Dry mounted permanent slides do not use a liquid mounting medium. The cover glass has to be held in place in another way.
When making a microscope slide, the specimen is usually surrounded either by water our by another mounting medium. There are also slides, however, that do not use any mounting medium at all. These are the dry-mounted slides. Here the specimen is surrounded by air. Mounting medium normally holds the cover glass in place, but in this case the cover glass must be fixed in some other way.
It is quite easy to make dry-mounted slides and therefore it is also a worthwhile beginner’s activity and useful for educational purposes in schools. It is also relatively safe, because you do not need any mounting media which contain (possibly harmful) solvents. A possible disadvantage is the lower image quality that is produced.
The presence of a mounting medium is generally necessary, if you want to obtain the best resolution with your microscope. Especially high numerical aperture objectives require the presence of a mounting medium and cover glass to produce the best resolution. Under certain circumstances it it also possible to observe the specimens without mounting medium at all. Dry-mounted slides can be observed with low power objectives, which work at low numerical apertures. With these objectives the loss of image quality is not as high.
Ideal specimens for dry-mounting
Dry mounting is useful for specimens which are free of water (and therefore do not decompose) and which are sufficiently stable so that they do not need the physical support of a surrounding mounting medium. It is also suitable for specimens which are water soluble (such as salt or sugar). These can not be observed in a wet mount as it would dissolve the substance. Specimens that can be dry mounted include:
- Pollen grains
- Sand samples
- Dry insect parts (wings, legs etc)
- Textile fibers
- Hair and fur
- various crystalline substances, such as salts or sugars
Disadvantages of dry-mounting
If dry mounting is so easy to do, why is it not done more often? As a matter of fact, there are some disadvantages. As is so often the case, much depends on the individual specimen used. Here are some possible disadvantages:
- Lower resolution: the refractive index of the mounting medium is important to produce a clear image. Without a mounting medium the specimen can appear quite dark. This is becasue it refracts the light differently.
- No preservation: the mounting media preserve the specimen from decomposition. Make sure that the specimen is completely dry, before mounting.
- No mechanical support: Dry mounted slides have to be stored horizontally, otherwise the specimen might move beneath the cover glass.
- Shrinkage: Some specimens might shrink and deform during drying.
Use tape to hold the cover glass in place
For making dry mounts, you can either use double sided adhesive tape or cut regular adhesive tape into thin strips to hold the cover glass in place. Using double sided adhesive tape has the advantage in that the tape itself acts as a spacer between the slide and the cover glass. This prevents the specimen from being crushed by the cover glass. The double sided tape is quite strong and therefore a misplaced cover glass will be very difficult to remove and there is the danger of the cover glass of cracking and the danger of injury. The pictures below show a method of using double sided tape, which I found to work quite well.
It is also possible to seal the cover glass and stick it to the slide using clear nail polish or glue, but there is the possibility for these liquids to be drawn under the cover glass. For this reason the sealant should be quite viscous.