Dr BREWSTER on certain new Phenomena of Colour 



to a physical analysis. So little attention, indeed, have they ex- 

 cited, that HAUY, MOHS, and other writers, describe them as ly- 

 ing in planes parallel to the faces of cleavage ; and in this cir- 

 cumstance HAUY finds an easy explanation of their origin, by as- 

 cribing them to accidental fissures between the natural joints of 

 the mineral *. 



Although Labradorite abounds in fissures, I have never disco- 

 vered any parallel to the general plane of changeable colour, and 

 I possess a specimen in which the colours lie in various curve 

 planes, cutting, at a great angle, all the natural joints of the 

 crystal. 



The first point which I was desirous of determining, was the 

 position of the plane of changeable colour. For this purpose, I 

 effected a tolerably good cleavage parallel to P, Fig. 3, and ha- 

 ving placed the crystal on the goniometer, I turned it round in 

 azimuth till the white image reflected from the face of cleavage, 

 and the mass of coloured light from the plane of changeable co- 

 lour, were both in the plane of reflection, the latter being formed 

 by rays nearer the perpendicular. Let the surface of cleavage 

 P, Fig. 3, be represented by DC, Fig. 5, and let RC be a ray of 

 light from a candle incident at C. This ray will be refracted in 

 the direction CA ; and if CDQ is the inclination of the plane of 

 changeable colour, the refracted ray CA will be reflected at A in 

 the direction AB, and will emerge from the spar in the direction 



* Elles proviennent, comme dans 1'opal, des legeres fissures qui interrompent le 

 tissue de la pierre ; mais Topale etant fendillee dans toutes sortes des directions, pre- 

 sente des reflets qui se succedent, a mesure qu'on la fait mouvoir, au lieu que dans le 

 feldspath, dont les fissures coincident sur un seul plan ; en sorte qu'ils se montrent tout 

 entiers, lorsque la lumiere est reflechie par ce plan, sous Tangle favorable pour la 

 renvoyer a Toeil, et disparoissent, des qu'on donne a la pierre un inclination diffe- 

 rente. J'ai reconnu en observant un morceau de feldspath opalin de Norwege, qui 

 m'a etc envoye par M. ESMARK, que les plans d'ou partoient les reflets dont je viens 

 de parler, etoient dans le sens des faces T qui sont les plus etendues. Traite de 

 Mineralogie, torn. ii. p. 613. 



