In field artillery, the accuracy of indirect fire depends on the use of aiming points.
refers to holding the intersection of the cross hairs on a bombsight when fixed at a specific target.
An aiming point would be a sharply defined and easily distinguished feature, such the edge of an obvious building.
However, this presents problems in featureless areas, in bad visibility or at night and putting lights on distant aiming points is seldom practical.
During that war the British introduced their first paralloscope, which was a horizontal mirror placed a few feet from the gun; the layer aimed his sight at its reflection.