4.') 4 Goldschmidt and Nicol — New Forms of Sperrylite. 



necessary in order to be able to separate here the certain from 

 the uncertain. One must also, by measuring numerous crystals, 

 accustom one's self to the peculiarities of the kind of crystal at 

 hand. On the one hand, there is the temptation to symbolize 

 the positions of the diffused signal reflections. This may lead 

 to great confusion. On the other hand, there is the danger of 

 demanding too great precision, and therefore of finding here 

 only the principal forms c, p, e. It was attempted by careful 

 discussion of the measurements and their picture in the gnomo- 

 nic projection, to obtain as many certain and typical forms as 

 possible. The forms here given as certain may be accepted. 

 Those indicated by (?) viz. z and D, possess a high degree of 

 probability. Under these circumstances a greater variation than 

 usual must be allowed between the positions of the faces as 

 determined by measurement and by calculation. 

 The new forms are characterized as follows : 



a = + $ (103) observed on crystal No. 8 with one face, 



measured: <£p = 269°14; 71° 35 

 calculated : <f> p — 270° 00 ; 71° 34 



The form is a faint or weak one, but certain. 



g # = — f (205) observed on crystal No. 1 with one small face. 

 On account of the minute size of the face the signal reflec- 

 tion was somewhat extended in the zone c b e ; however, 

 it w T as possible to fix the position. This gave : 



Distance from pole, p in zone c b e = 21° 36 

 calculated, =21° 48 



t>' = — f (203) observed on crystal No. 1 in the same zonal 

 part as g\ The signal reflection of the exceedingly small 

 face gave in the zone c b e the distance from pole 35° 22 

 (calculated = 33° 41). This would not suffice to establish 

 the form. However the form occurs with a better face on 

 the same crystal. This gives a good signal reflection show- 

 ing the position : 



measured : <\> P — 89° 27 ; 56° 08 

 calculated: cf>p=90° 00 ; 56° 18 



b" occurs also on crystal No. 4, with one face having the posi- 

 tion : 



measured : c£ P = 1° 43 ; 34° 31 

 calculated : ^ = 0°00; 33° 41 



The form may be considered as established. The simultane- 

 ous occurrence of g* and b' in the same zonal part corroborates 

 the symbol from the fact that the law of the number series is 



