ABSTRACT
The image which we see with a microscope is the result not only of the effects of the optical system of the instrument on the light used to form the image, but also of the interactions of the light with the specimen itself. These latter are often forgotten by microscopists, but it is important to consider them, since they form the basis of the rational choice of contrast technique. In many cases, these effects or their consequences on the image may be manipulated by the user in order to enhance or modify image contrast. This may be done either by altering some of the characteristics of the specimen, manipulating the illuminating system at the lamp or condenser and/or manipulating the imaging systems within or after the objective. It is also possible to manipulate the recording or storage medium which receives the final image. Even though a microscope user might not have made a deliberate choice to use a particular contrast technique, one or more of the effects to be described will be occurring and influencing the nature of the image. In order to choose the most appropriate contrast technique, it is essential for the microscopist to understand fully the way in which any particular combination of illumination, specimen, and imaging device act together. It is equally important to appreciate the role of the various conjugate planes of the microscope in image formation, since suitable manipulation of phenomena in these planes will allow alteration of contrast in the final image.
