284 Mr. L. Fletcher's Crystallographie Notes. 



edges AC C A and passes through the centre of the crystal, 

 the rhombs TBTB TBiTBj of figs. 2 and 3 will be per- 

 pendicular to the twin-plane, and will each represent the com- 

 position-plane of Haidinger. If, jiow, by simple translation 

 without rotation the half TBTB AC above the rhomb of 

 fig. 2 be associated with the half TBiTBi A A below the 

 rhomb of fig. 4 in such a way that the two rhombs coincide, 

 we get the composition represented in fig. 5. This figure 

 will therefore be that of a growth according to the law of 

 Haidinger. For convenience of direct comparison with the 

 results following from the law as given by Naumann and 

 Sadebeck, the same growth as fig. 5 is shown in fig. 6 as it 

 would be seen either after a rotation of the whole figure 

 through two right angles about the vertical axis, or by an e}*e 

 placed at the back of the paper. 



Again, let the rhombs MBMB M B^ B x of figs. 2 and 3 

 be the traces on the octahedron-faces of a plane parallel to the 

 twin-plane, and therefore the composition-plane of Naumann. 

 If now, just as before, the half M B M B C A above the rhomb 

 of fig. 2 be associated with the half M B x M B t A& below the 

 rhomb of fig. 4 in such a way that the rhombs coincide, we 

 get the composition represented in fig. 7. This figure will 

 therefore be that of a growth according to the law of Naumann. 



With the help of the stereographic projection of fig. 1 we 

 can now investigate the differences of the* growths represented 

 respectively in figs. 6 and 7. 



Fig. 6. 

 Haidinger's law. 



* 1 = O> 1= :0.j- 



Fig. 7. 

 Naumann' s laic. 

 f a «, =/3 &!= salient 1° 2SV, 

 \ b &= a a x = reentrant 1° 23£'. 



Angle C M . M 0, = salient 1° 42', 

 „ A M . M A x = reentrant 1° 4S 



Angle between the trunca- 

 ting planes of C and G x 

 = 90° 51'. 

 The faces dycxB x are 

 in a zone. 



_ We have seen that in the projection the plane of composi- 

 tion is a plane of symmetry to the poles of the two individuals; 

 and we further perceive that in each case the plane of compo- 

 sition is a plane of symmetry to the faces actually shown by 



Edges CT TCi coincident, 

 » AT TA X „ 



Angle between the trunca- 

 ting planes of C and C x 

 = 89° 9'. 



The faces a /3 a x /?i are not 

 in a zone. 



