Napoleon, on a river bank, wanted to know the width of the stream. A young soldier faced directly across to stream and adjusted the visor of his cap until the tip of the visor was in line with his eye and the opposite bank. Next he did an about-face and noted the spot on the ground now in line with his eve and visor-tip. He paced off the distance to this method made his report, and earned a promotion. What postulate is this method based on? Draw a diagram to help you explain.