The eyepiece of a compound ocular microscope makes use of two or more convex lenses. These lenses are larger in the middle so that they bend light and focus it on a central point, according to Microbus.
The most basic form of optical microscope consists of a single magnifying lens and does not make use of an eyepiece. More powerful compound microscopes use a series of lenses and employ an eyepiece that makes use of a convex lens to provide greater levels of magnification. Compound microscopes require the lens in the eyepiece in order to properly focus the light from the objective lens.