How to prepare an onion cell slide

This activity is one of the first uses of an optical microscope that most
school children ever encounter in a school biology lab. Onions are used
because they have large cells that are easily visible under a student
microscope.This experiment is normally performed by children between the
ages of 11 - 14.

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

Method

  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 then add 1
     drop of dye (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 over the slide. Make sure
     there are no air bubbles.
  4. Put the slide onto the stage of the microscope. Make sure the objective
     lens is set on low power, and the microscope light is turned on.
  5. Look through the eyepiece lens and turn the focusing wheel until you
     can see the cells. They should look like lizard skin.
  6. Swap the objective lens for a high powered one so that you can see the
     cells at bigger magnification.You should be able to make out the
     nucleus.

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