Introduction to your microscope.

Using a modern light microscope requires minimal understanding of optics.  But, practically, you should know how to adjust a microscope both for optimal resolution as well as comfortable viewing.  [Care and cleaning the microscope]

Caveat:  Information on this page applies to routine bright-field microcopes, such as those commonly used in introductory histology labs.  For a brief description of additional optical technology, see "Specialized Microscopy Techniques" at the Molecular Expressions website, Florida State University.

Several points are worth noting:

1.  Always carry your microscope upright when it's out of its case.  Otherwise the eyepieces can fall out.  (Not all floors are cushioned with carpet like Room 206.)

2.  For any binocular microscope at least one eyepiece should be focusable.  After adjusting for interocular distance (how far apart are your eyes?), focus a specimen while looking through the fixed eyepiece.  Then adjust the other eyepiece so that both eyes are comfortably focused at the same plane.  

On the multi-viewer 'scopes in Lindegren 206, only the leader's station works this way.  For the other viewers, both eyepieces must be adjusted to match the focus set by the leader.

3.  The condenser is the optical arrangement beneath the stage. A properly adjusted condenser provides Kohler illumination.  Improper adjustment can significantly reduce the resolution of the objectives.  For Kohler illumination:

On most microscopes, the condenser should first be focused and centered, so that edge of the field diaphragm (in the base, beneath the condenser) appears sharp through the eyepiece.  Note that on many student 'scope, the condenser position is fixed, not adjustable.

The condenser diaphragm should then be adjusted to match the illuminated field to the size of the objective.  If the diaphragm opening is too large, the image will appear washed-out; if too small, resolution will be reduced.  Find the condenser lever which appears to affect image brightness; open all the way, and then close until a decrease in brightness first appears.  You can visualize this diaphragm by removing one eyepiece and looking down the open tube; the edge of the diaphragm should appear at the edge of the illuminated field of view.  This is the proper setting, and should (ideally) be adjusted whenever you switch objectives.

If you don't want to bother adjusting the condenser diaphragm every time you change magnifications, it is preferable to leave it wide open rather than closed down.

Do not use the condenser diaphragm to adjust image brightness.  For that purpose, use the rheostat (voltage regulator) on the light source.

[More on condenser adjustment for Kohler illumination.]

4.  Most modern microscopes have several objective lenses offering a choice of magnifications.  Our microscopes are all "parfocal" so that you should not need to lower the stage to change objectives.

5.  A note on immersion oil -- Do not use oil without good reason and proper instruction.  Most very high power objectives (60-100X) require immersion oil, a fluid with a high refractive index, placed between the coverslip and the lens.  Without the oil, an oil immersion objective will give an extremely poor image.  But the oil must be applied correctly and cleaned up carefully or it will do more harm than good and contaminate other lenses as well.

Care and cleaning. 

• Three rules for carrying a microscope:
  1. Your full attention is on the job, just as if you were carrying a baby.
  2. Use both hands.
  3. Always carry the microscope upright (otherwise the eyepieces can fall out).
      
• Three rules for cleaning a microscope:
  1. Don't!  It easy to damage a microscope by injudicious cleaning.  Unless you have been properly trained to clean a microscope, please do not try.
  • Never the touch the objective lens surface, unless you have bought and paid for the microscope yourself.
  • Clean eyepieces and other optical surfaces only when absolutely necessary.  It is better to ignore dirt than to cause permanent damage to the microscope. 
  2. If you can see dirt, use logic and experiment to determine where in the optical path the dirt is located.
  • Dirt on a slide moves when you move the slide.
  • Dirt on the condenser will go in and out of focus when you move the condenser up or down.
  • Dirt on an eyepiece will move when you rotate the eyepiece.
  • Dirt on an objective cannot be seen while viewing through the microscope, but manifests itself as a decrease in image quality. 
    
  3. If dirt is objectionable, use the least-intrusive procedure to make it go away.
  • Some dirt can blown away without touching the surface.  Use this procedure freely, but blow gently and be careful not to spit. 
  • Dirt on slides may be wiped off at will, using the same care you would use for eyeglasses.
  • Dirt on an eyepiece is often accentuated by having the condenser aperture too small; such dirt may become inconspicuous when the diaphragm is opened to the correct setting -- see Kohler illumination.
  • Dirt on the condenser may become inconspicuous if you lower the condenser (i.e., move the dirt out of focus).
  • If dirt must be removed --
    • Begin by simply blowing dust away.
    • Use clean lens paper and moisture from breath.
    • Wipe gently in one direction.  Do not use a circular motion; do not scrub.
    • Do not try to clean an objective lens; ask for help.
       
• Here are instructions for painstakingly proper cleaning of optical surfaces (video from JoVE, the Journal of Visualized Experiments).

Comments and questions: dgking@siu.edu

SIUC / School of Medicine / Anatomy / David King

https://histology.siu.edu/intro/mscope.htm
Last updated:  4 June 2022 / dgk