This is a system of lenses fitted to the top of the microscope and whose function is to form a virtual and amplified image from the real image created by the objective.
The eyepiece assembly contains two cross-hairs, and is slotted into the microscope tube so that the cross-hairs are orientated E-W and N-S, i.e. parallel to the vibration directions of the polariser and analyser.
Most eyepieces have a magnification of x8 or x10.
The focal plane of the new image is about 25cm from the upper lens, the normal distance of vision of the human eye.