ON PHYSICAL OPTICS. 349 



the greater angle of the crystal.* It appears from the experiments of 

 Hi5ygens,t confirmed and extended by Dr. Wollaston, that the medium, 

 which causes the unusual refraction, has a different refractive power, 

 according to the direction in which the light passes through it, and that 

 if an oblate or flattened spheroid be described within a crystal, its axis 

 being in the middle of one of the obtuse solid angles, and its principal 

 diameters in the proportion of 9 to 10, the refractive power, with respect 

 to light passing in any direction, will always be inversely as the diameter 

 of the spheroid which is parallel to it ; and where it is greatest, will be 

 equaLgto that of the medium which produces the usual refraction, of which 

 the index is - . A ray of light, falling perpendicularly on any surface of 

 the spar, its point of incidence being considered as the centre of the spher- 

 oid, will meef the surface of the spheroid at the point where it is parallel 

 to that of the spar ; and a ray incident on the same surface in any other 

 direction, will preserve a relation to the perpendicular ray, which is nearly 

 the same as in ordinary refraction. (Plate XXIX. Fig. 435.) 



It is also remarkable, that the two portions of light, thus separated, will 

 not be further subdivided by a transmission through a second piece, pro- 

 vided that this piece be in a position parallel to that of the first ; but if it 

 be placed in a transverse direction, each of the two pencils will be divided 

 into two others ; a circumstance which appears to be the most unintel- 

 ligible of any that has been discovered respecting the phenomena of double 

 refraction. 



The appearances of colours, which are produced by transparent plates 

 of different thicknesses, and of those which are seen in light variously 

 diffracted or inflected, will be more conveniently examined, when we in- 

 vestigate the intimate nature of light, since the general explanation of these 

 colours, which will then be given, will enable us to follow them through 

 all their varieties, with much more ease than could be done at present, 

 without the help of some theory respecting their origin. 



LECT. XXXVII. ADDITIONAL AUTHORITIES. 



Colour and dispersion. Castelli, Optica Colorum, 1740. Euler, Hist, et Mem. 

 de Berlin, 1753, p. 294. Acta Petr. i. I. 174. Nov. Com. Petr. xii. 166. Dol- 

 lond, Ph. Tr. 1759, p. 733. Beguelin, Mem. sur les Prismes Achromatiques, Hist, 

 et Mem. de Berlin, 1762, p. 66. Lambert's Farben-pyramide, 4to, Berl, 1772. 

 Rochon, Recueil de Mecanique, p. 279. Comparetti de Luce et Coloribus, 4to, 

 Pad. 1787. Gruber iiber die Strahlenbrechung, 4to, Dresd. 1787. Seebeck, 

 Schweig. Jour. 1810, p. 1. Mollweide, Demonstratio Prop, quse th. Col. Hewtoni 

 Fundamenti loco est, Lips. 1811. Hoppe, Versuch einer ganz neuen Theorie der 

 Entstehung Sammtlicher Farben, Breslau, 1824. Deal, Nouvelle Essai sur la Lum. 

 et les Couleurs, 1827. Talbot, Ph. Mag. iii. 45 ; iv. 112, &c. Brewster, Ph. Tr. 

 1836, &c. Helwag, Newton's Farbenlehre, Liibeck, 1835. Rudberg, Pogg. Ann- 

 ix. 483. An account of Amici's prismatic telescope will be found in Quetelet's Sup- 



* Bartholin on Iceland Crystals (quibus mira et insolita refractio detegitur), Co- 

 penhagen, 1669. Ph. Tr. v. 2039. 

 ' t Traite de la Lumiere par C. H. D. Z. A Leyde, 1690. 



I Ph.Tr. 1802, p. 381. On this subject see Beccaria, Ph. Tr. 1762, p. 486 i 

 Brewster, Edin. Ph. Jour. i. 289 ; ii. 167, &c. &c. See also Lect. XXXIX. 



