132 



J. 11. Pratt — Northupite, Pirssomte^ 



axial angle 2 E, and the character of the double refraction and 

 dispersion. The exceptional purity of the material at hand 

 rendered it possible to make more complete determinations. 

 From a plate cut parallel to 010 the extinction c /n c or Bx^ 

 was measured in yellow light and found to be about 14° 52^ 

 from the vertical axis, in the acute angle (3. For red the 

 angle was about one degree greater. 



These values cannot be regarded as very exact, as the speci- 

 mens, which were best adapted for optical work, did not show 

 very perfect crystal outline. They dilffer, however, only 

 slightly from the values given bv Des Cloizeaux,* c /\ c or Bx^ 

 for red=14°48^ and for blue =13° 8^ 



The acute bisectrix is normal to 010 and the following 

 values of the apparent axial angle were obtained : 



Red, Li. 

 2E at 26°C = 51°26' 



Yellow, Na. 

 52°7f' 



Green, TI. 

 52°47f' 



These values agree closely with those obtained by Arzrunijf 

 which are as follows : 



Red, Li. 

 2E = 51°25' 



Yellow, Na. 

 52°19' 



Green, Tl. 



52°58' 



Des Cloizeaux gives : 2E = 51° 38' for red and 52° 53' for blue. 



Crossed dispersion could be distinctly observed and the char- 

 acter was /o<'y. The double refraction is negative and strong. 

 By means of total reflection the following values of the indices 

 of refraction, for yellow light, ]^a, were obtained. 



a=l-4435 Na. ^=1-5156 Na. y==l-5233Na. 



From the foregoing values Yy was calculated and found to 

 be 17° 32', while 16° 53' was obtained from the values of 2Ey 

 and yS. 



Des Cloizeaux has shown that the axial angle varies consider- 

 ably with the temperature, and a series of experiments has 

 been made to illustrate this, the results of which are shown in 

 the followino: table : 



Temperature. 



Apparent Angle. 

 2Ey. Na. 



Temperature. 



Apparent Angle. 

 2 Ey . Na. 



22°C 



52° 6' 



60°C 



54° 7' 



30° 



52°32' 



70° 



54°43' 



40° 



52°54' 



80° 



55°13' 



50° 



53°26' 



90° 



55°48' 



The maximum variation from 22° to 90° C. is therefore 

 3° 42'. 



Ann. d. Mines, V, xiv, p. 400, 1858. 



f Zs. Kr., vi, p. 24, 1882. 



