In order to view small objects with a compound microscope, the object, or specimen to be viewed must be properly prepared, mounted on a slide, and sometimes stained to increase the contrast between the object and background. This technique is used for preparing eukaryotic cells for the microscope. In order to view bacteria (prokayotes), which are much smaller than plant and animal cells, a different technique is used called a bacterial smear.
* How to Prepare a Wet Mount Microscope Specimen *
Step One: Obtain a clean microscope slide.
Step Two: Place a drop of liquid on the slide. This is the “wet” part of the wet mount. The liquid used depends on the type of cell being viewed. If examining a plant cell, tap water can be used. If examining an animal cell, physiological saline must be used. If plain water is used, the cell will explode from osmotic pressure, since animal cells have no cell wall to structurally support them. When viewing cells with a home microscope rather than in a lab, contact lens saline solution can be used for wet mounting animal cells.
Step Three: Obtain the specimen to be used. Some introductory biology classics for viewing include:
* The skin of an onion bulb. In order to view the cells, a very thin layer of skin must be obtained. Take a single layer of onion and bend it towards the shiny side. After it snaps, pull gently, and a transparent layer of skin, similar to Scotch tape, will appear.
* Cheek cells. Human epithelial cells can be obtained by gently rubbing a toothpick on the inside of the mouth, and then swirling the toothpick in the physiological saline on the slide.
* Pond water. Obtaining some water from a pond makes wet mount preparation a breeze, since the water and the specimens are both included.
Step Four: Place the specimen into the liquid on the slide.
Step Five: If the specimen is transparent, such as onion skin or check cells, stain should be added to increase contrast. A drop of iodine or methylene blue is commonly used. Do not use stain if viewing photosynthetic cells (which already appear green due to chlorophyll) or living organisms, such as protozoans in pond water (stains will kill them).
Step 6: Place a coverslip over the specimen. This will sandwich the specimen between the slide and the coverslip. To avoid trapping air bubbles, set one edge of the coverslip on the slide, and then let the rest of the coverslip drop. As it drops from one side to the other, air will be pushed out, and this will reduce the number of bubbles in the wet mount. (See photos at end of article for example).
* Viewing a Prepared Microscope Slide *
The specimen is now ready to be viewed through the microscope. Once the slide is positioned on the microscope stage, start at a low power magnification, and work up to the level of magnification required to get a god view of the specimen.
