330 R. W. G. Wyckoff— Crystal Structures of 



distinguishing between the two possible units of structure. 

 ^ It may sometimes happen that, in order to get reflec- 

 tions from more planes than can be obtained from one 

 crystal section, it is desirable to use sections cut parallel 

 to some more complicated plane of the crystal. For in- 

 stance, in order to get reflections from more planes of cal- 

 cite and rhodochrosite, photographs were taken through 



Fig. 8. — The gnomonic net. If this figure is enlarged so that it is 70 cm. 

 on a side, it will serve as a net for sphere of radius of 5 cm. 



cleavage pieces. The indices of planes reflecting in such 

 a photograph may be found if several of the most import- 

 ant planes are recognized, by getting the indices of the 

 various zone axes ; or quite simply, the gnomonic net of H. 

 Hilton 13 may be employed. The projection has simply to 



13 H. Hilton, Mineralogical Mag. 14, 18, 1904. 



