ORDINARY REFRACTION OF ICELAND SPAR. 377 



the luminous object I observed, virtually at an infinite distance. Having fitted a 

 pair of cross fibres of silk* in the anterior focus of the object-glass of a telescope. 

 I carefully adjusted it to distinct vision on a star, so that, on moving the 

 eye up and down, its image remained fixed on the wh-es. The eye-piece being 

 then cautiously removed, the wires were illuminated by a lamp ; and the beam 

 of rays issuing from the object-glass having been directed upon the prism, the 

 optical axis of the collimator was made parallel to the horizontal limb of the 

 theodolite by means of adjusting screws. A common oil-lamp was used in 

 ascertaining the angles of the prisms ; but when the deviation of the refracted 

 rays was observed, the wires were illuminated with the homogeneous yellow 

 light of a spirit-lamp with a salted wick : and it must be regarded as a re- 

 markable proof of the perfect homogeneity of this light, that the refracted image 

 of a single fibre of silk was always distinctly visible with a good prism. 



I shall now give the results of the examination of Mr Nicol's prisms, to 

 which I shall refer according to the numbers he has attached to them. The an- 

 gle of the prism was generally determined by four, and the deviation of the re- 

 fracted rays by six observations. 



The prism marked No. 1 is cut out of the crystal, so that in the position 

 of minimum deviation the transmitted rays are parallel to the axis ; and Mr Nicol 

 has worked with such accuracy, that the images produced by the ordinary and 

 extraordinary I'ays coincide almost exactly in this position. The angle of this 

 prism was found to be 60° 8' 8", the deviation of the refracted rays 52° 14' 36". 

 and consequently /x = l'658367.t 



The plane of refraction in the prism No. 2 is perpendicular to the axis. Its 

 angle was found to be 44" 29' 20', and the deviation of the refracted rays 

 33° 17' 8". From which /x= 1-658366. 



Two other prisms, No. 3 and No. 4, were also examined, in which the trans- 

 mitted rays are perpendicular to the axis ; but in either case the prism is cut so 

 that the plane of refraction differs from that of No. 2. 



For No. 3, the angle of the prism was found to be 59° 36' 32", the deviation 

 of the refracted rays 51° 25' 25", and M= 1-658384. 



For No. 4, the angle of the prism was found to be 44° 55' 24", the deviation 

 33° 42' 34", and //= 1-658361. 



In No. 5, the transmitted rays are inclined 45° to the axis ; the refracting 

 angle of the prism was found to be 45° 3' 51", the deviation of the refracted rays 

 33° 50' 58", and /i = 1-658385. 



* Silk is not the most suitable material for the purpose, owing to its transparency ; but I could pro- 

 cure no better at the time. 



t I have also examined another prism, No. 1, and have found ^ = 44° 23' 2", 3 = 33° 11' 0". and 

 /i = l-658362. 



