in Calcareous Spar and Beryl. 491 



luminous body at their intersection, become rectilineal. In 

 this case the secondary rings are not visible. 



The phaenomena of colour accompanying the rings are very 

 interesting. In the plane of the principal section, when the 

 two rings are on different sides of the luminous body, the inside 

 of each ring is red and the outside violet. When one of the 

 rings passes to the other side of the luminous body, its inside 

 becomes violet and its outside red. The degree of dispersion 

 at any point of the rings increases with its distance from the 

 luminous body. The colour of the secondary rings diminishes 

 as they approach to the primary rings. When the rings inter- 

 sect one another, by inclining the rhomb in the plane of the 

 principal section they are perfectly white at their intersection, 

 and the colour gradually increases towards the middle part of 

 their different segments. 



These various phsenomena depend on the law of dispersion 

 in Iceland spar. I have long ago shown that the dispersive 

 power accompanying the extraordinary refraction is much 

 greater than the ordinary dispersive power of the mirror, a 

 result subsequently confirmed by Rudberg; and Malus* and 

 others have described the general phaenomena of dispersion 

 when the ray is incident upon the natural face of the rhomb. 

 At a perpendicular incidence the ordinarily refracted ray is 

 neither refracted nor dispersed, while the extraordinary one 

 is both refracted and dispersed, the violet rays being those 

 which are least refracted. At a few degrees of incidence in 

 the principal section both rays are refracted ; but in the ordi- 

 nary ray the red rays are least refracted, while in the extraor- 

 dinary ray they are most refracted. At an incidence of 16° 

 45', according to Malus, the extraordinary rays cease to be 

 refracted, but continue to be dispersed, the violet rays being 

 always the least refracted. At an incidence of about 40° both 

 rays are refracted, but the extraordinary one is not perceptibly 

 dispersed. At incidences above 40° both rays are refracted 

 and dispersed in the same manner, the violet rays being most 

 refracted. When these rings are produced by a well-polished 

 specimen of Iceland spar, and in a dark room with the sun as 

 the luminous body, they form one of the most beautiful opti- 

 cal phaenomena which can be seen. If the luminous body has 

 some considerable breadth, and is crossed with dark or differ- 

 ently-shaded or coloured spaces, the phsenomenon becomes 

 still more beautiful. 



The phaenomenon of luminous rings has been very recently 

 observed in a specimen of Beryl, brought from India by the 

 Marchioness of Tweeddale, who was so kind as to present it 

 * Malus, ut supra, p. 201. 

 2 K 2 



