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Microscope,
instrument used to obtain a magnified image of minute objects or minute
details of objects.
 II.
Optical Microscopes
The
most widely used microscopes are optical microscopes, which use
visible light
to create a magnified image of an object. The simplest optical microscope
is the double-convex lens
with a short focal length (see Optics).
Double-convex lenses can magnify an object up to 15 times.
The
compound microscope uses two lenses, an objective lens and an
ocular lens, mounted at opposite ends of a closed tube, to provide greater
magnification than is possible with a single lens. The objective lens
is composed of several lens elements that form an enlarged real image of
the object being examined. The real image formed by the objective lens
lies at the focal point of the ocular lens. Thus, the observer looking
through the ocular lens sees an enlarged virtual image of the real image.
The total magnification of a compound microscope is determined by the
focal lengths of the two lens systems and can be more than 2000 times.
Optical
microscopes have a firm stand with a flat stage to hold the material
examined and some means for moving the microscope tube toward and away
from the specimen to bring it into focus. Ordinarily, specimens are
transparent and are mounted on slides—thin, rectangular pieces of
clear glass that are placed on the stage for viewing. The stage has a
small hole through which light can pass from a light source mounted
underneath the stage—either a mirror that reflects natural light or a
special electric light that directs light through the specimen.
In
photomicrography, the process of taking photographs through a
microscope, a camera is mounted directly above the microscope's eyepiece.
Normally the camera does not contain a lens because the microscope itself
acts as the lens system.
Microscopes
used for research have a number of refinements to enable a complete study
of the specimens. Because the image of a specimen is highly magnified and
inverted, manipulating the specimen by hand is difficult. Therefore, the
stages of high-powered research microscopes can by moved by micrometer
screws, and in some microscopes, the stage can also be rotated. Research
microscopes are also equipped with three or more objective lenses, mounted
on a revolving head, so that the magnifying power of the microscope can be
varied.
III.
Special-Purpose Optical Microscopes
Different
microscopes have been developed for specialized uses. The stereoscopic
microscope, two low-powered microscopes arranged to converge on a
single specimen, provides a three-dimensional image.
The
petrographic microscope is used to analyze igneous
and metamorphic
rock. A Nicol prism or other polarizing device polarizes the
light that passes through the specimen. Another Nicol prism or analyzer
determines the polarization of the light after it has passed through the
specimen. Rotating the stage causes changes in the polarization of light
that can be measured and used to identify and estimate the mineral
components of the rock.
The
dark-field microscope employs a hollow, extremely intense cone of
light concentrated on the specimen. The field of view of the objective
lens lies in the hollow, dark portion of the cone and picks up only
scattered light from the object. The clear portions of the specimen appear
as a dark background, and the minute objects under study glow brightly
against the dark field. This form of illumination is useful for
transparent, unstained biological material and for minute objects that
cannot be seen in normal illumination under the microscope.
The
phase microscope also illuminates the specimen with a hollow cone
of light. However, the cone of light is narrower and enters the field of
view of the objective lens. Within the objective lens is a ring-shaped
device that reduces the intensity of the light and introduces a phase
shift of a quarter of a wavelength. This illumination causes minute
variations of refractive index in a transparent specimen to become
visible. This type of microscope is particularly effective for studying
living tissue.
A
typical optical microscope cannot resolve images smaller than the
wavelength of light used to illuminate the specimen. An ultraviolet
microscope uses the shorter wavelengths of the ultraviolet region of
the light spectrum to increase resolution or to emphasize details by
selective absorption (see Ultraviolet
Radiation). Glass does not transmit the shorter wavelengths of
ultraviolet light, so the optics in an ultraviolet microscope are usually quartz,
fluorite,
or aluminized-mirror systems. Ultraviolet radiation is invisible to human
eyes, so the image must be made visible through phosphorescence
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photography, or electronic scanning.
The
near-field microscope is an advanced optical microscope that is
able to resolve details slightly smaller than the wavelength of visible
light. This high resolution is achieved by passing a light beam through a
tiny hole at a distance from the specimen of only about half the diameter
of the hole. The light is played across the specimen until an entire image
is obtained.
The
magnifying power of a typical optical microscope is limited by the
wavelengths of visible light. Details cannot be resolved that are smaller
than these wavelengths. To overcome this limitation, the scanning
interferometric apertureless microscope (SIAM) was developed. SIAM
uses a silicon probe with a tip one nanometer (1 billionth of a meter)
wide. This probe vibrates 200,000 times a second and scatters a portion of
the light passing through an observed sample. The scattered light is then
recombined with the unscattered light to produce an interference pattern
that reveals minute details of the sample. The SIAM can currently resolve
images 6500 times smaller than conventional light microscopes.
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