n = nSlices;
  if (nImages!=2 || nSlices<2)
     exit("Two stacks expected");
  means = newArray(n);
  for (i=0; i<n; i++) {
     setSlice(i+1);
     getStatistics(area, mean);
     means[i] = mean;
  }
  run("Put Behind [tab]");
  if (n!=nSlices)
     exit("Stacks must be the same size");
  for (i=0; i<n; i++) {
     setSlice(i+1);
     mean = means[i];
     run("Divide...", "value=&mean slice");
  }