Auxilliiary lenses are used to increase or decrease the default magnification range of stereoscopes.
An aux lens with magnification blow 1x increases the field of view, extends teh working distance while reduces the total magniofication.For instnace a 0.5x aux lens will increase the field of view two times in X direction (horiuzontal) and two times in Y direction (vertical). Working distance is incraesed almost double. These lenses are nromally 0.75x, 0.5x, 0.33x or 0.25x depending on the microscope model.
An aux lens with higher magnification than 1x will uincrease the total magnification. These elnses are normally 1.5x and 2x.