380 



showing that, in the case supposed, the ratio of tan 2/3 to 

 cos 20 is independent of the angle of incidence. In the ex- 

 periments which I made with Mr. Grubb this azimuth was 

 always 45° ; and the following Table contains the results 

 of observation compared with those obtained by calculation 

 from formulae (f). The experiments were made upon a 

 small disk of speculum metal ; and in the calculations I have 

 taken m = 2.94, x = 64° 25'. 



The light used in these observations was that of a candle 

 placed at a short distance, and was admitted through small 

 apertures at the ends of the tubes. (See the description of 

 the instrument in the Proceedings, vol. i. p. 159). The Nicol's 

 prism in the first tube having been secured in a position in which 

 its principal plane was inclined 45° to the plane of incidence, 

 and the two arms having been set at the proper angle with 

 the surface of the metal, the Fresnel's rhomb and the Nicol's 

 prism in the second lube were moved simultaneously, until 

 the image of the candle became as faint as possible. Had 

 light perfectly homogeneous been employed, the image could 

 have been made to vanish altogether ; but instead of vanish- 

 ing, it became highly coloured, and our rule in observing was 

 to make the blue at one side of it, and the red at the other, 

 equally vivid, so as to get results which should belong, as 

 nearly as possible, to the mean ray of the spectrum. When 

 this was done, the angles and /3 (subject however to certain 

 corrections which will be hereafter explained) were respec- 

 tively read off from the divided circles belonging to the 

 rhomb and the prism. The observations were made at large 



