920 



over ail angle of 45° or interchange the x, y and z axis. The series 

 always show strong convergence; for the following numbers, the 

 calculation of 8 terms was only necessary in one case. 



The table given below gives the values calculated for He in this 

 way. F'or the cases in which the lattice is unstable, the permeability 

 is indicated in the unstable diiection. In the third to the fifth column 

 the values of a b and c used in the calculation are mentioned. 



We must remark, that for the three cases always occurring with 

 equal values of a, ft and c only one calculation was necessary. For we 

 can interpret them as belonging to one and the same division in 

 two groups, but with the exterior field successively in the three 

 directions of the axes. According to (5) the values of H, belonging 



to it will taken together be equal to ~:=M. Moreover on account 



of symmetry only two are always equal to each other, so that only 

 one must be calculated. The smallest value of H,; for each lattice is 

 printed in bold type, the others have importance mainly for the 



