A microscope slide was originally a 'slider' made of ivory or bone, containing specimens held between disks of transparent 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 (shown on the right) 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.
Since high power microscopes have a very narrow region within which they focus, the object to be viewed ("specimen") is typically placed on the middle of the slide with another, much thinner square (or circle or rectangle) of glass placed over the specimen. This smaller sheet of glass is called a cover slip or cover glass, and typically measures 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) it creates an even thickness (in wet mounts) for viewing. The thickness of the coverslip is crucially important for high-resolution microscopy.
Many objects that are going to be viewed on a 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 critters), the object being prepared for viewing is contained within water. Special slides are available for viewing 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 (see "Problems and solutions" below). There should be no excess water (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 clips or a mechanical stage arm.
Problems and solutions
- Mostly air under cover slip – too little water was used (increase by 1 drop) or cover glass was improperly dropped onto specimen. Adding a drop of water to the slide at the very edge of the cover glass will result in water being taken in under the cover glass (capillary effect). If numerous air spaces are still evident, it is best to start over.
- Too much liquid under cover slip – cover glass slides around easily on glass slide, and may fall off if slide not held perfectly level. Using a small piece of absorbent paper (paper towel or tissue), touch edge of paper to excess water at edge of cover slip; repeat until cover glass affixes to the slide.
- Cover slip "rocks" on 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, remove it and re-mount the specimen.
- Fingerprints or dust may have been on the slide already which may affect your 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 acqueous specimen.
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.