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.

Cover slips are available in a variety of widths, lengths, and thicknesses. They are usually sized so as to fit well inside the boundaries of the microscope slide, which typically measures 25 by 75 mm. Square and round slips are usually 20 mm wide or smaller. Rectangular slips measuring up to 24 by 60 mm are commercially available.

Cover slips are widely available in several standard thicknesses, identified by numbers:

The thickness of the cover slip is crucially important for high-resolution microscopy. Typical biological microscope objectives are designed for use with No. 1.5 cover slips (0.17 mm thickness). Mount is used to attach the cover glass to the slide. The use of cover slips that deviate from this intended thickness will result in spherical aberration and a reduction in resolution and image intensity. Specialty objectives may used to image specimens without coverslips, or may have correction collars that permit a user to accommodate for alternative coverslip thickness.

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