Experiment: The Leeuwenhoek Microscope and the Beginning of Our View into the Small
You see glass everywhere. Your windows, your drinking vessels, your smartphone. Now you can too can manipulate this material to make a microscope completely from scratch. Forge a ball lens, see worlds previously unknown, and join the ranks of scientists who have assembled their own instruments.
What will you learn?
You will learn the history of the microscope's development in Northern Europe during the Enlightenment and the physics of how balls lenses function. You will learn how to manipulate glass into a simple lens. You will also learn, because of a material's index of refraction, why a ball of glass works as a magnifying lens, but a ball of diamond does not.
- You are starting from zero. You are beginning the field of microscopy. There are no previous experiments to do.
In grade school you are taught that your body is composed of individual "lives" called cells. Your heart, your muscles, your brain, your stomach, and almost all the parts of your body are made up of the trillions of cells that are you. Perhaps you have even seen some examples of these cells from our previous Backyard Brains experiments or experiments in your school. You may have also heard from your loved ones to wash your hands because of indivisible little things called "germs." The existence of cells and microscopic life is common knowledge as it should be.
But there was a time when this was not common knowledge. There was a time when such was the very edge of science. This time was the 1660s and 1670s in England and Holland with work done by two scientists - Robert Hooke and Antonie Philips van Leeuwenhoek.
Robert Hooke was an excellent inventor and polymath. He is indeed the Hooke of "Hooke's Law" concerning the force on springs, and he did important work along with Galileo and Huygens verifying the rings of Saturn. As far as we know, he moved on to other investigations after he published Micrografia and did not turn to further investigating the microworld. However, across the English Channel, in the nation of Holland, a successful clothing vendor in Delft began cultivating an interest in optics. He fabricated small glass spheres and developed a metal casing for the spheres in a deceptively simple and elegant design for viewing samples at different angles by changing the position of various screws.
Leeuwenhoek would stare at samples through the sphere in bright daylight, and, one day beginning in 1674, viewing a drop of pond water, he observed things moving which he called "animalcules." This was the first documented view of the living microworld, that there are living things in the world that our naked eyes cannot see, but with the invention of magnifying tools, we can. He also did an experiment observing bacteria on scum on his teeth, which were not present after he drank hot coffee (supposedly killing/removing the bacteria). He did "organism-culture" experiments with pepper grains to determine the origin of the animalcules. While he never formally published his findings in monographs or books, he communicated his observations in many letters written in Dutch to the Royal Society in England, which are stilled preserved and archived in London.
We at Backyard Brains are certainly fans of scientists who are excellent tool builders, which is one of the reasons we study Leeuwenhoek here. But let's take a step back. How does a lens work? Moreover, what is even a lens?
A lens has three properties, it is clear, it is curved, and it bends light. The bending of the light is the key property that allows microscopes to magnify images. Light bends when it enters or exits transparent material at an angle, and the curved form of a lens allows the bending to either "diverge out" or "converge in" depending on the shape of the lens.
and the higher a material's index of refraction, the more the light ray bends.
This mathematical relationship of the changing angles between two materials was first derived geometrically by the Persian/Baghdad scholar Ibn Sahl in the 10th century. Sahl was interested in the geometry of "burning mirrors and lenses" that can converge light rays from the sun to allow localized increases in temperature and flames. The law was then independently discovered again by Willebrord Snellius in Leiden in Holland in the early 17th century. Science history recognizes it the equation above as Snell's law, though it was known during the Islamic Golden age by Ibn Sahl and the famous optics theorist Ibn al-Haytham.
With just this simple index of refraction equation, you can calculate how lenses behave. Remember that a lens needs be curved. With this curvature, you can cause light rays to diverge or converge. Let's look at the simplest example, a ball lens.
Optics is all geometry, and the equation for calculating focal length based on the geometrical bending only requires we only know the diameter of the sphere (d) and the index of refraction of the material the sphere is made out of (n).
Returning to the parameters of the ball lens:
You see that either lowering d (diameter) and increasing n (index of refraction) lowers the effective focal length and increases the magnification. A strange property thus reveals itself. If you have an index of refraction greater than 2, the effective focal length never surpasses more than 1/2 diameter, or the radius. Since diamond has an index of refraction of 2.6, you actually cannot make a ball lens out of diamond! Such will never be focused, as the focus plane is actually inside the lens. Normal soda-lima glass has an index of refraction of 1.5, so the focal plane is outside the lens - Lucky for Leeuwenhoek! But how do we calculate the magnification of a ball lens? We use an equation based on the general lens equation:
A 5 mm diameter lens, which has a focal length of 3.75 mm, thus has a 67x magnification. The range of useful diameters is:
Now enough with such lovely history and theory, let's step away from our magic glass screens where we read of the accomplishments of others and build something of our own. We will construct a "re-imagination" of Antonie van Leeuwenhoek's microscope by melting clear glass, forming small spheres, and using our eyes to view worlds unseen.
Here are the tools with which you will build the first microscope. In the most basic form, you will need:
How to build the basic LeeuwenScope
- A high temperature flame - it can be a camping torch or a crème broûlèe torch.
- Solid Glass Filament, we use McMaster-Carr borosilicate glass filament but you can try with your own different glass filament, such as crystal glass (higher index of refraction, lower melting point) or soda lime glass (same index of refraction, lower melting point, more susceptible to fracture).
- Basic Metal Probe tools - we have found dental picks work the best.
- Basic Needle Noise Pliers, forceps, or a jewelry toolset.
- Support for the sphere lens
- Plain Glass Slide
How to Fabricate the Ball Lens
- Turn on the flame. Respect it.
- Take a filament of glass, hold it over the flame and pull the filament apart until you get two thin points. Make sure to use solid glass filament, not hollow glass.
- Take one half of your now two glass pieces, and push the tapered end into the flame until it forms a small spheroid end.
- With your forceps, break off the spherical end over a flame.
- Put your dental probe fine end into your glass bit over the flame.
- With patience, work the glass spheroid bit with the flame, forming it and forming it, using the heat and gravity as your friend, in your attempt to make a glass sphere. It should be as round as possible and without any bubbles and minimal embedded black residue.
- Once formed and cooled, please the sphere in a small hole in our 3D printer support (or alternatively a piece of cardboard). If the the glass sphere doesn't fit, use a small scissor blade to enlarge the hole.
- Now prepare a slide sample or look for a pre-prepared slide sample.
Prepare a simple slide
- Hunt or Buy an onion.
- Remove the outer dark skin. Cut the onion in half.
- With a tweezer, take a bit of clear onion skin. This skin layer is only one cell layer thick!
- Place the onion sample on a glass slide. You are now ready to look at it with your lens.
- If you have access to methylene blue, you can also view your skin cheek cells. Scrape the inside of your cheek with a tooth pick. Rub toothpick on glass side. Apply a drop of methylene blue. Soak up excess.
Using your Ball Lens to view the sample
- Place the holder with your embedded glass sphere up to your eye, as if you are looking through a ... microscope.
- Turn on a lamp and look at the lamp through the microscope. You need a light source.
- Bring your slide up to the other end of the ball lens. Note that the focal length is very short, 0.3-1.0 mm away from the lens.
- With patience and steady hands, the image should come into focus. Note that your ball lens needs to be at least 1 mm in diameter or greater, otherwise the focal length will be too short and too hard to focus for you.
Using the Leeuwenscope
- With our LeeuwenScope that you can buy or build, place your plastic holder with its ball lens in the support stand on the LeeuwenScope.
- Turn LED light on.
- Place your smartphone over the lens.
- Focus sample by turning the focus knobs. The depth of field is very narrow on ball lenses.
- Sample should come into focus on your smartphone, and you should be able to take a picture.
- Now find some pond water and see what Leeuwenhoek also saw! The greener the water, the better, and if there is plant debris is the water sample, even more so!
How is the quality of your ball lens compared to our industry ball lens and our RoachScope? See image comparison below.