378 



MR SWAN ON ICELAND SPAR. 



In No. 6, one of the faces is a cleavage plane, and the principal section of the 

 prism is in the same plane with the axis. Therefore, since the cleavage plane is 

 inclined 45° 23' 25" to the axis, it follows that the inclination of the transmitted 

 rays in the position of minimum deviation is nearly 66° 51'. The angle of this 

 prism was found to be 44° 28' 29", the deviation of the refracted rays 33° 16' 22", 

 and /.( = 1-658389. 



These results are combined in the following Table : — 



From this summary it will be seen, that the greatest difference between the 

 observed index of refraction of any prism and the mean of the whole results is 

 only -000014 ; while the diiference of the greatest and least results is less than 

 •00003. So close an agreement in six essentially different cases, seems to render 

 it very probable that the index of refraction is really constant ; and the result of 

 the investigation thus confirms the accuracy of the Huygenian law. 



* As the term, principal section, is employed in more than one sense, it may be proper to obsei-ve, 

 in order to avoid ambiguity, that I use it to denote a plane perpendicular to both faces of the prism. — 

 See Sir John Herschel's Treatise on Light, in the Entyyckypcedia Metropolitana, p. 370, art. 197. 



