www.BioTopics.co.uk
Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

USING THE MICROSCOPE


Microscopes -
available on-line

Identify the various controls, especially if you have not used the microscope for some time:

watsoncrop.jpg microscope.jpg
  • coarse focussing adjustment
  • fine focussing adjustment
  • controls for moving slide (if present)
  • sub-stage condenser focussing control - usually not to be touched
  • sub-stage iris diaphragm control: sometimes a lever, sometimes a revolving disc with numbered holes
    (see below)
numbered ring to alter amount of light

DO NOT TOUCH OTHER CONTROLS, such as condenser centring screws and slide carriage stops.
Do not proceed further if you are unsure about anything.


Before placing anything on the microscope stage, check the illumination, as follows:


If you are using a separate light for the microscope:

Checking that the lowest power objective is clicked into position and that the iris diaphragm is fully open, position the light in front of the mirror and adjust the mirror's position (tilt and alignment) for maximum evenness of illumination.

If you are using a microscope with a built-in light:

Check that the lowest power objective is clicked into position and that the iris diaphragm is fully open. The view from the eyepiece should be evenly bright


Place a slide, either a permanent preparation (which often need dust wiped off) or a "wet prep" (which you must not wipe), onto the stage, securing it carefully with the clips. Do not flick the corner of the slide if using a slide carriage. Position the specimen as near as possible to the centre of the hole. Light from below may help here.

LOOKING IN FROM SIDEWAYS (not looking down the microscope yet!), bring the objective lens near to the specimen (say 3 mm.) using the coarse focussing adjustment. Note the direction of movementof the control - in the next stage you will perform the opposite movement.

LOOKING DOWN THE MICROSCOPE ALL THE TIME, rack the lens away from the slide, and stop when something comes into focus. Hopefully, this is what you are meant to be seeing, but remember: it could be something like dust on the bottom of the slide or the top of the coverslip, or even on the condenser lenses or the light bulb.

From now onwards, use only the fine focussing adjustment.

The light may be reduced using the sub-stage iris diaphragm if it is too bright, or if the specimen is too transparent (such as Amoeba). In many cases, more detail will be seen.

Observe and draw what you see, using the textbook for labelling.

When you want to go up to a higher magnification:

Centralise and focus accurately on the area you wish enlarged. Some people do not realise that if you do not centralise and re-focus, you may be looking at a gap alongside your intended target.

HAVE THIS CHECKED by whoever is in charge.

You should have noticed that higher power lenses are longer than low power ones. Most microscopes these days are parfocal which means that all the lenses ought to be (nearly) in focus, even though there will be less and less space between lens and specimen.

CAREFULLY swing the revolving nosepiece turret to the next highest lens, checking that it clicks into position.

Using only the fine focussing control, turn about half a turn in one direction, checking to see if anything comes into focus. If not, try turning back to the original position, then a little further in the other direction. If no luck after a couple of goes, return to low power and start again.

Do not touch lenses with fingers, or leave liquids on objective lenses.

After use, return the objective to low power, remove the slide and if necessary wipe clean the stage, then put it all back!

"WET PREPS"

The simplest way to prepare a specimen for the microscope:

Slides and cover slips should be handled only by the edges.

Place a cleaned slide on a paper towel on a flat surface.

Put a drop of liquid (water, stain, etc.) in the middle of the slide.

Place the specimen into the liquid, and spread it out if required.

Handling it only by its edges, lower a coverslip carefully from one side onto the slide, squashing gently if necessary.

playtriangle.gifHow to lower a cover slip onto a slide for a wet preparation
Absorb any excess liquid with filter paper, and wipe base of slide.

Keep a "wet prep" as level as possible, and remember that it will eventually dry out from the edges inwards. The microscope light also has a drying effect.

After use, "wet preps" can be collected in a container of water, and the coverslips will slide off.
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