Geoscience Reference
In-Depth Information
light. Similarly, unstained living or dead bacteria can be seen as bright objects under
the high power of the dark background microscope. Light is stopped from entering the
tube of the microscope from below and the objects are illuminated instead from the
side. Bacteria or other microorganisms reflect the light falling on them and become
revealed as bright objects against a dark background. This is often a beautiful sight,
especially when motile organisms are seen clearly swimming across the field of view.
Spiral-shaped organisms, such as, spirochaetes, also present a dramatic picture under
these conditions; the morphology of these and other microbes can be readily observed
in this way.
Living microorganisms can also be observed, without staining, by the phase con-
trast microscope provided that the refractive index of the specimen differs sufficiently
from that of the surrounding medium. In this technique, the specimen is illuminated
by a hollow cone of light in which the phase of the light waves coming through the
objective lens is shifted by a quarter of a wavelength by a device known as a phase
ring. This results in interference since light passing through the slide unretarded is
seen as normal white light whereas light passing through the specimen has a longer
light path and arrives in the eyepiece out of phase. Usually, the image appears dark on
a light background. It is possible to take photographs with all these systems of micro-
scopy.
The electron microscope uses an electron beam instead of a light beam and it is
focussed by lenses, which are either electromagnets or electrically charged plates, to
form a highly magnified electron image. The resolving power is very much greater
than that of the light microscope. (The light microscope can resolve objects 0.2 mi-
crons in size, whereas the electron microscope can resolve even those only 0.001 mi-
crons in size; one micron = one thousandth of a millimetre). As in a television tube,
the electron beam must be produced and focussed in a high vacuum because elec-
trons are readily stopped even by gas molecules, and the image appears on a spe-
cially coated fluorescent screen. An electron beam is produced by heating a tungsten
wire electrically near to a positively charged metal plate, i.e. the anode. If a hole is
bored in the anode, electrons can pass through it as a beam. There is a third, negat-
ively charged, electrode which, by repelling electrons serves to concentrate them into
a beam. The electrical or magnetic fields in electron lenses may be varied by chan-
ging the voltage applied. Magnetic lenses are generally used and they have a minim-
um focal length of less than one millimetre. A typical electron microscope has two
condenser lenses, an intermediate lens and a projector lens which projects the final
positive image on to the fluorescent screen. Alternatively the image can be received
