Dry Mount Slides – Principle, Preparation, Uses

A dry mount slide is the simplest type of slide preparation in microscopy. It is the process in which a dry specimen is placed directly on the clean glass slide without using any liquid medium. It is the air that remains between the specimen and the objective lens, and the refractive index of air is about 1.0. It is easy to prepare and it is mostly used for observing solid materials which are already dry and stable in normal conditions.

These are hair, feathers, pollen grains, small insect parts, dry leaf fragments and even fine paper or cloth fibers. A coverslip is not always required, but it is usually placed to keep the specimen flat and to protect the lens from touching the material. When the coverslip is sealed with nail polish or varnish, the slide can be stored for a longer time.

The idea of this preparation was used long before the modern slide was introduced. It is the early “sliders” made of bone or ivory that were used, where mica sheets held the specimen. These were inserted under the lens for viewing.

Later, the use of standardized flat glass slides replaced these older methods and formed the basis of the dry mount technique used today. Dry mount slides can also be observed under dark field microscopy which helps in viewing transparent and low-contrast specimens.

Purpose of Dry Mount Slide

Dry mount slides allow for direct observation of the surface features and structures of solid specimens. Since there is no liquid medium involved, the specimen’s natural appearance is preserved, and the details of the sample can be seen clearly.

Principle of Dry Mount Slide

The principle of a dry mount slide is based on the fact that the specimen is viewed in air without any liquid medium. It is the process in which a dry and stable material is placed directly on the clean glass slide, and the air acts as the only medium between the specimen and the objective lens. Air has a low refractive index (about 1.0) and this affects the contrast of the image formed. It is why only dry and inert specimens like hair, feathers, pollen grains or minerals are used, because living or moist samples will dry very fast and lose their structure. A coverslip can be placed to keep the specimen flat and to prevent the objective lens from touching the specimen during focusing. The low refractive index difference between many unstained samples and air results in low contrast under the brightfield microscope, making it difficult to observe the internal parts. It is in such cases where darkfield illumination is useful because the scattered light from the specimen helps in producing a clearer image against the dark background.

Material Required for Dry Mount Slide

To prepare a dry mount slide, you will need the following materials:

1. Clean glass slide for placing the specimen.
2. Dry specimen such as hair, pollen grains, feathers, dry leaf pieces or small insect parts.
3. Coverslip for keeping the specimen flat and protecting the objective lens.
4. Forceps or toothpick for transferring the specimen on the slide.
5. Scissors or a razor blade for cutting the specimen into small pieces.
6. Gloves for handling the slide and coverslip to avoid fingerprints.
7. Microscope for observing the prepared slide.
8. Cleaning materials like alcohol, water or lint-free tissue for removing dust.
9. Air blower for clearing loose particles from the slide.
10. Nail polish or varnish (optional) for sealing the coverslip to make a semi-permanent slide.

Ideal specimens for dry-mounting

It is the method in which the specimen is placed on a slide without adding any liquid medium. The specimen is kept in its dry state and covered with a coverslip. It is suitable for those materials which do not require moisture to maintain their structure and can be observed directly.

Some of the ideal specimens for dry-mounting are–

1. Pollen grains. These are commonly used because pollen grains remain intact in dry condition. It is used to observe pollen morphology and the characteristic outline of different plant species.
2. Sand samples. Sand particles from beach or desert can be directly mounted. It is useful to study the shape, size and composition of sand grains.
3. Dry insect parts. Wings, legs and exoskeleton of insects are stable in dry condition. This helps in observing venation pattern of wings and other external structures.
4. Textile fibres. Fibres remain unaffected when dry, so it is used to examine texture, colour and arrangement of fibres. It is useful in forensic and textile studies.
5. Hair and fur. Human or animal hair can be observed directly. It is used to study colour, thickness and structure of hair shaft.
6. Crystalline substances. Crystals of salts or sugars (NaCl, sucrose etc.) can be mounted dry. It helps in observing crystal shape and arrangement.
7. Mineral specimens. Rock fragments and small mineral crystals are used for studying physical properties and surface features.
8. Dry plant parts. Leaves, seeds, and small stems can be observed when dried. It is helpful in identifying venation, seed coat pattern and surface texture.
9. Dust particles. Dust collected from different environments is mounted to study composition and origin.
10. Non-living biological materials. Dried tissues and non-living cells are also mounted for studying structure when hydration is not required.

Dry mounting is simple and quick. It is used mainly when the specimen is stable in dry conditions and does not get damaged without mounting fluids.

Procedure of Dry Mount Slide

It is the simplest method of slide preparation where specimen is placed on a glass slide in dry state. The procedure is quick and easy and is done when the specimen does not require any mounting fluid.

A. Preparation and selection

1. Clean the slide and coverslip– The slide and coverslip must be clean and free of grease or dust. It is cleaned with soap and water or with 70% ethanol and allowed to dry. Handle only by the edges to avoid fingerprints.
2. Dry the materials– The cleaned glass are dried with lint-free cloth or air dried. It is important that no moisture remain on the surface.
3. Choose appropriate specimen– Select a solid, inherently dry specimen (hair, pollen, feather, small insect part, fibre, dust etc.). If high magnification is required the specimen is flattened or thinned in this step.

B. Transfer and placement

4. Transfer the sample– Use forceps, fine brush or toothpick to pick the specimen. In this step care is taken to avoid crushing delicate structure.
5. Place on slide– Place the specimen gently at the centre of the slide. The specimen is positioned so that it lies flat against the glass and does not overlap other pieces.
6. Adjust orientation– If necessary orient the specimen (wing venation, fibre direction, seed coat face) so diagnostic features are visible when viewed from above.

C. Coverslip application (optional)

7. Apply coverslip (if used)– A coverslip is usually applied to protect the specimen and objective lens. Hold the coverslip by an edge, place one side down adjacent to specimen and lower the opposite edge slowly so the coverslip settles without shifting the specimen. This action gently flattens the specimen.
8. No mounting medium– Do not add liquid. The coverslip is simply resting on the specimen. If specimen is very bulky a small paper wedge or tape can be used to support the coverslip.

D. Examination

9. Place on microscope stage– The prepared slide is placed on the stage and secured with stage clips. Begin observation under low power and then move to higher objectives as needed. In this step focus is adjusted using coarse then fine focus to avoid crushing sample.
10. Lighting and condenser– Adjust condenser and diaphragm to give appropriate contrast for dry specimen (increase contrast for surface features). In this step illumination is controlled to reveal texture and relief of the specimen.

E. Making semi-permanent and cleanup

11. To make semi-permanent (luting)– If the slide is to be stored, the coverslip edges is sealed with a luting agent (clear nail varnish or mounting varnish). Apply varnish around the edges and allow to dry. This prevents dust ingress and secures the coverslip.
12. Post-observation cleaning– For temporary slides remove coverslip and return specimen to storage or discard. Clean slide and coverslip with 70% ethanol and lens cloth before reuse. The microscope stage and objective are checked for any debris.

Uses of Dry Mount Slide Technique

• Examination of solid and dry specimens. It is used mainly for observing dry and inert materials which do not need any mounting medium for support. These include hair, feathers, pollen grains, dry leaf fragments, seeds, small insect parts and mineral dust. The specimens remain stable in dry condition and can be viewed directly.
• Study of non-biological particulate matter. It is used to observe fine particles like dust, sand grains, finely cut cloth or paper fibres. These samples do not require moisture and their surface outline is clearly seen.
• Introductory microscopy and teaching purpose. This technique is simple and quick, so it is used in school and college laboratories. It does not require staining or chemical reagents. It helps beginners learn basic handling of slides and microscopes.
• Quick and temporary observation. Dry mounts can be prepared within few seconds, so it is useful when temporary viewing is required. The slide is not permanent and can be dismantled after use.
• Surface study under low power. It is suitable for surface examination using low power objectives or stereo microscopes. The outer texture, edges and shape of specimens can be observed easily in this method.
• Use with special illumination techniques. Dry mounted samples can be studied under dark field illumination to increase contrast. It is useful for transparent fibres or small particles. Dry mounts can also be viewed under phase contrast or DIC to observe fine surface details.
• Specimen demonstration and identification. It helps in showing structural characteristics of dry plant parts, insect wings, pollen grains and textile fibres for identification work in laboratories.
• Preparation of reference slides. Dry mount slides are used to prepare quick reference slides for routine examination. They are sealed when long-term storage is required.
• Study of plant matter. Dry leaves, bark pieces, seeds and pollen are commonly observed to understand venation, texture and external morphology.
• Observation of fibres and forensic materials. Hair, feathers and textile fibres are examined for colour, thickness and texture which is useful in forensic investigations and textile analysis.

Advantages of Dry Mount Slide Technique

• Simple and easy method. It is the most basic slide preparation technique. The procedure is quick, easy to make, and does not require any complicated steps or reagents.
• Suitable for beginners. It can be prepared even by students with minimal training. No specialised skills are needed for handling the specimen or slide.
• Low material requirement. Only a slide, coverslip and the dry specimen are needed. No stains, chemicals or mounting medium is required which makes the method economical.
• Useful for general viewing. It is used for simple and rapid observations when only the external surface or outline of the specimen is required.
• Ideal for inherently dry specimens. It is suitable for materials such as pollen, hair, fibres, insect parts, dry leaf fragments and fine mineral particles. These specimens remain stable in air and do not require hydration.
• Protection of objective lens. When a coverslip is used it acts as a protective layer. It prevents the objective lens from touching the specimen, especially in high power where working distance is very small.
• Helps in securing the specimen. The coverslip also keeps the specimen fixed in place on the slide which prevents movement during focusing.
• Compatible with special illumination techniques. Dry mounted samples show improved visibility under dark field illumination. It gives good contrast for transparent fibres and small particles without the use of staining.
• Effective for surface texture study. Under DIC the dry specimen shows clear topography and surface details with a relief-like appearance which helps in analysing texture.
• No specimen distortion due to liquid. As no mounting medium is used, delicate dry specimens remain in their natural state without swelling or distortion.
• Faster preparation for temporary observation. Since it does not involve drying, staining or chemical treatment, it is ideal for quick temporary observations in laboratory work.

Limitations of Dry Mount Slide

• Unsuitable for living specimens. Living cells or aquatic samples cannot be used because they dry out quickly in absence of a liquid medium. The technique is restricted to dry materials only.
• Restricted specimen type. It is used only for solid and inert specimens such as hair, feathers, pollen, dry plant parts or mineral dust. Moist or soft biological tissues cannot be prepared by this method.
• Temporary preparation. Dry mounts are not permanent. They remain temporary unless the coverslip edge is sealed with varnish or nail polish for long-term preservation.
• Low contrast in brightfield. Air is the medium in this method, and it has low refractive index. Transparent specimens appear faint and show poor contrast under normal brightfield illumination.
• Difficulty in observing fine structures. Intricate structures like fine cell walls or delicate surface patterns are often difficult to see because they do not stand out clearly in dry condition.
• Need for special illumination. To observe transparent dry specimens clearly, techniques like dark field or DIC may be required. These give better contrast but need special equipment.
• Thickness sensitivity. For high-power viewing, the specimen must be thin and flat. Thick materials cause blurred images and cannot be resolved properly.
• Risk of spherical aberration. When the specimen is uneven or bulky, the light path is disturbed. This leads to spherical aberration, and the image becomes distorted.
• Risk of objective crashing. Thick specimens may touch the objective lens during focusing, especially with high-power objectives which have very short working distance.
• High contamination sensitivity. Since no medium is used, dust, fingerprints or small debris are easily seen. These contaminants affect the clarity of the image.
• Image quality compromised by slide condition. Any residue or grease on the slide or coverslip produces artefacts. It reduces visibility and interferes with observation of the specimen.

FAQ

1. What is a dry mount slide?
A dry mount slide is the simplest slide preparation where a solid and dry specimen is placed directly on a clean slide in air medium and viewed under the microscope. It does not use any liquid or stain.

2. How do you prepare a dry mount slide?
It is prepared by placing a clean and dry specimen at the centre of the slide. The specimen is flattened if needed and then a coverslip is gently lowered over it to keep it in place.

3. What specimens are suitable for dry mounting?
It is suitable for dry and inert materials like hair, feathers, pollen, dry leaf fragments, fibres, mineral dust and small insect parts.

4. What materials are needed for dry mount slides?
A clean glass slide, coverslip, forceps or toothpick for handling, and the dry specimen are usually needed. No chemicals or mounting medium are required.

5. What are the advantages of dry mount slides?
It is simple, quick and does not need any reagent. It is suitable for beginners and useful for observing surface features and dry specimens. A coverslip protects the lens and keeps specimen fixed.

6. What are the disadvantages of dry mount slides?
It gives poor contrast for transparent specimens. It is temporary unless sealed. It cannot be used for living organisms or wet samples. Dust or fingerprints can interfere with the image.

7. Do you use a coverslip for dry mount slides?
A coverslip is usually used. It protects the objective lens and keeps the specimen secured but the mount can still be viewed without it if the specimen is large.

8. How do you make a permanent dry mount slide?
To make it permanent the coverslip edges is sealed with a luting material like clear nail varnish. It prevents movement and protects the specimen for storage.

9. Can you stain dry mount slides?
Stains are generally not used in dry mounts because no liquid medium is present. If staining is required it becomes a wet mount.

10. What is the difference between wet mount and dry mount slides?
A wet mount uses a liquid medium like water or glycerine and is suitable for living or moist specimens. A dry mount uses air as the medium and is suitable only for dry materials.

11. How do you make a dry mount of hair?
A strand of hair is cleaned if needed and placed flat at the centre of a slide. A coverslip is gently lowered so the hair remains straight and then it is observed.

12. How do you make a dry mount of an insect?
A small insect or insect part is placed on a clean slide with forceps. The wings or legs are arranged flat if required and a coverslip is placed carefully over it.

13. What is a temporary dry mount slide?
It is a dry mount prepared for quick observation without sealing. The slide can be dismantled after use and reused again.

14. How do you prepare a dry mount slide without a coverslip?
The specimen is placed directly on the clean slide and observed as it is. This is done for thick or rigid specimens where coverslip is not needed.

15. How do you avoid air bubbles when preparing dry mount slides with mounting medium?
Dry mounts normally do not use mounting medium. If any medium is added the coverslip is lowered slowly from one edge so that air escapes and no bubbles are trapped.