490 Sir D. Brewster on the Phenomenon of Luminous Rings 



spectrum with a violet disc in its centre. As the inclination 

 increases the spectrum becomes a ring, which gradually ex- 

 pands and approaches to the luminous body or to the primary 

 ring in which it is inclosed, the red rays being on the outside and 

 the violet on the inside. The other secondary ring or segment 

 lies beyond the primary rings. It has a curvature of contrary 

 flexure at its extremities, and gradually approaches the lumi- 

 nous body, or the primary ring beyond which it lies. It is also 

 highly dispersed, but less so than the other secondary ring, 

 and its violet side is on the outside of the ring. These two 

 secondary rings are very feeble in the intensity of their light ; 

 but their intensity increases with the angle of incidence, and 

 at the greatest possible inclination their intensity becomes 

 equal, and finally superior, to that of the two primary rings, 

 which almost disappear. 



The ring E, or that formed by the Extraordinary image of 

 the spar, vanishes at an incidence of 19° from the perpendi- 

 cular, or 109° from the surface of the rhomb between the 

 point of incidence and the obtuse angle of the rhomb, and in 

 the plane of the principal section. The ring O, or that formed 

 by the Ordinary image, vanishes at an incidence of 33° or 123°, 

 similarly reckoned. The two rings E and O are equal at an 

 incidence of 26° or 116°. The inner secondary ring com- 

 mences or disappears at an incidence of 57°. These results 

 prove that, the fibres or tubes, as I have shown them to be, 

 are parallel to one of the edges of the primitive rhomb. 



The two secondary rings are produced by the duplication 

 of the ordinary and extraordinary pencils when they are re- 

 flected from an interrupting line or stratum, in the manner 

 which I have described in the Philosophical Transactions for 

 1815*. 



The light of the two primary rings is polarized like that of 

 the two pencils from which it proceeds ; the light of the com- 

 plete secondary ring is polarized like that of the ring which 

 incloses it ; and the light of the secondary segment is polarized 

 like that of the adjacent primary ring, in conformity with the 

 results given in the paper now referred to. 



If we incline the rhomb in a plane perpendicular to that of 

 the principal section, commencing at the incidence where the 

 two primary rings are equal, both of them expand equally, 

 the rings intersecting each other, and the point of intersection 

 keeping in the plane of inclination. The remote portions of 

 both rings soon disappear, and the visible portions, with the 



* On the Multiplication of Images, and the Colours which accompany 

 them in some specimens of Iceland Spar, p. 270. See also Malus, Theorie 

 de la Double Refraction, p. 194. 



