The plate P carries a mirror M. The mirror mount has two points resting in a fixed groove, F, and at the other end has a single point resting on the object whose displacement one is measuring. Raising the object through a distance will tilt the mirror through an angle or radians (approximately) but will turn the light beam through an angle .
Hence
Note that with the telescope nearly perpendicular to the scale at the
beginning then is close to the telescope, and the difference between
two elongations (
) is very accurately given by
where is the scale reading. This relation holds so long as is small enough that .