PALACHE. 



CRYSTALLOGRAPHY OF LEADHILLITE. 



447 



The calculation of the elements proceeded according to the method 

 of Goldschmidt 4 as follows. For each of the best faces measured the 

 two quantities, 



x' — sin </> tan p 



y' = cos </> tan p 



were calculated, <£ and p being the measured angles for each face and 

 x' and y' the rectangular coordinates of the projection point of the 

 face in gnomonic projection. 



Now in the monoclinic system the following relations hold: 



i'= p p + e ) j 

 E ' = -P Po + e ) 



x' = 

 -x' 



r ' - q q 



where p and q are rational multiples of the elements p and q (coor- 

 dinates of the unit form) and e = cot p.. 



Since p. could not be measured directly on our crystals, it was neces- 

 sary to calculate both e and p n in equations I and q in equation II, 

 these three quantities being the elements of the mineral which it was 

 desired to determine. 



4 Ueber Lorandit von Allchar in Macedonien, Zeit. fiir. Kryst., 30, 281 

 (1898). 



