Experiment: The Leeuwenhoek Microscope and the Beginning of Our View into the Small
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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 microscopes.

Time 30 minutes
Difficulty Beginner

What will you learn?

You will learn the history of the microscope in Northern Europe during the enlightenment and the physics of how balls lenses function. You will learn how 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.

Prerequisite Labs

  • You are starting from zero. You are beginning the field of microscopy. There are no previous experiments to do.


Background

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 as 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.

In 1664, a 29-year-old Robert Hooke was commissioned by the Royal Society of England to write and publish "Micrografia – Or some Physiological Descriptions of the Minute Bodies Made by Magnifying Glasses With Observations and Inquiries Thereupon" Using a compound microscope, he made a famous observation of a slice of cork, showing that the tissue of the plant was made up of individual elements he called "cells," after their appearance to the cells of honeycombs.

Robert Hooke was an excellent inventor and polymath (he is also the Hooke of "Hooke's Law" concerning the force on springs, and did important work along with Galileo and Huygens verifying the rings of Saturn, thus he moved on to other investigations after he published Micrografia and, so far as we know, 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 began fabricating small glass spheres developed a metal casing for the spheres in a deceptively simple and elegant design for viewing samples at multiple angles, with multiple screws to change the position, orientation, and focus of the sample.

Leeuwenhoek would then stare through the sphere in bright daylight, and, one day, viewing a sample of pond water, beginning in 1674, he observed things moving which he called "animalcules." This was the first documented view of the 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 also 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 English Royal Society, where are stilled preserved and archived in London.

We at Backyard Brains are 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 approaches the material at an angle, when it is perpendicular (normal) to the material it will not bend). The curved form of a lens allows the bending to either "diverge out" or "converge in" depending on the shape of the lens.

We often think of the speed of light as a constant that can never be surpassed, but light actually travels at different velocities depending on the material in which it is passing through. Because of this difference in speed of light between two materials, and given light's peculiarities, when a ray of light, traveling in vacuum or air, encounters a new material, the angle will change so that light "spends less time" in the material. This level of bending is defined as an "index of refraction."

And moreover, the higher a material's index of refraction, the more the light ray bends.

This mathematical relationship between an "index of refraction" and the angle of deflection of a light ray 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 with curved mirrors and glass lenses to allow localized increases in temperature and flames. The law was independently discovered again by Willebrord Snellius in Leiden in Holland in the early 17th century. Science history recognizes it 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 the 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 curve. Let's look at the simplest example, a ball lens.

You can see there are three measures: the diameter, the effective focal length, and the back focal length. The back focal length is simply the difference between half the diameter (radius) and the effective focal length. Optics is all geometry, and the equation for calculating focal length simplifies to:

But how does the lens actually enlarge the image and cause the magnification. See the image below, and you will recognize why the curvature of the lens is the fundamental key. Note: The two drafts below are being improved and will be updated soon!

Now let's look at the parameters of the ball lens again.

You can know see that the only two variables in the equation are n and D. The lower D (diameter), the lower the effective focal length, and the higher the n, or index of refraction, the lower the effective focal length as well. 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, and the focal plane is outside the lens - Lucky for Leeuwenhoek! But how much is the magnification of a ball lens? We can calculate it using an equation based on the general lens equation:

A 5 mm diameter lens, which has a focal length of 3.75 mm, thus a 67x magnification. The range of useful diameters is:

The smaller the lens, the more the magnification! The caveat is that the focal length starts becoming unmanageable around 1 mm. At 1 mm diameter, the focal length is 0.75 from the center of the sphere, or 0.25 mm outside the lens! This is reaching into the thickness of cover slip glass (~0.2 mm), and it will appear you will never be able to focus your sample. At 2 mm diameter lens, the focal length is 0.5 mm outside the lens, and at 5 mm, you have an even more comfortable 1.25 mm focal length outside the edge of the sphere, but of course, you magnification is less.

Now enough with suck lovely history and theory, let's step away from our magic glass screens where we read of the previous accomplishments 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.

Video

Procedure

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

  1. A high temperature flame - it can be a camping torch or a crème broûlèe torch.
  2. Solid Glass Filament, we use McMaster-Carr borosilicate glass filament but you can try with your own different glass filament, such as crystal glass (high index of refraction, lower melting point) or soda lime glass (same index of refraction, lower melting point, more susceptible to fracture).
  3. Basic Metal Probe tools - we have found dental picks work the best.
  4. Basic Needle Noise Pliers or a jewelry toolset.
  5. Support for the sphere lens
  6. Plain Glass Slide
  7. Onion

How to Fabricate the Ball Lens

  1. Turn on the flame. Respect it.
  2. 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.
  3. Take one half of your now two glass pieces, and push the tapered end into the flame until it forms a small spheroid end.
  4. With your needle nose pliers, break off the spherical end over a flame.
  5. Put your dental probe fine end into your glass bit over the flame.
  6. 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 or embedded black residue.
  7. 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.
  8. Now prepare a slide sample or look for a pre-prepared slide sample.

Prepare a simple slide

  1. Hunt or Buy an onion.
  2. Remove the outer dark skin. Cut the onion in half.
  3. With a tweezer, take a bit of clear onion skin. This skin layer is only one cell layer thick!
  4. Place onion sample on a glass slide. You are now ready to look at it with your lens!
  5. 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

  1. Place the holder with your glass bead up to your eye, as if you are looking through a ... microscope.
  2. Turn on a lamp and look at the other end through the microscope. You need a light source.
  3. 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.
  4. 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

  1. With our LeeuwenScope that you can buy or build place your plastic holder with its ball lens in the support stand on the LeeuwenScope.
  2. Turn LED light on.
  3. Place your smartphone over the lens.
  4. Focus sample by turning the focus knobs. The depth of field is very narrow on ball lenses.
  5. Sample should come into focus on your smartphone, and you should be able to take a picture.
  6. 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.

Happy exploring the previously invisible worlds!

Notes

  • You can see the microscope that Robert Hooke used for his studies at the National Museum of Health and Medicine in Washington, D.C. as part of an exhibition - The Evolution of the Microscope. This museum also has the revolver that killed Abraham Lincoln.
  • Scans of Hooke's gorgeously drawn book are available online.
  • Leeuwenhoek's microscope are rare, and most museum exhibitions only show replicas. But...if you want to see original Leeuwenhoek microscopes, your road leads to the Boerhaave Museum in Leiden, the Netherlands. We have heard rumors that another is stored at the Delft University of Technology.
  • Scans of Leeuwenhoek's letters to the Royal Society are available online.
  • If you want to visit the resting places of the scientists mentioned here, Antonie van Leeuwenhoek is buried at the "Oude Kerk" (Old Church) in the small town of Delft. Willebrord Snellius is buried at the Pieterskerk church in Leiden. The resting places of Robert Hook and Ibn Sahl, to our knowledge, are unknown and have been lost to history.
  • The compound microscope that Hooke used should have been good enough to see at least unicellular organisms. Perhaps he never looked a pond water as Leeuwenhoek had? Or perhaps the optical quality was not high enough? We will not know short of looking ourselves at pond water with Hooke's microscope (we are trying to obtain permission).
  • The recent re-make of Cosmos, hosted by Neil Dygrass Tyson, has an episode talking about Ibn al-Haytham's work, who was a contemporary of Ibn Sahl. Go to time mark 8:20 of episode 5 "Hiding in the Light" (available on Netflix and iTunes).
  • If you want to dive into the original text and understand how Ibn Sahl derived the "Sahl's Law" (Snell's Law), you can read French scholar Roshdi Rashed's examination of Ibn Sahl's original Arabic text.
  • Paul de Kruif's famous 1926 book The Microbe Hunters has an excellent early chapter on Leeuwenhoek. The late chapters on 19th century disease science unfortunately propagate late 19th century racial stereotypes.
  • It is easier to make microscopes out of glass than out of mirrors, and it is easier to make telescopes out of mirrors than glass. Why is this? Think about the sizes and weights involved.
  • It is interesting to think that light travels 2.6x less when it is traveling through diamond, with its index of refraction of 2.6. Could we imagine a material with an infinite index of refraction? A Black Hole? Can we imagine a science fiction world with indices of refraction less than 1, or, gasp, less than zero?
  • How Leeuwenhoek built his microscope is still unknown and only hypothesized as he never revealed his techniques, typical of scientists of his day (also Galileo never revealed how he built his telescopes). They were not very open-source, but we try.
  • Science Fair / Research Project Ideas

  • Why did it take until the enlightenment for Europe to invent the microscope and telescope? A history examination into the development of glass shaping techniques would be a valuable endeavor. The Venetians made advancements in shaping glass and mirrors in the 1300s-1400s, which they militantly kept a trade secret. Glass had been manipulated since antiquity (Romans), but it appears glass clear enough to be used as corrective eyeglasses occurred in the 1300s in Italy. The first European telescopes were invented in Holland in 1608 by Hans Lippershey. Why you think it took so long between eye glasses and telescopes? If you can make eye glasses, you are almost there.....Our hypothesis is that truly "clear" glass was difficult to manufacture until the 17th century, and optic theory was still not understood well.

    Acknowledgements

  • This project was developed in collaboration with biologist Daniela Flores of the Chilean Science outreach group MicroMundo.

    References

  • John N. Davis. 2007. Measuring the magnification of homemade simple microscope lenses. Micscape magazine.
  • Howard Gest. 2004. The discovery of microorganisms by Robert Hooke and Antoni Van Leeuwenhoek, fellows of the royal society. Notes Rec. R. Soc. Lond. 58 (2), 187–201
  • Edmund Optics. Understanding Ball Lenses.
  • Roshdi Rashed. 1990. A pioneer in anaclastics: Ibn Sahl on burning mirrors and lenses. Isis, Vol. 81, No. 3. pp. 464-491.
  • Lesley Robertson. 2015. van Leeuwenhoek microscopes—where are they now? FEMS Microbiology Letters, 362, 2015, fnv056.
  • Maria Rooseboom. 1939. Concerning the optical qualities of some microscopes made by leeuwenhoek. Journal of Microscopy. Vol 59,3, Pgs 177–183.
  • J. Van Zuylen. 1981. The microscopes of Antoni van Leeuwenhoek Journal of Microscopy. Vol. 121, Pt 3, pp. 309-328