THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



SUPPLEMENT to VOL. XXXIII. THIRD SERIES. 



LXXII. On the Phenomenon of Luminous Rings in Calcareous 

 Spar and Beryl, as produced by tubular cavities containing the 

 two ?iew Fluids. By Sir David Brewster, K.H., LL.D., 

 F.R.S., and V.P.R.S. Edin.* 



IN a paper read at the meeting of the British Association at 

 York in 1844, I showed that the two rings seen to pass 

 through a luminous body, when viewed through certain spe- 

 cimens of calcareous spar, were produced by a great number 

 of minute tubes parallel to one of the edges of the primitive 

 rhomb. When the light is incident on a natural face of the 

 rhomb, two rings are generally seen, varying in magnitude as 

 the angle of incidence varies. At one incidence in the plane 

 of the principal section, one ring vanishes by contracting into 

 a luminous point ; and at a different incidence the other ring 

 vanishes in a similar manner, the rings always passing through 

 the luminous body, and vanishing in a point coincident with 

 it. At another incidence the luminous rings are equal, the 

 place of the luminous body being at their point of contact, 

 and a ring on each side of it. If we now incline the rhomb 

 in the plane of the principal section, one of the rings will 

 vanish, as already described, while the other enlarges itself; 

 and by continuing the inclination, the vanished ring will re- 

 appear within the other ring, and touching it at the place of 

 the luminous image. As the inclination increases (the refracted 

 ray approaching the axis of the rhomb) the rings increase in 

 size, the lower ends disappear, and the upper segments, where 

 they are in contact, become almost rectilineal. 



Beside these two primary rings there are two secondary 

 ones ; one of them a complete ring, and the other only a large 

 segment of one. The complete ring commences at the centre 

 of the lesser ring in a red spot at an incidence of 57°; and as 

 the angle of incidence increases, yellow, green, blue, and violet 

 tints successively emerge, till we have a beautiful circular 

 * Communicated by the Author. 



Phil. Mag. S. 3. No. 225. Suppl. Vol. 33. 2 K 



