How to prepare an onion cell slide

From Academic Kids

Image:Make an onion cell slide.png
Fig. 1
forceps, dye, pin, onion membrane, slide

Image:Microscope diagram.png
Fig. 2

This activity is one of the first uses of an optical microscope that most students encounter in a biology lab. Onions are used because they have large cells that are easily visible under a microscope and the preparation of a thin section is very straight forward..

An onion is made of many concentric layers. Each layer is separated by a thin skin or membrane. In this experiment you will make a wet mount or slide and look at the cells of the membrane under a microscope.


  1. Take a small piece of onion and using forceps (tweezers), peel off the membrane from the underside (the rough side).
  2. Lay the membrane flat on the surface of a clean glass slide, and then add one drop of dye solution (iodine or methylene blue). Be very careful; these dyes will stain your skin and clothes.
  3. Using a pin, lower a thin glass cover slip or cover glass onto the slide. Make sure there are no air bubbles.
  4. Make sure the lowest power objective lens (the shortest lens if there are several present) is in line with the optical tube, and the microscope light is turned on. Then place the prepared slide onto the stage of the microscope.
  5. Looking from the side (NOT through the eyepiece), lower the tube using the coarse focus knob until the end of the objective lens is just above the cover glass. Do this carefully so as not to crack the cover glass (and possibly damage the objective lens).
  6. Now look through the eyepiece and turn ONLY the smaller, fine focusing knob to move the optical tube upwards until you can see the cells on the onion. They should look something like lizard skin.
  7. Swap the objective lens for a higher powered one so that you can see the cells at greater magnification. You should be able to make out the nucleus.

Proper use of the microscope—intended to prevent damage to the objective lenses—requires that the following techniques be followed:

  • Never use the coarse focus knob while looking through the eyepiece. The point of focus will be very near the cover glass. Looking from the side, lower the optical tube until the objective lens is as close as you can get it to the cover glass without actually touching it. Starting with the low power objective lens is the fastest way to achieve proper focus.
  • Initially, slowly focus back (turn the fine focus knob to raise the optical tube) while looking through the eye piece. Once the specimen comes into focus, you can make fine adjustments up or down with the fine focus knob without fear of damaging the slide or the microscope.
  • If the specimen does not come into view (does not focus), raise the tube a little with the coarse focus knob and attempt to focus again with the fine focus knob. Once the object is in focus, switching objective lenses (to a higher power) should be possible without any further coarse adjustments.

A microscope, such as the one shown in Figure 2, can be tilted back relative to its base for easier viewing through the eyepiece. However, the tilt should be minimal (no more than 15 degrees) to avoid problems with the microscope slide not remaining in place on the stage. Further adjustments for comfort should come from raising one's seat.

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