324 R. W. G. WycUff— Crystal Structures of 



tained on a piece of 50 X 65 cm. drawing paper. These 

 tables, then, hold for a sphere of projection of 5 cm. 

 radins. They are calculated as follows (fig". 4) : 



CO = distance from crystal to plate (3, 4, or 5 cm.). 

 C'A = distance from any spot to central spot on photo- 

 graphic plate. 



0=y 2 tan -1 pYy = angle of the reflection. 



BC = distance on projection corresponding to spot dis- 

 tant x from the central spot. 

 BC = 5cot#. 



The manner of making the projection is the same 

 whether the crystal is set to give a symmetrical or an un- 

 symmetrical photograph. As already stated, an unsym- 

 metrical photograph is more useful in interpreting the 

 structure of crystals because in this case the same kind of 

 a plane occurs at different distances from the center of the 

 photograph and reflects X-rays of different wave lengths. 

 The amount of dissymmetry desired for this purpose 

 is in most cases relatively slight, however, and does 

 not produce enough distortion in the projection to cause 

 trouble in determining the crystallographic indices (fig. 

 7). The accompanying figures illustrate the manner of 

 projection and the way of obtaining the indices of planes. 

 (Figures 6, 7 and 2.) 



If the indices of the reflecting planes are to be obtained 

 from the coordinates of their projected positions, it is 

 necessary that one of the axes be approximately normal 

 to the plane of the projection. For cubic, tetragonal, and 

 orthorhombic crystals X-rays may be conveniently passed 

 through sections cut normal to any of the three axes. 

 The coordinate systems on the projection in these cases 

 are always rectangular, though the unit lengths along the 

 axes will be different if the system is orthorhombic, or if 

 the rays travel along an a-axis of a tetragonal crystal. 

 The lengths of the axial units in every case can, of 

 course, be obtained in the usual manner from a knowledge 

 of the axial ratio. If the crystal is hexagonal, as it is 

 in the present case, the photograph is best taken with the 

 rays normal to a basal section. The coordinate axes are 

 then of equal length and make an angle of 60° with one 



