Cover slip

A cover slip, coverslip or cover glass is a thin flat piece of transparent material, usually square or rectangular, about 20 mm (4/5 in) wide and a fraction of a millimetre thick, that is placed over objects for viewing with a microscope. The object is usually held between the cover slip and a somewhat thicker microscope slide, which rests on the microscope's stage or slide holder and provides the physical support for the object and slip.

The main function of the cover slip is to keep solid specimens pressed flat, and liquid samples shaped into a flat layer of even thickness. This is necessary because high-resolution microscopes have a very narrow region within which they focus.

The cover glass often has several other functions. It holds the specimen in place (either by the weight of the cover slip or, in the case of a wet mount, by surface tension) and protects the specimen from dust and accidental contact. It protects the microscope's objective lens from contacting the specimen and vice-versa; in oil immersion microscopy or water immersion microscopy the cover slip prevents contact between the immersion liquid and the specimen. The cover slip can be glued to the slide so as to seal off the specimen, retarding dehydration and oxidation of the specimen. Microbial and cell cultures can be grown directly on the cover slip before it is placed on the slide, and specimens may be permanently mounted on the slip instead of on the slide.