Specimens that are to be observed under a compound microscope are secured on a microscope slide. This is then inserted into the microscope to be made viewable by the observer. They are standard tools for scientific research.
Slides provide two primary functions, the first, to protect the objective lens of the microscope from touching the specimen, and the second, to create an even thickness (in wet mount slides) for viewing.
This article looks at how to prepare microscope slides for observation.
History Of The Microscope Slide
Originally, a slide was known as a ‘slider’, in Victorian England, slides were made from ivory or bone, with the specimens being held under transparent mica.
Later on, glass slides became widely used, along with mica coverslips, or some users simply used two slides with the specimen positioned in-between.
Much of what we know about the historical usage of slides during the 20th century is unknown. During the second world war, many important scientific objects were destroyed.
In Berlin, for example, a research institute located in the region contains a wealth of different artifacts, which contain not only bones, wall charts and models, but approximately 30,000 microscopic specimens.
Professor Ilana Loewy, intent on finding more information about early 20th century slides, found what she was looking for when she visited the Museum für Naturkunde, which contains around 80,000 specimens currently being used by scientists studying developmental biology.
The Vermes collection in the same museum contains around 30,000 slides, including ones which would have been used in private showrooms.
Microscope Slide Appearance
Most microscope slides are made from either glass or plastic material. They are approximately 1×3 inches and range in thickness from 1mm-1.2mm.
There are two widely used types of slides used for microscopic inspection, these are flat and concave slides. Below, we look at the features of each of these slides.
Flat Glass Slides
These slides are the most commonly used and are fairly basic. They are traditionally made from either plastic or glass and are normally 1 to 1.2mm thick. They are designed to fit under the stage clips on a microscope stage.
Glass is normally preferred to plastic for observing specimens under the microscope. This is because glass allows for easier light transfer through the object.
Glass is known as having a low refractive index, which means that light bends less as it passes through a substance.
The corners of the glass are normally rough and are a sharp 90 degrees, this can make them potentially hazardous if not handled with care.
The bottom and top edges are usually frosted, which allows the user to easily mark the sample for identification or orientation. Sometimes the frosted edges appear in a variety of colors, making the process of identification and organization easier.
It is possible to purchase glass slides with rounded edges to avoid potential injury. To achieve this, the corners are slightly clipped for comfortable handling.
Prepared Slides
These types of slides are ideal within a teaching environment. Prepared slides consist of sets of slides which have already been prepared by a professional. Typically, they consist of around 50 to 100 slides per pack.
These specimens within the object are killed by a process of preservation. To achieve this, the specimens are fixed with a preservative that prevents deterioration.
After this, the specimen is soaked in a paraffin substance and left to harden. It is then cut into smaller pieces and mounted on the slide. After being coated with permount, it is covered by a slip.
Used widely to teach students, these kinds of slides are a good way of providing an introduction to microscopic techniques.
Instead of conducting more advanced work involving slide preparation, they can simply focus on observation and microscope adjustments.
Concave Slides
These kinds of slides are considerably expensive when compared to their flat counterparts. Concave slides consist of surface depressions which make them suitable for viewing large specimens or liquid solutions.
Depending on the variety of concave slides, they normally contain either one or two depressions, which are traditionally 0.5 and 0.8mm deep. They are thicker than normal slides, which makes it possible to hold more liquid.
These slides are ideal for viewing thick specimens which cannot be placed in smaller glass slides. Specimens which are larger avoid being crushed during the observation process as they have more freedom to move around.
Charged Microscope Slides
Charged microscope slides feature an additional chemical, or charge, which is added to the slide. This allows specific kinds of tissue to fix in an adhesive manner to the slide.
Usually only used for specific types of applications, they are primarily used by scientists researching pathology, cytology and histology.
Additional features
Some slides contain additional features that make them suitable for specific types of observation. For example, transparent mica slides are commonly used with high-powered light and electron microscopes.
They are a good option for the production of particle imaging, cellular structure and carbon support films.
Some slides also contain an etched grid system or graticule, which allows researchers to keep track of certain areas. These slides also help with hand sketching and the plotting of geographic areas.
Cover Slips
A cover slip consists of a thin piece of transparent glass. Normally a fraction of a millimeter thick and either rectangular or square, it is placed over the specimen after it has been mounted on the slide.
The two types of cover slips available are number 1 and number 2. Number 1 slips are between .13-.17mm thick, and number two covers are between .17-.25mm thick.
Number 1 slips are designed for high-resolution microscopy, as well as being used for oil immersion preparations. Number 2 slips are for general usage.
Cover slips are traditionally made from either silicate glass or borosilicate, with their main function being to protect samples from unwanted movement and contamination.
In addition to this, cover slips also serve to protect the microscope itself by preventing direct contact between the specimen and the lens.
Sometimes, other materials are used for cover slips, such as quartz and different varieties of plastic. Cover slips also sometimes come with grids, etched lines and other thicknesses.
When Should I Use A Cover Slip?
Sometimes, it can be tricky to know when exactly you need to use a cover slip during observation.
The primary way to determine when a cover slip should be used is by which microscope you are using. Below, is a list of some different varieties of microscopes and whether or not they require a cover slip.
Microscopes That Need A Cover Slip
- Compound Microscope – When using a compound microscope, both a cover slip and slide are required.
- Polarizing Microscope – These microscopes are typically used to view thinner sections of different rocks, powders and minerals. Depending on the sample, cover slips are usually used to contain and flatten the sample.
- Upright Biological Microscope – These microscopes can sometimes be referred to as compound microscopes. Both a slide and cover slip are required when using this microscope.
Microscopes That Don’t Need A Cover Slip
- Stereo Microscope – With these microscopes, the sample sits directly on the stage of the microscope and does not need to be placed on a slide.
- Metallurgical Microscopes – Both inverted and upright metallurgical microscopes do not require a cover slip. Samples sit directly on the stage.
- Inverted Biological Microscopes – These microscopes involve the use of a petri dish, which does not require a cover slip.
Preparation Techniques
Preparing A Dry Mount
The most basic method of placing a sample on a microscope slide, the dry mount method involves placing an uncut, thin slice of the item in the center of the slide and then placing a cover slip over the sample.
These kinds of mounts are suitable for inorganic and dead matter, such as hair, pollen, feathers and plant materials. If attempting to place the living matter on a dry mount, the sample will dry out and expire quickly.
Preparing A Wet Mount
In the wet mount technique, a drop of water is used to suspend the specimen between the slide and cover slip. Traditionally used for aquatic samples, they can be suspended in fluids such as brine, immersion oil, or water.
To prepare a wet mount sample, the specimen must be placed in the mount using tweezers, after a drop of liquid has been applied. The cover slip must then be placed at an angle over the slide, making contact with the edge of the drop of liquid.
Covers must be lowered gradually to avoid creating air bubbles. Finally, any excess water can be removed using a paper towel.
This type of method is great for allowing users to view live samples without them drying out. As well as aquatic samples, blood, saliva and other cells are also ideal for observation with a wet mount.
Preparing A Smear Slide
These slides provide an effective, simple and cheap manner of examining marine sediments. It is an essential method for identifying and classifying sediment types. These types of slides should be performed in an atmosphere with a moderate temperature.
To prepare this sample, the user must first begin by using a toothpick to take a sample from the surface of the core. This only needs to be around 1-2mm3.
After this, it must be placed on the slide and dispersed using a few drops of water. The sample is smeared out by using the toothpick until it becomes a thin film.
Finally, the cover slip is placed carefully, avoiding air bubbles. If any of the sample appears beneath the glass, too much specimen has been used.
Preparing A Squash Slide
A squash slide is designed primarily for soft samples, normally fruits and fungi.
In order to prepare a squash slide, a drop of water must be placed on the slide, followed by a small piece of the specimen. After this, the cover slip is placed on top.
Afterwards, the sample must be pressed using a soft round object (such as an eraser) to test if the specimen is sufficiently soft. The sample should be pressed carefully until it begins to look like a thin, transparent film.
It is important to note that samples which may be too solid must be softened before performing this method of slide preparation.
Plant material can sometimes be softened by using boiling water. If this doesn’t soften the sample sufficiently, it can be heated with an acid such as diluted HCI or acetate.
When using substances such as acetate, be sure to take the necessary safety precautions as they can prove dangerous when not handled carefully. After treating the specimen with acid, rinse and compress.
Preparing Slide Staining
Microscope cell staining involves the enabling of better cell visualization under the microscope. Stains provide a better view of areas such as the nucleus or cell wall. There are two different types of stains, some involving living cells, and others involving fixed cells.
In terms of types of staining solutions used for performing these samples, methylene blue, iodine, and crystal violet are all good options.
To stain a sample, a very straightforward process, a drop of staining solution needs to be added to the edge of one side of the cover slip. Following this, an edge of a paper towel must be positioned at the opposite end. Finally, allow the dye to be pulled across the sample.
Traditionally, stains are used in fields such as pathology and virology. This allows for the diagnosis of disease.
Conclusion
With such a wide variety of different methods for preparing slides, observers can look at an immense variety of different samples.
Microscope slides allow for observation of a plethora of different specimens, be that through mounting the samples manually or purchasing prepared slides.
Glass slides open up a new world of possibilities for those who are interested in scientific research.
Ever since I was a little girl, I have been fascinated by science. Every holiday I would ask for the newest science kit, and spend weeks learning with it.
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