Physical Principles of Electron Microscopy: An Introduction to TEM ...

An Introduction to Microscopy 7
eyepiece
objective
transparent
specimen
half-silvered
mirror
reflecting
specimen
(a) (b)
Figure 1-4. Schematic diagrams of (a) a biological microscope, which images light
transmitted through the specimen, and (b) a metallurgical microscope, which uses light (often
from a built-in illumination source) reflected from the specimen surface.
individual components (organelles) within each biological cell. Because the
light travels through the specimen, this instrument can also be called a
transmission light microscope. It is used also by geologists, who are able to
prepare rock specimens that are thin enough (below 0.1 �m thickness) to be
optically transparent.
The metallurgical microscope (Fig. 1-4b) is used for examining metals
and other materials that cannot easily be made thin enough to be optically
transparent. Here, the image is formed by light reflected from the surface of
the specimen. Because perfectly smooth surfaces provide little or no
contrast, the specimen is usually immersed for a few seconds in a chemical
etch, a solution that preferentially attacks certain regions to leave an uneven
surface whose reflectivity varies from one location to another. In this way,