We can derive the value of a cubic crystal from any line, by means of eq. The answer would not however turn out to be constant because of certain systematic errors, but the error, as we have seen, will be less for lines with high . The only problem is to decide what function of should be used to give a straight-line extrapolation. It is tabulated by Lipson and Steeple. A typical example is given in Fig. 5.
Cubic crystals are the easiest to deal with.
For lower symmetries extra problems arise. Complete rules cannot be given here, but basically the principle is to use lines with high h index for a , high k index for b , and high l index for c . Complete details of suggested procedures are given by Lipson and Steeple.