1907 Ross manual: The front end of the rear sight base is threaded to engage a ring for fine adjustment. Both its front and rear ends project over the barrel in such a manner that they secure the handguards. Fine adjustment is obtained by the ring already mentioned which is termed the "micrometer thimble." It engages the front ends of the index slides and when revolved gives them a forward or backward movement carrying with them the sight slide and raising or lowering the rear sight leaf. It is graduated in tens from 0 to 100, the even numbers being marked in plain figures and the odd numbers being indieated by a line. These graduations represent a horizontal increase or decrease of ten yards at any range, the curve of the sight leaf giving the extra height of backsight required.* The micrometer is at zero when the 0 rests on the cross line cut on the rear sight base from which position an increase or decrease of 50 yards range can be given. The value of each graduation on a vertical target is roughly 1 inch per 100 yards.