PQ 



4:1° 



PR 



46° 



QR 



36° 



70 C. S. Hastings — Double Refraction in Iceland Spar. 



Table III gives the double angles of deviation as measured 

 for the extraordinary ray for each refracting angle. As has 

 already been stated, the deviation is that belonging to D 2 , ex- 

 cept in the case of the edge QR, where, on account of the 

 small dispersion, the sodium line was set upon as a single line. 



As before, the observations enclosed in brackets are rejected. 

 These were reduced in quite the same way as were the devia- 

 tions for the ordinary ray, with the following resulting values 

 for A £ : 



3' 58"-23±0 // -39+3' / -60(£-20°) 



58' 45"-69±0 // -24-i-3 // -60(i5-20 o ) 



1' 39 // -2]±0"-21-l"-58(«~20°) 



In order to reduce the angle of deviation for QR to what it 



should be for D 2 , the angular distance between D x and D 2 for 



dA 

 the ordinary ray was determined, and half its product by 



for this region of fhe spectrum, was taken as an additive cor- 

 rection. The value of the correction was found to be 3 //# 85, 

 whence the deviation for the extraordinary ray D 2 for QR be- 

 comes 



36° 1' 43"-06±0"-21 



(7) Principal indices of refraction. 



The crystalline axis has been found to make an angle of less 

 than V with the plane bisecting the refracting angle QR ; 

 hence we may apply the ordinary formula connecting the index 

 of refraction with the angles of minimum deviation and refrac- 

 tion, namely, 



. A+a 

 sin , — \ — 

 2 



sin - — 



2 



The resulting indices will be the principal indices for calcite at 

 a temperature of 20° C. 



forPQ l-658393±2 



PR 1*658387±2 



QR 1"658387±2 



l-658389±l-2 

 The single value of ju e is 



l-486450±l-4 



(8) Test of JTuyg hens' s Law. 



We are now in a position to test the law of extraordinary 

 refraction from the principal indices of refraction and the 



