The object lens, focusing lens, diaphragm and eye piece are all mounted on an optical axis
called the line of collimation or the line of sight.
This is an imaginary line which joins the optical centre of the object lens to the centre of
the cross hairs.
When looking through the eye piece of the surveying telescope, a set of lines called the
cross hairs can be seen. These are used for taking measurements from the staff. These
cross hairs are etched on a small sheet of glass known as the diaphragm.