In this practical you use a light microscope to observe a plant or animal cell, such as onion epidermis or cheek cells. First, place a thin layer of the specimen on a slide and add a drop of a stain, such as iodine solution, which makes the structures easier to see. Lower the coverslip slowly at an angle, using a mounted needle, so that no air bubbles are trapped.
Place the slide on the stage and start with the low power objective lens. Use the coarse focus wheel to bring the specimen roughly into focus, then the fine focus to make it sharp. Only then switch to a higher power objective and adjust with the fine focus. The total magnification is the eyepiece magnification multiplied by the objective magnification, so a ×10 eyepiece and a ×40 objective give ×400.
Make a scientific drawing in pencil with clear, continuous lines, no shading and no colouring. Draw large and label the structures with ruled label lines, which must not cross. Give the drawing a title and write the magnification beside it.
To find the real size of a cell, use magnification = image size ÷ actual size, so actual size = image size ÷ magnification. Use the same unit for both sizes. For a drawing 60 mm wide at ×400, the actual width is 60 ÷ 400 = 0.15 mm, which is 150 µm.
