Using the Microscope.
Maladjustments: Locating and Correcting.
8 of 9
1 of 1
Troubleshooting Setup Errors.
There are many potential sources of error in setting up a microscope, and tracking down their precise cause is not always easy.
Apart from the more obvious failures to produce an image caused by faulty lamp electrics, a knocked mirror etc, the most common problem could be described as a generally poor quality image, especially on the highest powers and oil immersion.
A poor quality image obtained with dry objectives is often accompanied by a tendency for the image to shift back and forth as the fine focus adjustment is applied. In a correctly set up microscope, all points on the image should go symmetrically in and out of focus as the fine adjustment is used. The shifting effect is almost always due to one or more optical components being out of centre, or some object encroaching upon the illumination or image forming rays.
The latter possibility is best checked by removing the eyepiece and observing the objective back lens. The image of the substage diaphragm should be concentric with the objective back lens, and and any obstruction to the image forming rays should be visible if present. A common cause of obstruction is a substage stop carrier which is partly obstructing the rays entering the condenser, or less commonly, some object which has fallen into the upper part of the objective.
Errors in the centration of the optical components of the microscope must be remedied by repeating the appropriate steps in the Köhler setup procedure.
Dark patches or objects which are visible with the eyepiece replaced may be located by carefully rotating each component of the optical system in turn, and noting whether the object rotates as well.
Start with the eyepiece, which is the easiest component to rotate and a common location of annoyingly obtrusive dirt particles. The objective may be carefully unscrewed and rotated, but if nothing was seen when the back lens was checked, this is less often the cause of problems.
The condenser may be unclamped and rotated, though condensers can get quite grubby before they seriously degrade a brightfield image.
The remaining possible location of dirt and obstructions affecting the image is the lamp.
The dirt may be located on the condenser or the lamp envelope. Light modifying filters close to the lamp must also be kept quite clean. In short; the lamp, its bulb, condenser, diaphragm and any associated filters must be kept clean at all times, as any debris located there will be reimaged in the plane of the specimen (or its image) all the way through the system.
Techniques for removing dirt from lenses, once located, will be dealt with in a later update of these notes.
Poor image quality in oil or water immersion setups is often caused by obscuration of the objective front lens by air bubbles in the immersion medium. This can be verified by removing the eyepiece and carefully examining the objective back lens at various levels of focus using a phase telescope. This will clearly identify any bubbles if present. The remedy consists of removing the bubbles, along with the original oil or water, and reimmersing the objective.
Immersion objectives in which the frontlens is concave or somewhat recessed may require that a small drop of the immersion medium is applied to the frontlens as well as the specimen to avoid the formation of bubbles.