Wet mounts should ideally have no air bubbles because beginners may have trouble distinguishing the bubbles from the specimen when looking under the microscope. The presence of bubbles can also keep live organisms from moving freely. Another problem is that large bubbles may lower the viewing resolution.
Microscopes used for viewing wet mounts are designed to provide the highest-quality resolution when the specimen is submerged in water. If a large air bubble surrounds the specimen instead of water, the quality of the image becomes lowered. Another issue with air bubbles in wet mounts is that they appear to have a dark ring surrounding their edges underneath the microscope. This dark ring can cover up a portion of the specimen, making it harder to view.
Certain types of specimens are more prone to air bubbles than others. Bubbles can easily become trapped underneath specimens that are in the shape of large sheets, such as slices of onion skin. Porous specimens are also prone to air bubbles because their pores may be filled with air prior to creating the wet mount. A vacuum is useful for sucking out the air from the pores. Specimens with fatty surfaces form air bubbles easily as well, because they repel water. Applying alcohol to the specimen to remove the fatty layer can help prevent air bubbles.