"Before the Back-Quadrants were invented when the Forestaff was most in use there was not one Old Master of a ship amongst twenty but what was blind in one Eye by daily staring in the Sun to find his Way."  The cross staff required that one look directly into the sun to take measurements to determine its elevation above the horizon. With the back-quadrant or back-staff one stood with his back to the sun and determined the elevation by a reflection in a mirror.