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 :— 
Inclination of the 
plane of incidence, 
or of the principal Inclination of the Index of refraction Difference of each 





Pai aden oe en | ne Se 
0 the optical axis 
of the crystal.* 
No. 1 0° 0° 1:658367 — 0:000008 
2 90° 90° 1:658366 — 0:000009 
3 0° 90° 1658384 +0:'000009 
4: 45° 90° 1658361 — 0-000014 
5 0° 45° 1658385 + 0:000010 
6 0° 66° 51’ 1:658389 + 0:000014 
Mean 1.658375 0-000011 

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 difference 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 observe, 
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 Encyclopedia Metropolitana, p. 370, art. 197. 
