(4) First divide the total three-dimensional region V into k bins. When there are N total electrons detected, there will be N k electrons in the kth bin. If we add the number of electrons in all the bins up to l(∆V ), there will be l(∆V ) k=1 N k in these bins, and the three-dimensional volume occupied by these bins will be ∆V . If there are m electrons generating the observed pattern, there will be 3m coordinates for the electrons, three spatial coordinates for each of the electrons. The vector x thus denotes x = (x 1 , x 2 , . . . , x 3m ). Thus in the last line of (4), the index i labels the electrons, and j labels the spatial position in rectangular coordinates x, y, z of the electrons, The mean µ ij of the density ψ( x) is given by