328 LECTURE XXXVI. 



fcl. Par. 1638. Zucchius, 2 vols. 4to, Lugd. 1652-6. Thomasius, 4to, l/)53. 

 Lichtner, 4to, 1653, and 4to, 1654. Balthasar, 4to, 1656. Mancini, Bolog.*1660. 

 De la Chambre, La Lumiere, Paris, 1662. Vossius de Nat. Lucis, 4to, Amst. 1662. 

 Kohlhausen, Lips. 1663. Grandorgeeus, 4to, Cad. 1664. Fabri, Synopsis Optica, 

 Lyon, 1667. Saggi del Acad. di Cimento, 1667. Cherubin, Dioptrique, fol. Paris, 

 1671. Kirchmaier de Luce, Misc. Acad. Nat. Cur. 1677, App. p. 219. ' Moly- 

 neux, Dioptrica Nova, 4to, Lond. 1694. Hartsoeker, Essai de Diop. 4to, Par. 

 1694. Gregorii (D.) Catop. et Diop. Oxf. 1695. Huygens, Op. Post. Lugd. 1704. 

 Craig< Optica Analytica, 1708. Rizetti de Luminis Affectionibus, Ven. 1727. 

 Smith's Optics, 4to, Camb. 1738. (German by Kastner, Altenb. 1775.) Euler, 

 Dioptrica, 3 vols. 4to, Petrop. Martin's Optics, 1750. Courtivron, Traite, Paris, 

 1752. LacaiUe, 1756. Deincken, Alt. 1757. Bischoff, Ulm, 1760 & 1772. Al- 

 garotti (trans.}, The Philosophy of Sir I. Newton explained in Six Dial, on Light, 

 12mo, Glasg. 1765. Thomin's Traite d' Op. Paris, 1769. Harris's Optics, 4to, 

 1775. Scherfer, 4 vols. 4to, Vindob. 1775. Biirja, Berlin, 1793. Karstens, 

 Lehr. derMathem. Theil viii. Rampinellius, Optique, Brix. 1760. Emerson's Op- 

 tics, 1768. Ferguson's, 1770. Stack's, Dub. 1811. Settele, Elem. di Ottica, 2 vols. 

 Rom. 1818-19. Nobili, Milan, 1820. Maseres, Scriptores Optici, 4to, Lond. 

 1823. Bourgeois, Manuel d'Optique Experimental, 2 vols. 12mo. 1823. Brew- 

 ster's Optics, Edin. Encyc., Cab. Cyc., and Encyc. Brit. Herschel's, Encyc. Me- 

 tropolit. (complete and good), Transl. in French by Verhulst, with Supplement by 

 Quetelet, 3 vols. 1826. Amondieu, Lehr. der Optik, Leipz. 1827. Rottger, Halle, 

 1828. Prechtl. Wien, 1829. Higgins, Lond. 1829. Coddington on Reflection 

 and Refraction, Camb. 1829. Littrow, Dioptrik, Wien, 1830. Wood's Optics, Camb. 

 v. y. Lloyd on Light, 1831. Powell's Optics, Oxf. 1833. Schmidt, Gott. 1834. 

 Johnson's Optical Investigations, Oxf. 1835. Phelps's Optics, Camb. 1835. Grif- 

 fin's, Camb. 1840. Bartlett's, New York, 1841. 



LECTURE XXXVI. 



ON OPTICAL INSTRUMENTS. 



AMONG the great variety of instruments depending on optical principles, 

 it is most consistent with our plan to attend first to those which may be 

 denominated optical measures, which are calculated either for the determi- 

 nation of the quantity or intensity of light itself, or for the examination of 

 the properties of various material substances with respect to light. Reflect- 

 ing quadrants and circles, which are often used in astronomical and 

 nautical observations, although they derive their utility in some measure 

 from optical laws, may most properly be considered as belonging to the 

 subject of practical astronomy. 



It is a problem of frequent occurrence in economical investigations, to 

 compare the intensity of the light afforded by any two luminous objects. 

 For this purpose it is necessary to assume as a principle, that the same 

 quantity of light, diverging in all directions from a luminous body, remains 

 undiminished at all distances from the centre of divergence. Thus, we 

 must suppose that the quantity of light falling on every body is the same 

 as would have fallen on the place occupied by its shadow : and if there 

 were any doubt of the truth of the supposition, it might be confirmed by 

 some simple experiments. It follows that since the shadow of a square 



