A darkening of the corners of an image relative to its center (see example below) is likely a result of vignetting, which is an expected effect with many lenses. There are different types of vignetting, including optical, natural, and mechanical vignetting. For a good summary of vignetting go to http://www.vanwalree.com/optics/vignetting.html#optical.
One way to reduce or eliminate this effect is to buy a better lens, such as a C- or CS-mount lens. You can get these from companies such as Pentax, Fujinon, Computar and Edmund Optics. Another option is to buy a larger format lens. For example, if you are using a PL-B952, which uses a 1/3" CCD, a lens built for a 2/3" CCD might reduce this effect. However, be aware that focal lengths can differ significntly between different lens formats.
The most common causes of vignetting in microphotography and macrophotography:
Optical axis of camera and microscope not in line
Camera lens limits the light path
No optical adjustment between the microscope and the camera
An unsuitable projection lens is already integrated in the microscope’s photo tube or C-mount connection
The optical system is off-centre