Vol. 5] Louderback.—Benitoite. 343 



(O==40°18'; likewise a prism face at each (<^+n60)° with p 

 = 90°. To illustrate the nature of the measurements obtained, 

 results are here given for two crystals. In general it has been 

 found that all of the individual faces of a form are not meas- 

 urable on the same crystal. 



Crystal No. 3 



Measured. Reflection. Calculated. 



<t> P 



0° 00' 40° 18' 



60° 00' 40° 18' 



120° 00' 40° 18' 



240° 00' 40° 18' 



300° 00' 40° 18' 



0° 00' 90° 00' 



240° 00' 90° 00' 





4> 



P 





P 1 



0° 00' 



40° 18' 



excel. 



7T- 



• 59° 59' 



40° 14' 



poor 



p3 



119° 56' 



40° 18' 



good 



wi 



broken 







pa 



240° 00' 



40° 18' 



excel. 



ir<< 



299° 54' 



40° 18* 



fair 



mi 



0° 00' 



90° 00' 



good 





240° 00' 



89° 58' 



fair 



Crystal No. 1 



Measured. Reflection. 









P 









pi 



0° 00' 



40° 19' 





excel. 





7T- 



60° 05' 



40° 19' 





poor 





P 3 



120° 04' 



40° 22' 





fair 





IT* 



180° or 



40° 22' 





good 





V s 



239° 58' 



40° 21' 





good 





ir'< 



300° or 



40° 21' 





fair 





/111 



0° 00' 



90° 00' 





fair 





M" 



60° 04' 



90° 00' 





fair 







120° 05' 



90° 00' 





fair 





m s 



240° 01' 



90° 00' 





fair 









Measured. 





Calculated. 











P 



4> P 





r (10T2) 



0° 02' 



23° 



00' 



0° 00' 22° 58: 





a (1121) 



89° 59' 



30° 



03: 



90° 00' 30° 00' 







90° 00' 



30° 04' 







d (2241) 



71° 07' 



30° 



03' 



71° 12' 30° 00' 







71° 12' 



30° 



00' 





For a and d the two sets of readings are averages from two 

 crystals. 



For x the average on one crystal was </>=55° 22' p=41° 48'; 

 on another <£=54° 49' p=41° 39'. The calculated angles for 

 — iy ]() are <£=55° 07' P =41° 48'. 



