History of the Microscope: How the light microscope evolved.
By Mary Bellis, About.com Guide
Light 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
A light microscope, even one with perfect lenses and perfect illumination, simply cannot be used to distinguish
objects that are smaller than half the wavelength of light. White light has an average wavelength of 0.55
micrometers, half of which is 0.275 micrometers. (One micrometer is a thousandth of a millimeter, and there are
about 25,000 micrometers to an inch. Micrometers are also called microns.) Any two lines that are closer
together than 0.275 micrometers will be seen as a single line, and any object with a diameter smaller than 0.275
micrometers will be invisible or, at best, show up as a blur. To see tiny particles under a microscope, scientists
must bypass light altogether and use a different sort of "illumination," one with a shorter wavelength.
The introduction of the electron microscope in the 1930's filled the bill. Co-invented by Germans, Max Knoll
and Ernst Ruska in 1931, Ernst Ruska was awarded half of the Nobel Prize for Physics in 1986 for his
invention. (The other half of the Nobel Prize was divided between Heinrich Rohrer and Gerd Binnig for the
STM.)
In this kind of microscope, electrons are speeded up in a vacuum until their wavelength is extremely short, only
one hundred-thousandth that of white light. Beams of these fast-moving electrons are focused on a cell sample
and are absorbed or scattered by the cell's parts so as to form an image on an electron-sensitive photographic
plate.
Power of the Electron Microscope
If pushed to the limit, electron microscopes can make it possible to view objects as small as the diameter of an
atom. Most electron microscopes used to study biological material can "see" down to about 10 angstroms--an
incredible feat, for although this does not make atoms visible, it does allow researchers to distinguish individual
molecules of biological importance. In effect, it can magnify objects up to 1 million times. Nevertheless, all
electron microscopes suffer from a serious drawback. Since no living specimen can survive under their high
vacuum, they cannot show the ever-changing movements that characterize a living cell.
Light Microscope Vs Electron Microscope
Using an instrument the size of his palm, Anton van Leeuwenhoek was able to study the movements of onecelled organisms. Modern descendants of van Leeuwenhoek's light microscope can be over 6 feet tall, but they
continue to be indispensable to cell biologists because, unlike electron microscopes, light microscopes enable
the user to see living cells in action. The primary challenge for light microscopists since van Leeuwenhoek's
time has been to enhance the contrast between pale cells and their paler surroundings so that cell structures and
movement can be seen more easily. To do this they have devised ingenious strategies involving video cameras,
polarized light, digitizing computers, and other techniques that are yielding vast improvements in contrast,
fueling a renaissance in light microscopy.
Here are the important compound light microscope parts...
Eyepiece: The lens the viewer looks through to see the specimen. The eyepiece usually contains a 10X or 15X
power lens. (NPHS: 10x)
Diopter Adjustment: Useful as a means to change focus on one eyepiece so as to correct for any difference in
vision between your two eyes.
Body tube (Head): The body tube connects the eyepiece to the objective lenses.
Arm: The arm connects the body tube to the base of the microscope.
Coarse adjustment: Brings the specimen into general focus.
Fine adjustment: Fine tunes the focus and increases the detail of the specimen.
Nosepiece: A rotating turret that houses the objective lenses. The viewer spins the nosepiece to select different
objective lenses.
Objective lenses: One of the most important parts of a compound microscope, as they are the lenses closest to
the specimen.
A standard microscope has three, four, or five objective lenses that range in power from 4X to 100X. When
focusing the microscope, be careful that the objective lens doesn’t touch the slide, as it could break the slide and
destroy the specimen. (NPHS: 4x, 10x, 40x)
Specimen or slide: The specimen is the object being examined. Most specimens are mounted on slides, flat
rectangles of thin glass.
The specimen is placed on the glass and a cover slip is placed over the specimen. This allows the slide to be
easily inserted or removed from the microscope. It also allows the specimen to be labeled, transported, and
stored without damage.
Stage: The flat platform where the slide is placed.
Stage clips: Metal clips that hold the slide in place.
Stage height adjustment (Stage Control): These knobs move the stage left and right or up and down.
Aperture: The hole in the middle of the stage that allows light from the illuminator to reach the specimen.
On/off switch: This switch on the base of the microscope turns the illuminator off and on.
Illumination: The light source for a microscope. Older microscopes used mirrors to reflect light from an
external source up through the bottom of the stage; however, most microscopes now use a low-voltage bulb.
Iris diaphragm: Adjusts the amount of light that reaches the specimen.
Condenser: Gathers and focuses light from the illuminator onto the specimen being viewed.
Base: The base supports the microscope and it’s where illuminator is located.
How Does a Microscope Work?
All of the parts of a microscope work together - The light from the illuminator passes through the aperture,
through the slide, and through the objective lens, where the image of the specimen is magnified.
The then magnified image continues up through the body tube of the microscope to the eyepiece, which further
magnifies the image the viewer then sees.
The parts of a microscope work together in hospitals and in forensic labs, for scientists and students,
bacteriologists and biologists so that they may view bacteria, plant and animal cells and tissues, and various
microorganisms the world over.
Compound microscopes have furthered medical research, helped to solve crimes, and they have repeatedly
proven invaluable in unlocking the secrets of the microscopic world.
Anatomy of an Optical Microscope
Discussing the different components comprising a microscope is necessary to
explain how a microscope works.
Simply, a microscope is a framework to contain the lenses and serve as a platform
for stable viewing and a platform for other peripheral components that are used to
improve the image quality.
The Lenses
The lenses are the essence of the microscope and are at the heart of the question - how does a microscope work?
A compound microscope has two or more lenses. The eyepiece or ocular lens sits atop the body tube. Many
microscopes are binocular and have two ocular lenses.
Additionally, a binocular head will have a prism, either in the head or the body tube, to split the image and
direct it to both oculars. The oculars have different available magnifications, but usually less than the power of
the objective lenses.
The objective lenses are at the bottom of the microscope tube nearest the
specimen; they gather and focus the light transmitted from the specimen.
Usually three or four objectives of different strengths will reside in a
revolving turret, and magnification may be changed by turning the turret to
line up a different lens with the body tube.
Objective magnification strengths usually range from 10X to 100X. Fine and
coarse focusing adjustments are accomplished with focusing knobs located on
the body of the microscope.
The Stage
The specimen sits directly below the objectives on the microscope’s stage.
Clips on the stage hold the specimen slides in place for stable viewing.
A mechanical stage allows precise movement of the specimen along the X and Y coordinates and graduated
markers allow the viewer to note the location of features on the slide.
A hole or aperture in the stage allows light to illuminate the specimen.
The Light
Below the stage, a diaphragm, condenser, and light source control light emission and distribution to the
specimen. At the bottom of the optical train is the illumination source.
In a simple microscope, the light source may be ambient light collected and reflected upwards into the aperture
by a small mirror.
The type of illumination source will increase in sophistication as a microscope’s complexity increases.
Tungsten-halogen, mercury arc, and metal halide lamps, as well as LED illumination provide different types of
light to meet certain viewing requirements.
Some microscopes have a condenser directly above the light source. This device is actually a lens that gathers
light and focuses it into a cone directed at the specimen. A diaphragm controls the diameter of the light beam
before it finally passes into the specimen. Different types of condensers are manufactured for specialized
viewing needs.
How Does a Microscope Work? - Optical
To be useful, a microscope must accomplish three things: it must magnify the object you are trying to view,
resolve the details of the object, and make these details visible.
Understanding these ideas is the first step to learning how a microscope works. The optical or light microscope
uses visible light transmitted through, refracted around, or reflected from a specimen.
Light waves are chaotic; an incandescent light source emits light waves traveling in different paths and of
varying wavelengths. Some of the lenses in a microscope bend these light waves into parallel paths, magnify
and focus the light at the ocular.
How Does a Microscope Work? - Magnification
The power to enlarge the image of the specimen when viewed through a microscope is known as the
magnification and is dependent upon how much the lenses bend the light waves.
Magnification is expressed in numeric multiples of how much enlargement occurs with a lens. If the
magnification of a lens is 2X then it roughly doubles the size of the image of the object.
With a compound microscope, the total magnification can be determined by multiplying the magnifications of
the objective and ocular lenses. Consequently, an ocular lens of 10X coupled with a 40X objective yields a total
magnification of 400X.
However, the higher the magnification the closer the lens must be to the specimen. Since a higher magnification
lens bends light more severely, the specimen is brought into focus a shorter distance from the lens and this is
known as the focal length.
Generally, a lens providing higher magnification will also provide better resolution. These two factors working
together are very important in determining how a microscope works?
How Does a Microscope Work? – Resolution
The resolution of a specimen is highly dependent upon the light waves. The shortest
distance between two points that the microscope can define as clearly being separate
points is the resolution of the microscope.
Pictured left: microscope image of salt crystals.
Resolution is perhaps more important than magnification in understanding how a
microscope works? If the points cannot be clearly focused then they are closer together
than the resolution of the microscope and, regardless of the magnification, the image
quality will be poor.
The resolution is determined by the frequency of the light waves illuminating the
specimen and the quality of the lens. A rule of optical physics is that the shorter the
wave length the greater the resolution.
Usually expressed in microns, the best resolution a light microscope can produce is 0.2 microns or 200
nanometers. Discounting the light source, a lens having a resolution of 0.5 microns won’t resolve points as close
together as a 0.3 micron lens.
How Does a Microscope Work? - Contrast
Contrast is another important ingredient in how a microscope works.
High magnification and resolution won’t guarantee that you will actually see the image of the specimen. If all of
the light passes through a cell no details will be visible. Some light frequencies must be absorbed to different
degrees by structures inside the cell and this allows you to see the specimen.
Using a microscope’s condenser or diaphragm, the size and intensity of the light beam can be modified. A
narrow beam provides higher contrast. Staining the specimen may be necessary to obtain the contrast you need
to view the details of your sample.
How Does a Microscope Work? - Electron
As previously mentioned, optical microscopes are limited in resolution by the frequency of the light waves.
Electron guns emit a flow of electrons of a considerably shorter wave length than visible light and this fact
allows an electron microscope to have higher resolution and magnification.
In many ways, an electron microscope functions similarly to an optical scope except that, instead of visible
light, a stream of electrons is used to illuminate the specimen. The electron beam is focused with magnetic
lenses. Changes to the electron beam inside the specimen are recorded and an image is formed based upon these
changes.
How to Use and Adjust a Compound Microscope
Step by Step.....Safely and Easily
Here are step-by-step instructions on how to use and adjust a compound microscope:
1. Turn on the illuminator. When using the dimmer, it is best slowly increase the light intensity as the
lamp heats up quite quickly.
2. Place a slide or specimen on the stage with the sample directly above the aperture and, if possible, fasten
it to the stage with the stage clips. Reminder: A cover slip is always
needed to allow for the best quality image.
3. Ensure the iris diaphragm is completely open, allowing the
maximum amount of light to reach the slide and the lenses. Caution:
Do not use the iris diaphragm to control the light, it is to control
resolution and contrast - use the dimmer instead.
4. Rotate the nosepiece so that the objective lens with the lowest level
of magnification is directly above the sample. Reminder: Using lower
magnifications first helps to select the part of the specimen of interest
and then adjust further.
5. Look through the binocular eyepieces and adjust the iris diaphragm
until the amount of light is satisfactory. More light is better than less light, but the comfort of the
viewer’s eyes should also be taken into account.
6. Turn the coarse adjustment knob until the specimen comes into broad focus. Caution: you should not
use the coarse focus with a high magnification objective for fear of the objective making contact with the
slide.
7. Turn the fine adjustment knob until the specimen comes into sharp focus. Caution: should not take a
long time to find focus, otherwise the high magnification objective could also hit the slide. If you are
having a difficult time to find focus then restart with the lower magnification objective.
8. The viewer should then be able rotate the nosepiece to higher settings and bring the sample into more
and more detail with a minimal amount of refocusing.
Some compound microscopes come with what is called a rack stop. A rack stop prevents the objective lenses
from being lowered into the slide.
However, some older microscopes do not have a rack stop, so it is always advisable to check, just in case.
Lowering an objective lens into a slide could easily break the slide and damage the sample.
In order to move your microscope safely, one hand should be under its base for support and the other at its arm.
Be sure to only switch off the microscope when the dimmer is set to the lowest intensity and always turn off the
lamp before moving the microscope.
So now you know how to use and adjust a compound microscope. Well done. Happy Viewing!
http://inventors.about.com/od/mstartinventions/a/microscope_2.htm
http://www.microscope-microscope.org/basic/microscope-parts.htm
http://www.microscopemaster.com/parts-of-a-compound-microscope.html
MICROSCOPE: History, Parts, & Function
CLASS SET: DO NOT WRITE ON THIS PAGE!
Answer the following questions on your own paper.
1) How did Galileo contribute to the formation of the light microscope we use today?
2) Why is Anton van Leeuwenhoek considered the father of microscopy?
3) Why can electron microscopes magnify an object better than light?
4) List a positive and negative of using electron microscopes to enlarge an object.
5) MICROSCOPE LABELING: use the scope image in the article to help you label your half page scope drawing.
6) How is the fine adjustment knob different than the coarse adjustment knob?
7) What are the 2 parts that actually enlarge or magnify the image on the slide?
8) What is the function of the iris diaphragm?
9) The microscopes at NPHS have 3 objective lenses. How do you change from one magnification to another?
10) What three things must a microscope accomplish in order to be useful? Define each of them briefly.
11) When using a compound light microscope, the iris diaphragm should start in what position?
12) Which objective lens do you start with?
13) How will you bring an object into focus?
MICROSCOPE: History, Parts, & Function
CLASS SET: DO NOT WRITE ON THIS PAGE!
Answer the following questions on your own paper.
1) How did Galileo contribute to the formation of the light microscope we use today?
2) Why is Anton van Leeuwenhoek considered the father of microscopy?
3) Why can electron microscopes magnify an object better than light?
4) List a positive and negative of using electron microscopes to enlarge an object.
5) MICROSCOPE LABELING: use the scope image in the article to help you label your half page scope drawing.
6) How is the fine adjustment knob different than the coarse adjustment knob?
7) What are the 2 parts that actually enlarge or magnify the image on the slide?
8) What is the function of the iris diaphragm?
9) The microscopes at NPHS have 3 objective lenses. How do you change from one magnification to another?
10) What three things must a microscope accomplish in order to be useful? Define each of them briefly.
11) When using a compound light microscope, the iris diaphragm should start in what position?
12) Which objective lens do you start with?
13) How will you bring an object into focus?