When viewing any slide with a microscope, a small square or circle of thin glass called a coverslip is placed over the specimen. It protects the microscope and prevents the slide from drying out when it's being examined.
The coverslip is lowered gently onto the specimen. A mounted needle can be used to hold the specimen in place as the coverslip is lowered. It is important that no air bubbles are trapped underneath.
Most cells are colourless. Stains are used to add contrast. Certain stains are also used to stain specific cell structures or cell products.