AbstractIn a previous article [12] in thisReviewthe writer explained the technique of solving this problem byCondition Equations. Since the method ofObservation Equationsby Variation of Coordinates is now in much more general use, the present article sets out the general principles involved. It has been pointed out that there was an error in the adjustments given in the first article. The opportunity is now taken to explain how this occurred and to give revised results. Some interesting new techniques have also been produced as a result of the present investigation.