Microscope slide

Microscope slides and cover slips. A single, standard 3-inch long glass slide is shown with a cover glass in place.

A microscope slide was originally a slider made of ivory or bone, containing specimens held between disks of translucent mica. These were popular in Victorian England until the Royal Microscopical Society introduced the standardized microscope slide in the form of a thin sheet of glass used to hold objects for examination under a microscope. A standard microscope slide is 75 x 25 mm (3" X 1") and about 1.0 mm thick. A range of other sizes is available for various special purposes: 75 x 50 mm and for geological use 46 x 27 mm (petrographic) and even 48 x 28 mm (thin sections)

Since high power microscopes have a very narrow region within which they focus, the object to be viewed, or specimen, is typically placed on the middle of the slide with another, much thinner sheet of glass placed over the specimen. This smaller sheet of glass is called a cover slip or cover glass, and typically is rectangular, measuring between 18 and 25 mm on a side, and 0.085 to 0.25 mm thick. The cover glass serves two purposes: (1) it protects the microscope's objective lens from contacting the specimen, and (2) in wet mounts (see below), it creates an even thickness for viewing. The thickness of the cover slip is crucially important for high-resolution microscopy.

Mounting

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Handling

Microscope slides and coverslips must be held by the edges, as fingerprints on the slide or coverslip will result in a difficulty seeing the object of study clearly. This is especially crucial when viewing a specimen underwater, as air bubbles and fingerprints will render it nigh impossible to properly observe the studied specimen.

Dry Mount

The specimen is simply placed on the slide, and then a cover slip is placed on top.

Wet mount

Many objects that are going to be viewed on a compound light microscope slide are prepared as a wet mount using water. Other materials are used when a permanent slide is being prepared for viewing and storage. In a wet mount, the specimen is placed at the center of the slide with one or two drops of water and the cover glass placed over the specimen. In some preparations (such as looking at pond water for microscopic organisms), the object being prepared for viewing is contained within water. Special slides are available for viewing wet mounts that require more than one or two drops of water.

Once the specimen and water are combined on the slide, the cover glass is added. The cover glass should be placed at an angle to the slide, one edge touching the slide, and then lowered as if hinged there. If done properly, the water will force out any air as the cover glass closes over it, and no bubbles will be trapped beneath the glass. Although an occasional bubble might be tolerated, large numbers will make viewing the specimen difficult. Adhesive forces between the liquid and the glass will hold the cover glass firmly in place. Generally, only one drop of water is sufficient. Adding too much water will create a problem, as the affixing of the cover slip to the slide will depend on much weaker cohesive forces. There should be no excess water outside the cover slip, and the cover slip should remain in place when the slide is moved to the stage of the microscope, where it is held in place by stage^{[clarification needed]} clips or a mechanical stage arm.

An example application is the vaginal wet mount (or vaginal smear), which is a gynecologic test wherein a sample of vaginal discharge is observed by wet mount microscopy to find the cause of vaginitis and vulvitis.^[1]

Problems and solutions

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• Mostly air under the cover slip – too little water was used or the cover slip was improperly dropped onto specimen. Adding a drop of water to the slide at the very edge of the cover slip will result in water being taken in under the cover glass via capillary action. If numerous air spaces are still evident, it is best to start over.

• Too much liquid under the cover slip – the cover slip slides around easily on the glass slide, and may fall off if the slide is not held perfectly level. A small piece of absorbent paper may be touched to the edge of the cover slip to remove the excess water, until the cover glass affixes to the slide.

• The cover slip rocks on the specimen or is clearly not laying flat – either the specimen or something in the sample (a grain of sand, for example) is preventing the cover slip from coming down far enough to adhere to the slide. Focusing on this slide will be difficult. Possibly the specimen is not thin enough or evenly sliced. If a grain is present, it may be removed and the specimen re-mounted.

• Fingerprints or dust may have been on the slide already which may affect the final results. In the case of dark field microscopy, dust particles and minute scratches can cause intolerable problems as sources of unwanted glare in the image. In such cases, freshly-cleaved mica may be used instead of glass to support and cover the specimen.

Cover slip standard thicknesses

• 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. 2 – 0.19 to 0.23 mm thick

Graticule slide

Vector rasterized grid (low quality)

The text above describes a standard microscope slide. However, there are many special purpose slides. For example, a graticule slide is a microscope slide that is marked with a grid of lines (for example, a 1 mm grid) that allow the size of objects seen under magnification to be easily estimated. Such grids are perhaps more often employed in counting exercises. Sometimes one square of the grid will itself be subdivided into a finer grid.

References

The Wikibook Constructing school science lab equipment has a page on the topic of Graticule slides