Leeuwenhoek's Microscopes
A Dead-end Design
His microscope was a superior design because it solved his problems better than the alternatives. It was a dead-end design because it was too hard to make and much harder to use than the double-lens microscope.
Let's go back to the late 1660's...
Our friend Antony has made some very small glass beads, almost spherical. He learned to grind little shards of glass until they were barely a millimeter or two in diameter, not much larger than the thread end of a common sewing needle. He polished them just the right amount so that they would be smooth but still keep their uniform hemi-spherical shape on each side.
Now what does he do?
What problems did Leeuwenhoek have to solve in order to use his lenses?
How to ...
- hold the lens
- keep the specimen within the very short focal length
- focus the specimen
- retain the focus
- light the specimen
- keep his hands free
See the summary on the page "Dead-end design" (right sidebar) to learn how his microscope design solved these problems.
As a surveyor, Leeuwenhoek knew the usefulness of low-power telescopes to see distant landmarks. As a cloth merchant, he knew the usefulness of low-power magnifying glasses to count threads. These thread-counters (dradentellers; image on right) magnified three to five times.
What were his options?
Logically, he had few options:
- hold the lens in one hand, the specimen in the other
- fix the lens and hold the specimen
- fix the lens and the specimen and make them both adjustable
Models to follow: microscopes in the mid 1660's
What compound (multiple-lens) microscopes in the late 1660's could have served as models for Leeuwenhoek?
The style of microscope that Galileo used had a lens at each end of a tube and one in the middle. The style of microscope that Robert Hooke used had a lens at each end of the tube. It also had a removable lens in the middle of the tube. Hooke solved part of the lighting problem by positioning a burning glass to focus the light from an oil lamp onto the specimen.
These microscopes magnified, at most, 50 times. (source?) The Related pages listed below explain these microscopes in more detail.
Leeuwenhoek's bead of glass was small enough that it magnified far more than that, with good resolution. But it had only one lens with a very short focal length, so these compound microscopes were not helpful.
Single-lens microscopes
What single-lens microscopes in the late 1660's could have served as models for Leeuwenhoek?
A single-lens microscope is just a very small magnifying glass. Magnifying glasses were well known; in fact, Leeuwenhoek probably used one in his linen shop. The difference is the size of the lens and what it reveals, the questions it answers. People using low-power (3x to 10x) magnifying glasses tend to ask questions only about things they can see with their eyes.
Amsterdam mayor Johannes Hudde was apparently the first person to use a single lens that was so small he could see things he could not see with his eyes, microscopic things. We have references to this microscope and the time is right, the 1660's. Unfortunately, we have no description or image of Hudde's microscope.
Jan Swammerdam also used a single lens microscope. The sketch above right is from his March 1678 letter to Melchisedec Thévenot, who also corresponded with Leeuwenhoek a decade later. The recent rendering of Swammerdam's microscope with a graphics program on the left (by Michael W. Davidson for Molecular Expressions) gives a better idea of what it may have looked like.
Swammerdam visited Leeuwenhoek in the 1670's. We don't know whether a few years earlier, Swammerdam could have showed one of his devices to Leeuwenhoek while he was developing his solutions. On the other hand, Leeuwenhoek could have shown his to Swammerdam, who then made his own version without Leeuwenhoek's complicated system of screws for positioning, focusing, and braking. As Swammerdam drew it, at least, it worked best for liquids in capillary tubes.
A more likely model for Leeuwenhoek's design came from Robert Hooke.
What did Leeuwenhoek choose?
In Micrographia (1665), Robert Hooke told how to make a glass bead lens over a candle flame. Then he told how to mount the lens.
if one of these be fixt with a little soft Wax against a small needle hole, prick’d through a thin Plate of Brass … or any other Metal, and an Object, plac’d very near, be look’d at through it, it will both magnifie and make some Objects more distinct then any of the great Microscopes.
This is the design that Leeuwenhoek used. The first letter he sent to the Royal Society in April 1673 extended Micrographia's observations of mould, the stinger of a bee, and an eye of a bee in the same order. That makes it highly likely that Leeuwenhoek was familiar with the large-format, magnificently illustrated book, a best-seller at the time.
In fact, Hooke's description addressed only one of Leeuwenhoek's problems: how to hold the lens. The phrase "placed very near" does not indicate how to accomplish this placement.
It is a large leap from Hooke's cursory description to the microscope on the right, the 167x silver microscope in the Deutsches Museum, Munich.
While Leeuwenhoek did not invent the microscope, he did invent the system of screws to solve some of the other problems. We do not have access to his trial-and-error design process. However, by 1673, Leeuwenhoek was using such a microscope. It worked well enough that he stayed with this same design for the next half-century, the first, last, and only person to publish observations made with such a device.
Leeuwenhoek was the world's first microscopist, not to be equaled until the nineteenth century. The range and quality of his work is awe-inspiring. He was at the very beginning of a new technique that could be applied in many fields. He had to feel his way, and describe the undescribed.
Very few could follow him, mainly because of the technical state of the instrument and sheer lack of understanding in others, because microscopy takes the human consciousness into an entirely new world, as with a newborn child.
G. L’E. Turner, Museum of the History of Science, Oxford
