Many objects that are going to be viewed on a compound light 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 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.