HISTORY OF MICROSCOPE
During that historic period known as the Renaissance, after the "dark" Middle
Ages, there occurred the inventions of printing, gunpowder and the mariner's compass,
followed by the discovery of America. Equally remarkable was the invention of the light
microscope: an instrument that enables the human eye, by means of a lens or
combinations of lenses, to observe enlarged images of tiny objects. It made visible the
fascinating details of worlds within worlds.
Invention of Glass Lenses
Long before, in the hazy unrecorded past, someone picked up a piece of
transparent crystal thicker in the middle than at the edges, looked through it, and
discovered that it made things look larger. Someone also found that such a crystal would
focus the sun's rays and set fire to a piece of parchment or cloth. Magnifiers and "burning
glasses" or "magnifying glasses" are mentioned in the writings of Seneca and Pliny the
Elder, Roman philosophers during the first century A. D., but apparently they were not
used much until the invention of spectacles, toward the end of the 13th century. They were
named lenses because they are shaped like the seeds of a lentil.
The earliest simple microscope was merely a tube with a plate for the object at one
end and, at the other, a lens which gave a magnification less than ten diameters -- ten
times the actual size. These excited general wonder when used to view fleas or tiny
creeping things and so were dubbed "flea glasses."
Birth of the Light Microscope
About 1590, two Dutch spectacle makers, Zaccharias Janssen and his son Hans,
while experimenting with several lenses in a tube, discovered that nearby objects
appeared greatly enlarged. That was the forerunner of the compound microscope and of
the telescope. In 1609, Galileo, father of modern physics and astronomy, heard of these
early experiments, worked out the principles of lenses, and made a much better
instrument with a focusing device.
Anton van Leeuwenhoek (1632-1723)
The father of microscopy, Anton van Leeuwenhoek of Holland, started as an
apprentice in a dry goods store where magnifying glasses were used to count the threads
in cloth. He taught himself new methods for grinding and polishing tiny lenses of great
curvature which gave magnifications up to 270 diameters, the finest known at that time.
These led to the building of his microscopes and the biological discoveries for which he is
famous. He was the first to see and describe bacteria, yeast plants, the teeming life in a
drop of water, and the circulation of blood corpuscles in capillaries. During a long life, he
used his lenses to make pioneer studies on an extraordinary variety of things, both living
and non-living and reported his findings in over a hundred letters to the Royal Society of
England and the French Academy.
Robert Hooke
Robert Hooke, the English father of microscopy, re-confirmed Anton van
Leeuwenhoek's discoveries of the existence of tiny living organisms in a drop of water.
Hooke made a copy of Leeuwenhoek's light microscope and then improved upon his
design.
Charles A. Spencer
Later, few major improvements were made until the middle of the 19th century.
Then several European countries began to manufacture fine optical equipment but none
finer than the marvelous instruments built by the American, Charles A. Spencer, and the
industry he founded. Present day instruments, changed but little, give magnifications up
to 1250 diameters with ordinary light and up to 5000 with blue light.
Beyond the Light Microscope
The Light Microscope
Light microscopes are used to examine cells at relatively low magnifications.
Magnifications of about 2000X are the upper limit for light microscopes. The highest
resolution of a light microscope is about 0.2 μm. The use of blue light to illuminate a
specimen gives the highest resolution. It is because blue light is of a shorter wavelength
than white or red light. For this reason, many light microscopes come fitted with a blue
filter over the condenser lens to improve resolution.
The common light microscope used in the laboratory is called a compound
microscope. It is because it contains two types of lenses; ocular and objective. The ocular
lens is the lens close to the eye, and the objective lens is the lens close to the object. These
lenses work together to magnify the image of an object.
Parts of Compound Microscope
There are twelve parts in a compound microscope. They are as follows:
Parts of Binocular Medical Microscope with built-in Illumination
Illuminator (Light Source)
A mirror or electric bulb is provided as the source of light rays. The function of the
mirror is to provide reflected light from a lamp or sunlight. Most microscopes today have
built-in lamps that provide necessary illumination.
You can turn on and off the light source using a switch and adjust the illumination
intensity by turning the light adjustment knob. This knob is calibrated with a scale of 1 to
10; 1 is low intensity, and 10 is high intensity.
Diaphragm (Iris)
Many microscopes have a rotating disk under the stage known as the diaphragm
or iris. The diaphragm has different-sized holes that control the amount of light passing
through it. Based on the transparency of the specimen, adjustment of the diaphragm
setting to achieve a needed degree of contrast is possible.
Iris is used to increase or reduce the condenser aperture. Iris is closed for about
two-thirds for 10X objective, Iris is open more for 40X objective, and iris is fully open for
100X objective. One should use lamp brightness control, not the iris, to reduce the
illumination intensity. If the condenser aperture is closed too much, there will be a loss of
detail (resolution) in the image.
Condenser
Beneath the stage is a group of lenses that comprise the condenser. The condenser
accepts parallel light rays produced by an illuminator and condenses them into a strong
beam. It causes light rays from the light source to converge on the microscopic slide. The
clarity of the image increases with the higher magnification of the condenser.
Aperture
It is the hole present in the microscopic stage. Through the aperture, the
transmitted light reaches the stage from the source.
Stage
The stage is a flat platform positioned about halfway up the arm. It is the part
that holds the slides in place using simple or mechanical stage clips and
enables them to be examined in a controlled way. The specimen can be moved
systematically up and down and across the stage, i.e., X and Y movements.
The stage is moved up or down using a sub-stage adjustment knob. An operator
can move the slide around during a microscopic examination using stage control knobs.
An integral, smooth-running mechanical stage, preferably with vernier scales to enable
specimens to be easily located, is needed for smooth microscopic operations in a
laboratory.
Objective lens
These are primary lenses that magnify the specimens. Four objective lenses are present
in the compound light microscope. The shortest lens has the lowest power. Similarly, the
longest one is the lens with the greatest power. The higher power objective lenses are
retractable, i.e., when they hit a slide, the end of the lens will push in, thereby protecting
the lens and the slide.
Objective lens of a microscope
• (4X): It is a scanning objective lens. It also provides the lowest magnification
power of all objective lenses.
• (10X): It is a low-power lens. Lower magnifications locate specimen samples in
certain areas on a microscope slide.
• (40 X): It is a high-power lens. 40X objective lens is used for examination of wet
preparations e.g, hanging drop, and ova and cyst examination in the stool.
• (100 X): It is the oil-immersion lens. The lenses on which oil is used are
called oil-immersion lenses. Visualization of bacteria generally requires
immersion oil with 100X objective (i.e. total magnification of 1000X).
Magnification of 1000X is sufficient for the visualization of fungi, most
parasites, and bacteria but is not enough for observing viruses that require
magnification of 100,000X or more. Electron microscope provides such
magnification.
Most ocular lenses magnify the image ten times. So, the total magnification of a
microscope is calculated by multiplying the power of the objective lens by the power of
the eyepiece (10x). For example, if you are observing an object by a scanning objective
lens (4x), you are observing a 40 times magnified image (10x eyepiece lens multiplied by
4x scanning objective lens).
Body Tube
It transmits the image from the objective lens to the ocular lens.
Ocular Lens (eye-piece)
Ocular lens of a microscope
It is located at the top of the microscope, and
the ocular lens or eyepiece lens is used to look
through the specimen. It also magnifies the
image formed by the objective lens, usually ten
times (10x) or 15 times (15x). Usually, a
microscope has an eyepiece of 10x
magnification power. Advanced microscopes
have eyepieces for both eyes and are called
binocular microscopes.
A binocular microscope lets the user see the image with both eyes at once. It
improves the quality of microscopical work as it is more restful, particularly when
examining specimens for prolonged periods.
The eyepiece tube, also known as the eyepiece holder, holds the eyepiece lens
together. They are flexible in the binocular microscope that rotates for maximum
visualization. They are not flexible in the monocular microscopes.
Revolving Nose Piece
The revolving nosepiece holds several objective lenses of varying
magnification. It is movable, and the user can rotate it to achieve desired magnification
levels. Ideally, a microscope should be parfocal, i.e. the image should remain focused
when objectives are changed.
Coarse and Fine Adjustment Knob
Coarse Adjustment Knob
The coarse adjustment knob located in the arm of a microscope moves the stage
up and down to bring the specimen into focus. The coarse adjustment helps to get the first
focus. The gearing mechanism of the adjustment produces a large vertical movement of
the stage with only a partial revolution of the knob. Because of this, the coarse
adjustment should only be used with low power (4x and 10x objectives) and never
with high power lenses (40x and 100x).
Fine Adjustment Knob
A fine adjustment knob is generally
present inside the coarse
adjustment knob. It helps in
bringing the specimen into sharp
focus under lower power. It also
helps for overall focusing when
using a high-power lens.
Arm
The arm of the microscope supports the tube and connects it with the base. The
arm as well as the base help to carry the microscope. In the case of high-quality
microscopes, an articulated arm with more than one joint is present.
Base
The base is the bottom of a microscope. It helps to support the microscope. A
microscopic illuminator is also present in it.
In summary, the parts of the microscope and their functions are
explained below in the table:
Name of the parts Function
Connects ocular tube and base. It also helps carry the
Arm (limb)
microscope
Base Provides support to help microscope stand upright
Coarse adjustment Focus of image under high power and moving the stage up and
knobs down.
Forming a cone of all the dispersed light rays from the
Condenser
illuminator
Diaphragm (Iris) Controls the intensity of illuminating light
Eyepiece (ocular
Magnification of image produced by objective lens
lens)
Fine adjustment
Focus the image when viewing under high power
knobs
Illuminator Provides high-intensity light at the field aperture
Mirror Reflects light from an external source
Objective lens Primary magnifier of microscope
Maintains the correct distance between the ocular and
Ocular tube
objective lens
Holds the objective lens. Its rotation helps to change the power
Revolving nose piece
of the objective lens
Stage Place for holding sample
Stage clips Keeps the slide with a specimen in place on the stage
Download Microscope Parts Worksheet
