The ability of a lens system to show fine details of the object being observed. It is used to make actual measurements of the size of objects seen through the microscope. Learn more here . An independent light that usually connects to the microscope body and gives off a ring of light. See some here.
Lenses are never perfect.
If you were looking at something perfectly flat, you might find that much of the center part of your field of view is in focus but out on the edges it is fuzzy and a bit out of focus. Semi-plan lenses improve this deficiency by showing sharper images and less aberrations in the perimeter of the field of view. They are better than standard achromatic lenses but cost quite a bit more. A flat glass or plastic rectangular plate that the specimen is placed on. It may have a depression or well to hold a few drops of liquid.