Cover slip
Encyclopedia
A cover slip 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 culture
s 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.
, such as silicate or borosilicate glass
, but specialty plastic
s are also used. Fused quartz
cover slips may be used where ultraviolet
transparency is required, e.g., for fluorescence microscopy. Some cover slips may have reticles etched on them.
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 coverslips (0.17 mm thickness). Use of coverslips that deviate from this intended thickness will result in spherical aberration
and a reduction in resolution and image intensity. Specialty objectives may be intended for use without coverslips, or may have correction collars that permit a user to accommodate for alternative coverslip thickness.
, depending on the sample. A solvent-free sealant that can be used for live cell samples is "valap", a mixture of vaseline
, lanolin
and paraffin
in equal parts.
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
. The object is usually held between the cover slip and a somewhat thicker microscope slide
Microscope slide
A microscope slide is a thin flat piece of glass, typically 75 by 25 mm and about 1 mm thick, used to hold objects for examination under a microscope. Typically the object is placed or secured on the slide, and then both are inserted together in the microscope for viewing...
, 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
Image resolution
Image resolution is an umbrella term that describes the detail an image holds. The term applies to raster digital images, film images, and other types of images. Higher resolution means more image detail....
microscopes have a very narrow region
Depth of field
In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...
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
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
) 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
Oil immersion
In light microscopy, oil immersion is a technique used to increase the resolution of a microscope. This is achieved by immersing both the objective lens and the specimen in a transparent oil of high refractive index, thereby increasing the numerical aperture of the objective lens.Immersion oils are...
or water immersion microscopy
Water immersion objective
In light microscopy, a water immersion objective is a specially designed objective lens used to increase the resolution of the microscope. This is achieved by immersing both the lens and the specimen in water which has a higher refractive index than air, thereby increasing the numerical aperture of...
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
Dehydration
In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...
and oxidation of the specimen. Microbial and cell culture
Cell culture
Cell culture is the complex process by which cells are grown under controlled conditions. In practice, the term "cell culture" has come to refer to the culturing of cells derived from singlecellular eukaryotes, especially animal cells. However, there are also cultures of plants, fungi and microbes,...
s 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.
Materials
Cover slips are usually made of glassGlass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
, such as silicate or borosilicate glass
Borosilicate glass
Borosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion , making them resistant to thermal shock, more so than any other common glass...
, but specialty plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
s are also used. Fused quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
cover slips may be used where ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
transparency is required, e.g., for fluorescence microscopy. Some cover slips may have reticles etched on them.
Dimensions
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:
- No. 0 – 0.085 to 0.13 mm thick
- No. 1 – 0.13 to 0.16 mm thick
- No. 1.5 – 0.16 to 0.19 mm thick
- No. 1.5H – 0.17 to 0.18 mm thick**
- No. 2 – 0.19 to 0.23 mm thick
- No. 3 – 0.25 to 0.35 mm thick
- No. 4 – 0.43 to 0.64 mm thick
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 coverslips (0.17 mm thickness). Use of coverslips that deviate from this intended thickness will result in spherical aberration
Spherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...
and a reduction in resolution and image intensity. Specialty objectives may be intended for use without coverslips, or may have correction collars that permit a user to accommodate for alternative coverslip thickness.
Sealing
The cover slip is often glued to the microscope slide, in order to seal off the specimen from contamination and decay. A number of sealants are in use, including commercial sealants, laboratory preparations, or even regular clear nail polishNail polish
Nail polish, or nail varnish, is a lacquer applied to human fingernails or toenails to decorate and/or protect the nail plate.-History:...
, depending on the sample. A solvent-free sealant that can be used for live cell samples is "valap", a mixture of vaseline
Vaseline
Vaseline is a brand of petroleum jelly based products owned by Anglo-Dutch company Unilever. Products include plain petroleum jelly and a selection of skin creams, soaps, lotions, cleansers, deodorants and personal lubricants....
, lanolin
Lanolin
Lanolin , also called Adeps Lanae, wool wax or wool grease, is a yellow waxy substance secreted by the sebaceous glands of wool-bearing animals. Most lanolin used by humans comes from domestic sheep...
and paraffin
Paraffin
In chemistry, paraffin is a term that can be used synonymously with "alkane", indicating hydrocarbons with the general formula CnH2n+2. Paraffin wax refers to a mixture of alkanes that falls within the 20 ≤ n ≤ 40 range; they are found in the solid state at room temperature and begin to enter the...
in equal parts.