A major problem in using the Cross-Staff was in having to look at the sun. This led to blindness or at least damaged eyesight for navigators. To solve the problem, John Davis (sometimes spelled "Davies") invented an instrument in 1594 (published description--1595) that used the shadow of the sun instead of the direct view of the sun to obtain the altitude. It also eliminated the need to look in two directions at the same time. Now a navigator could look at the horizon and line up the shadow of the sun with the horizon at the same point on the instrument.
The observer adjusted the shadow vane, the upper, left scale in the diagram, so that the sun would cast its shadow on the horizon vane, the lower, left-hand object. Sighting from the back (right-hand side in the diagram), the observer would adjust the eyepiece at the back of the Back-Staff so that the horizon and sun's shadow were aligned. He would then read the scale of the shadow vane, add it to the scale of the eyepiece and thereby obtain the altitude of the sun. As with other height-measuring methods, this number was used to obtain the latitude of the observer.
Davis invented the instrument in 1594, but both he and others made a number of modifications of his original instrument. Of course, it could not be used to measure the altitude of Polaris. Why? No shadow. The Back-Staff was very popular and soon most mariners used it in place of the astrolabe or Cross-Staff. While it was more accurate than either of those instruments, it would be replaced in the mid-1700s by the Octant or Sextant, which would be even easier to use and more accurate.