This is found immediately below the ocular. It may be incorporated or removed at will. It can only be used when convergent light is used to observe the property called the Interference figure (3).

The Bertrand lens magnifies and focusses interference figures but the Bertrand lens does not produce the interference figure but modifies the focal plane of the image formed by the objective to allow it to be focussed and amplified by the ocular.

An alternative means of viewing interference figures is to remove the eyepiece and look down the microscope tube at the highe-power objetive lens, preferably wiyh the aid of a pin-hole stop inserted at the top of the tube.