A microscope slide is a thin flat piece of glass, typically 75 by 25 mm (3 by 1 inches) and about 1 mm thick, used to hold objects for examination under a microscope. Typically the object is placed or secured ("mounted") on the slide, and then both are inserted together in the microscope for viewing. This arrangement allows several slide-mounted objects to be quickly inserted and removed from the microscope, labeled, transported, and stored in appropriate slide cases or folders.
Microscope slides are often used together with a cover slip or cover glass, a smaller and thinner sheet of glass that is placed over the specimen. Slides are held in place on the microscope's stage by slide clips, slide clamps or a cross-table which is used to achieve precise, remote movement of the slide upon the microscope's stage (such as in an automated / computer operated system, or where touching the slide with fingers is inappropriate either due to the risk of contamination or lack of precision)
The origin of the concept was pieces of ivory or bone, containing specimens held between disks of transparent mica, that would slide into the gap between the stage and the objective. These "sliders" were popular in Victorian England until the Royal Microscopical Society introduced the standardized glass microscope slide.
Dimensions and types
A standard microscope slide measures about 75 mm by 25 mm (3" by 1") and is about 1 mm thick. A range of other sizes are available for various special purposes, such as 75 x 50 mm and for geological use, 46 x 27 mm for petrographic studies, and 48 x 28 mm for thin sections. Slides are usually made of common glass and their edges are often finely ground or polished.
Microscope slides are usually made of optical quality glass, such as soda lime glass or borosilicate glass, but specialty plastics are also used. Fused quartz slides are often used when ultraviolet transparency is important, e.g. in fluorescence microscopy.
While plain slides are the most common, there are several specialized types. A concavity slide or cavity slide has one or more shallow depressions ("wells"), designed to hold slightly thicker objects, and certain samples such as liquids and tissue cultures. Slides may have rounded corners for increased safety or robustness, or a cut-off corner for use with a slide clamp or cross-table, where the slide is secured by a spring-loaded curved arm contacting one corner, forcing the opposing corner of the slide against a right angled arm which does not move. If this system were used with a slide which did not incorporate these cut-off corners, the corners would chip and the slide could shatter. 
A graticule slide is marked with a grid of lines (for example, a 1 mm grid) that allows the size of objects seen under magnification to be easily estimated and provides reference areas for counting minute objects. Sometimes one square of the grid will itself be subdivided into a finer grid. Slides for specialized applications, such as cell counting, may have various reservoirs, channels and barriers etched or ground on their upper surface. Various permanent markings or masks may be printed, sand-blasted, or deposited on the surface by the manufacturer, usually with inert materials such as PTFE.
Some slides have a frosted or enamel-coated area at one end, for labeling with a pencil or pen. Slides may have special coatings applied by the manufacturer, e.g. for chemical inertness or enhanced cell adhesion. The coating may have a permanent electric charge to hold thin or powdery samples. Common coatings include poly-L-lysine, silanes, epoxy resins, or even gold.
The mounting of specimens on microscope slides is often critical for successful viewing. The problem has been given much attention in the last two centuries and is a well-developed area with many specialized and sometimes quite sophisticated techniques.
In a dry mount, the simplest kind of mounting, the object is merely placed on the slide. A cover slip may be placed on top to protect the specimen and the microscope's objective and to keep the specimen still and pressed flat. This mounting can be successfully used for viewing specimens like pollen, feathers, hairs, etc. It is also used to examine particles caught in transparent membrane filters (e.g., in analysis of airborne dust).
Wet mount or temporary mount
In a wet mount, the specimen is placed in a drop of water or other liquid held between the slide and the cover slip by surface tension. This method is commonly used, for example, to view microscopic organisms that grow in pond water or other liquid media, especially when studying their movement and behavior. Care must be taken to exclude air bubbles that would interfere with the viewing and hamper the organisms' movements. It is also used for rapid investigations where a permanent record is not required, as well as to examine physiological liquids like blood, urine, saliva, semen (in semen analysis), and vaginal discharge (in vaginal wet mount).
Prepared mount or permanent mount
For pathological and biological research, the specimen usually undergoes a complex histological preparation that may involve cutting it into very thin sections with a microtome, fixing it to prevent decay, removing any water contained in it, staining specific parts of it, clearing to render it transparent, and impregnating or infiltrating it with some transparent solid substance. As part of this process the specimen usually ends up firmly attached to the slide.
The mounting medium is the solution in which the specimen is embedded, generally under a cover glass. Simple liquids like water or glycerol can be considered mounting media, though the term generally refers to compounds that harden into a permanent mount. Popular mounting media include Permount, glycerol jelly, and Hoyer's mounting medium. Properties of a good mounting medium include having a refractive index close to that of glass (1.518), non-reactivity with the specimen, stability over time without crystallizing, darkening, or changing refractive index, solubility in the medium the specimen was prepared in (either aqueous or non-polar, such as xylene or toluene), and not causing the specimen stain to fade or leach.
Examples of mounting media
Popularly used in immunofluorescent cytochemistry where the fluorescence cannot be archieved. The temporary storage must be done in a dark moist chamber. Common examples are:
- Glycerol-PBS (9:1) with antiquench e.g. any of the following
- propyl gallate
- 1,4-Diazabicyclo (2,2,2)-octane (DABCO) (very popular)
- Ascorbic acid
- Mowiol or Gelvatol
Used when a permanent mount is required
- Canada balsam
- DPX (Distrene 80 - a commercial polystyrene, a Plasticizer e.g. dibutyl phthalate and Xylene)
- DPX new (with Xylene but free of carcinogenic Dibutyl phthalate)
- Entellan™ (with Toluene)
- Entellan™ new
- Neo-Mount™ (compatible with aliphatic Neo-Clear® but not compatible with aromatic solvents like Xyene)
Contrasting with other types/meanings of "mounting"
In contrast to mounting necessary for glass coverslips, somewhat similar mounting can be done for bulkier specimen preservation in glass containers in museums. However an entirely different type of mounting is done for Electron_microscope#Sample_preparation which can be for biological or nonbiological materials and is further subdivided into "hot"(compressive) and "cold" (castable) type mounting processes. Though named "mounting", it is more akin to embedding in histology and should not be confused with the mounting described above. The term mounting in other fields has numerous other meanings which are partially indexed in this disambiguation page on Mounting.
- Quartz Microscospe Slides and Cover Slips from a commercial website (Ted Pella). Accessed on 2010-01-23.
- Quartz Microscope Slides and Cover Slips catalog page from a commercial website (SPI Supplies). Accessed on 2010-01-23.
- Histology and Light Microscopy catalog page from a commercial website (EMS). Accessed on 2010-01-23.
- Microscope Slides catalog page from a commercial website (TEKDON). Accessed on 2010-01-23.
- Gold Coated Microscope Slides and DNA Imaging Kit catalog page from a commercial website (Asylum Research). Accessed on 2011-08-31.
- Medical Laboratory Science : Theory And Practice By Ochei & Kolkatker, p. 446.
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