Photometric, 

 2 vols. Erlangen, 1803-5. Leslie's Photometer, Nich. Jour. iii. 461. Ritchie's, 

 Ph. Mag. v. 139. Potter's, ibid, new series, iii. 284. Xavierde Maistre's, Bibliot. 

 Univ. 1832, p. 323. Osann's, Pogg. Ann. xxxiii. 405. Steinheil's, ibid, xxxiv. 644. 



Measurement of Refractive Powers. J. A. Porta, De Refractione, 4to, Neap. 

 1583. Lahire on the Refraction of Ice, Hist, et Mem. ix. 328, x. 172 ; of Oil and 

 Water, ix. 382, 577. Lowthorp on the Refraction of Air, Ph. Tr. 1699, p. 339. 

 Cassini on do. Hist, et Mem. 1700, p. 78, H. 112. Hauksbee on the Refraction of 

 Fluids, Ph. Tr. 1710, p. 204. Euler on do. Hist, et Mem. de Berlin, 1756, p. 

 235; of Glass, 1766, p. 202. J. A. Euler, ibid. 1762, pp. 279, 302, 318, 328. 

 Cadet and Brisson, Hist, et Mem. 1777, p. 541. Biot and Arago, Memoire sur 

 les Affinites des Corps pour la Lumiere, Mem. de 1'Institut, 1806, ii. 301. Frau- 

 enhofer, Bestimmung des Brechungs und Farbenszerstreuungs-vermbgens Ver- 

 schiedener Glasarten Schum. Astron. Abhandl. 1815. Hartmann, in Schum. Astr. 

 Nachr. vii. 265. Malus, in Gilb. Ann. xxxi. 225. Marx, in Schweigger's Jour. v. 

 385, Ixi. 46. Arago, Annales de Chimie, vol. i. &c. Dulong, Mem. sur les Pouvoirs 

 Refringens des Fluides Elastiques, Mem. de 1' Acad. 1825. 



Construction of Mirrors. Mudge on the Best Composition of Metals, Ph. Tr. 

 1777, p. 296. Potter on Improvements in Casting and Working, Ph. Mag. 1831, 

 iv. 13, vi. 228. Lord Oxmantown, ibid. ix. 213. 



Theory of Lenses. Kastner, Com. Gott. i. 185, ii. 183. Lagrange surlaTheorie 

 des Lunettes, Hist, et Mem. de Berlin, 1778, p. 162. Bohnenberger, Zeitschrift fur 

 Astron. i. 277, 385. Von Munchow, ibid. ii. 448. Gauss, ibid. iv. 345. Mobius, 

 inCrelle's Jour. v. 113 ; Schleiermacher, in Poggendorf's, xiv. 1. Schulten, Supp. 

 a la Theorie des Verres Simples, Vedensk Aph. 1821, p. 265. Herschel, Ph. Tr. 

 1821, p. 222. Hamilton on a System of Rays, Tr. Roy. Ir. Ac. 1824, &c. Barlow, 

 Ph. Tr. 1827, p. 231. Santini, Teoria degli Stromenti Ottici, Padua, 1828. Lub- 

 bock, Ph. Mag. vii. 161. 



Reflecting Goniometer. Wollaston, Ph. Tr. 1809, p. 253. Malus, Mem. d'Ar- 



* Savery and Dollond, Ph. Tr. xlviii. 165, 178, 551. 



t Ramsden, Description of two new Micrometers, Ph. Tr. 1779, Ixix. 419. 



