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
.