372 LECTURE XXXIX. 



[pass through it, the second only vertical ones. Another remarkable pro- t 

 perty of crystals was discovered by M. Arago, that of depolarizing light. 

 A plate of Iceland spar cut perpendicular to the axes, and placed between 

 two tourmalines, exhibits a beautiful series of concentric rings broken 

 by a dark or bright rectangular cross. This complex phenomenon admits 

 of the readiest explanation. Suppose the axes of the tourmalines at right 

 angles to each other. The light which has passed through the first con- 

 sists of horizontal vibrations only. These fall on the plate of Iceland 

 spar, which being symmetrical relative to its axis, those vibrations which 

 fall perpendicularly on it pass through without suffering any modification. 

 They are subsequently stopped by the second tourmaline, and hence a 

 dark horizontal band. For a nearly similar reason there is a dark vertical 

 band. The direction of motion of the particles in these cases is either 

 coincident with or at right angles to a plane which passes through the ray 

 and the axis of the crystal. But in other places, the direction of motion 

 is oblique to such a plane, and the ray is doubly refracted, so that on 

 emergence it consists of two, which on being united no longer form a 

 polarized ray as before. The second tourmaline, consequently, is incapa- 

 ble of wholly absorbing this ray, and thus we are presented with brightness. 

 Moreover, the distance from the centre at which the maxima of brightness 

 occur depends on the length of the wave ; these maxima are therefore 

 recurring, and form rings, which, since the waves of different colours are 

 different in length, must be coloured. 



Such is an outline of the explanation afforded by the principles of this 

 lecture to the phenomena of double refraction and polarization. To enter 

 into detail exceeds our limits. We must refer the reader to Airy's 

 Mathematical Tracts, or Lloyd's Lectures on Light, the latter being a 

 popular treatise. In Gehler's Physikalisches Worterbuch, art. Undula- 

 tions (1842), is a tolerably complete analytical investigation of the subject.] 



LECT. XXXIX. ADDITIONAL AUTHORITIES. 



Diffraction. Hooke, Ph. Tr. 1672. Newton's Op. lib. 3. Maraldi, Hist, et 

 M6m. 1723, p. 111. Dutour, Mein. des Sav. Etr. v. 636. Stratico, Saggi di Pa- 

 dova, ii. 185. Jordan on the Inflections of Light, Lond. 1799. Arago, Ann. de 

 Chimie, i. 199, 332; Sur la Scintillation des Etoiles, xxvi. 431. Rapport sur 

 quelques Mem. xi. 5. Fresnel, Mem. de 1'Acad. v. Annales de Ch. xi. 246, 337. 

 Frauenhofer, Neue Modification des Lichtes, Miinchen, 1818. Gilbert's Ann. Ixxiv. 

 337. Mayer, Phaenom. ab Inflexione Luminis pendent. Com. Gott. 1820, p. 49. 

 T. Young on Frauenhofer's Experiments, Ed. Jour, of Sc. New Series, i. 112. 

 Airy, Camb. Tr. vol. v. 



Coloured Rings. Boyle, Experiments touching Colours, 1663. Hooke, Microg. 

 and Birch's Hist. iii. 29, 53. Newton, Op. lib. 2. Jordan on the Colours of thin 

 transparent Bodies, 1800. T. Young on the Colours of thin Plates, Jour, of the 

 Roy. Inst. i. 241. Introduction to Medical Literature, p. 556. W. Herschel, Ph. 

 Tr. 1807, pp. 180, 189, 338; 1810, p. 365. Knox, ibid. 1815, p. 161. Arago, 

 Mem. d'Arcueil, iii. 223. Brewster, Ed. Tr. 1815, xii. 191. Airy on a Modifica- 

 tion of Newton's Rings, Camb. Tr. iv. 219, 409. 



Colours by Reflection. Brewster on the Optical Phenomena of Mother-of- Pearl, 

 Ph. Tr. 1814, p. 397. Colours of grooved Surfaces, ibid. 1829, p. 301. 



Miscellaneous. Babinet, Mem. d'Op. Compte Rendue, 1837, p. 638. 



ON POLARIZED LIGHT. 

 Apparatus. Biot on Tourmaline, Ann. de Chimie, 1815. Seebeck, in Biot's 



