To obtain the best possible image when using a compound microscope, it is necessary to correctly adjust the optical system to focus both the objective lens and the light source onto the specimen.
The first stage is to focus the objective onto a sample slide. Use a blank slide with a cross drawn on it. Select the lowest power objective lens and focus onto the cross. Once a focussed image of the cross can be seen, the eyepieces on a binocular microscope should be adjusted so that the image is focussed for both eyes, thus compensating for any difference of vision in each eye.
The second stage is to focus the light source by adjusting the condenser.
- First adjust the condenser by racking it up to the top (take care not to damage/displace the slide).
- Move the specimen (cross) to one side to clear the field of view.
- Open the condenser diaphragm fully
- Close the lamp diaphragm (if fitted) so that the edge can be seen when looking down the microscope. If there is no lamp diaphragm, a dissecting needle placed against the lamp provides a focussing aid.
- Lower the condenser lens until the image of the edge of the lamp diaphragm, or alternatively the dissecting needle, is in focus.
- Open the lamp diaphragm (if fitted) until it just clears the field of view.
- Check the adjustments with the higher power objectives and fine tune if necessary.
Once experience has been gained, the dissecting needle can be dispensed with and the condenser can be focussed using the image of either the lamp element or the texture of the ground glass plate covering the lamp, depending on the microscope design.