THE 



LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AXD 



JOURNAL OF SCIENCE. 



SUPPLEMENT to VOL. XXV. FOURTH SERIES. 



LXVL On Asterism and Brewster's Luminous Figures. 

 By Prof, ton Kobell*. 



[With Three Plates.] 



THE beautiful phenomena of asterism, which were long only 

 known in the case of sapphire and garnet, have been 

 further studied in the investigations of Brewsterf, Babineti, and 

 Volger5, and have been shown to exist in many minerals. Babinet 

 has described them as phenomena of gratings; and the simpler 

 of them may indeed be easily produced by cutting suitable 

 systems of close parallel lines either in a smooth copper plate, or 

 by ruling them on a glass plate covered with copper or silver. 

 Bv means of a candle in an otherwise dark room, and with one 

 system of such lines, one band of light is seen which cuts the 

 lines at right angles ; with two systems of lines crossing at right 

 angles, a rectangular luminous cross is seen, or, if the bands cut 

 at an acute angle, it is oblique ; with three systems drawn to the 

 sides of a triangle, a six-rayed luminous star ; with radial lines 

 proceeding from a centre, a parhelial circle is seen under certain 

 angles of incidence. 



The last phenomenon is often seen through a piece, about an 

 inch long, of an ordinary glass rod (one-third of an inch thick), 

 the ends of which are polished quite smooth. Viewed at a cer- 

 tain distance from a candle-name, the glass being slightly 

 inclined, a circular luminous ring is visible through these ter-. 

 minal faces, in which the flame always stands in a point of the 

 periphery. In polarized light such glass cylinders show the 

 cross through the faces mentioned. A cylinder of homogeneous 

 glass which does not polarize does not give the phenomenon ; but, 

 on the other hand, it is not every polarizing glass which does. In 



* Translated from the Berichte derKonigl. Baierischen Acad. derWissen- 

 schaftea. 1S62. 



t Edinburgh Transactions., vol. xiv. 1837- Phil. Mag. Jan. 1853. 



% PoggemlorfFs Aanalen, vol. xli. 1837. 



^ Sitzungsberickte der Wiener Akademie, vol. xix. 185(J. 



Phil. Mag. S. 4. No. 164. Suppl Vol. 24. 2 L 



