Testing the Optical Square:
Observe the ranging rod at A with optical square stationed at some intermediate point C and aligned along line AB (Figure(a). Mark a point as perpendicular offset at C. Further, observe the ranging rod at B, although remaining instrument stationed at C and aligned along AB, and mark the offset point b. If the optical square is accurately adjusted then it points a and b will be coinciding. If this is not then, the instrument requires adjustment. The distance ab is bisected at D. Then CD will be true offset. Remain the ranging rod at D. And rotate mirror M2, till image of A and B, i.e. point a or b coincides along with D as depicted in Figure (a). A process is repeated till both the observations, through optical square, provide a single offset position D within place of a and b.
Figure(a) : Testing the Optical Square