Phase Specimens and Amplitude Specimens Specimens that do not
possess much color but a different refractive index that the
surrounding mounting medium can be referred to as phase
specimens as they cause a phase-shift in of the light. The
unaided human eye is not capable of detecting this phase shift.
This phase shift is then converted into a brightness difference
by the optics of the phase-contrast microscope. Bacteria are a
good example here. They are nearly completely transparent but
nevertheless appear darker or brighter (depending on the optics)
than the background. Amplitude specimens possess a color and are
able to decrease the brightness of the passing light all on
their own. These specimens are best observed using bright-field
microscopy. Pigmented structures (such as chloroplasts) and
specimens that are selectively stained are examples. Many
specimens are a combination of these two. Here the choice of the
right kind of microscope is important in order to see that what
one wants to see. Phase contrast