From the one specimen, more than one type of image can be obtained in the microscope. Worrying though this may seem, this knowledge can be used to great advantage by the discerning microscopist. Resolving power (minimum resolved distance) always remains a major consideration, but for the biologist, poor contrast is often a detriment to satisfactory observation, and contrast improvement has been a major source of development in microscope technique. At its simplest, contrast, is profoundly influenced by the way that light is directed on to the specimen, an advantage that is made use of in darkfield illumination.

In general, the illumination for the microscope should have an adequate intensity, provided through a separate intensity control. The observed field should be completely and evenly illuminated. Controls for the area of illumination and the angle of illumination should be learned to maximize microscope performance.

Key concepts:

• A research microscope usually has at least two iris diaphragms in the light path. Their separate functions must be recognized.
  
• Light intensity should be controlled by a transformer, not the condenser iris diaphragm.
  
• For measurement, a scale inserted in the eyepiece can be superimposed on the microscope image.
  
• No one set of instructions will explain the workings of all models of microscopes.
  
• For many biological applications poor contrast of the image is limiting to satisfactory observation.
  
• Brightfield (Köhler) Illumination – Theory.