Cover Glass

COVER GLASS, a thin glass plate used for covering microscopic specimens. A very important requirement for cover glass is a completely flat and even surface and uniform thickness throughout its entire extent. The thickness of cover glass ranges from 0.10 to 0.25-0.30 mm; on average it is usually 0.15-0.18 mm; microscope objectives are usually corrected for this thickness. With a different thickness of cover glass, it is necessary to use, especially at high magnifications with dry or water-immersion objectives, a correction mount by setting the pointer to the corresponding mark. The thickness of cover glass is measured directly with a thickness gauge (see Microscope). When using homogeneous (oil) immersions, the thickness of cover glass is not of essential importance. However, with a thick cover glass, the free working distance of the objective may be less than the given thickness of the glass, and in this case it will be impossible to prepare the specimen. In some cases (e.g., in counting chambers) polished mirror cover glasses are used, which are significantly thicker than indicated above (up to 0.5 mm); this limits the choice of objective, whose free working distance must be greater than the specified value. The standard size of cover glass is 18×18 mm, but glasses of smaller sizes are also used, as well as significantly larger sizes (up to 30×40) for covering serial sections. There are also round cover glasses, however they have no particular advantages over square ones. When it is difficult to obtain cover glass, they can be replaced with transparent plates of mica, cellophane, and celluloid; in particular, one can use movie films from which the light-sensitive layer has been removed. (See also Microscope and Microscopic Technique.)