a cover glass is a piece of very thin glass or plastic used to cover material on a microscope slide. It protects the microscope and prevents the slide from drying out when it's being examined.
A microscope slide is a thin flat piece of glass, typically 75 by 26 mm and about 1 mm thick, used to hold objects for examination under a microscope. Typically the object is mounted on the slide, and then both are inserted together in the microscope for viewing.
