Starting with the low power objective lens is the fastest way to achieve proper focus. 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. The point of focus will be very near the cover glass. Never use the coarse focus knob while looking through the eyepiece.Proper use of the microscope - intended to prevent damage to the objective lenses - requires that the following techniques be followed: Be very careful these dyes can stain your skin and clothes.You should be able to make out a nucleus in each cell. Swap the objective lens for a higher powered one so that you can see the cells at greater magnification.The cells should look something like lizard skin. Now look through the eyepiece and turn ONLY the smaller, fine focusing knob to move the optical tube upwards until an image comes into focus.Do this carefully so as not to crack the cover glass (and possibly damage the objective lens). 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.Then place the prepared slide onto the stage of the microscope. 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.Using a pin, lower the thin glass cover slip or cover glass onto the slide.Add a drop of Iodine solution to the onion skin.Place the membrane flat on the surface of the slide.Take a small piece of onion and using tweezers, peel off the membrane from the underside (the rough side).First add a few drops of water or solution on the microscope slide to avoid dryness and wilting.In this exercise, you will make a wet mount on a microscope slide and look at the cells of the onion membrane magnified by the high power, compound microscope. An onion is made of layers, each separated by a thin skin or membrane. The cells are easily visible under a microscope and the preparation of a thin section is straight forward. Tissue from an onion is a good first exercise in using the microscope and viewing plant cells.
1ĭye, pin, onion membrane, slide, and cover slip Fig. A wet mount of the onion peel under the microscope stained with methylene blue at 50X zoom Fig.
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