TABLE OF CONTENTS Chapter 1 contained an overview of various forms of microscopy, carried out using light, electrons, and mechanical probes. In each case, the microscope forms an enlarged image of the original object (the specimen) in order to convey its internal or external structure. Before dealing in more detail with various forms of electron microscopy, we will first examine some general concepts behind image formation. These concepts were derived during the development of visible-light optics but have a range of application that is much wider.
Clearly, an optical image is closely related to the corresponding object, but what does this mean? What properties should the image have in relation to the object? The answer to this question was provided by the Scottish physicist James Clark Maxwell, who also developed the equations relating electric and magnetic fields that underlie all electrostatic and magnetic phenomena, including electromagnetic waves. In a journal article (Maxwell, 1858), remarkable for its clarity and for its frank comments about fellow scientists, he stated the requirements of a perfect image as follows
For each point in the object, there is an equivalent point in the image.
The object and image are geometrically similar.
If the object is planar and perpendicular to the optic axis, so is the image.
Besides defining the desirable properties of an image, Maxwell's principles are useful for categorizing the image defects that occur (in practice) when the image is not ideal. To see this, we will...
