In the book 'Emergency Navigation' by David Burch ( http://www.amazon.com/Emergency-Navigation-Improvised-No-Ins... ), the method is explained more clearly. With a clear Iceland spar crystal 1 to 2 inches on a side, the direction of the sun can be pinpointed to within a few degrees. You can also get the same effect using a piece of cellophane and the lens of a pair op polarized sunglasses. You do need a patch of clear sky at a 90 degrees angle from the sun direction to observe the change in intensity.