Using the Microscopes

With difficulty, van Leeuwenhoek
used his microscopes to make
the world of microorganisms visible.

Ingenuity

Like so many of his countrymen, Antony van Leeuwenhoek was ingenious. The Dutch people of his time developed innovative:

uses for the windmill
designs for ships
methods of financing ocean voyages
ways of depicting light with pigment

During the 1600's, no one did those things as well as the Dutch.

Van Leeuwenhoek, the curious, clever draper from Delft, made his own innovations. He learned how to make and mount tiny glass lenses with minimal aberration that still had enough magnifying power and resolution to make his specimens visible. No one did those things as well as van Leeuwenhoek.

He looked at salt crystals or a pig's liver or a dragon-fly's eyes with lenses magnifying up to two hundred times. He was able to view and describe bacteria which were less than 2 µm in diameter. If the lens was small enough, round enough, and clear enough and turning the screws let the specimen come into focus, magnification and resolution were possible. Visibility, however, was a separate and more difficult problem.

Lower in this column:

Secrets | Cabinet of Wonders
Solutions | Specimen Preparation
Illumination | Additional Magnification

Visibility

The constraints of his design on visibility created problems that van Leeuwenhoek had to solve with his viewing techniques and specimen preparation methods.

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Secrets

There is much speculation about van Leeuwenhoek's secrets. The microscopes themselves were not secret. He showed them to people all the time. To grind his lenses, he did the same as any other lens grinder, only smaller. The other lens-making methods were no secret; Hooke's Micrographia, detailing the method of making lenses from drops of molten glass, was one of the best selling books of the time.

Yet early in his career, on October 9th, 1676, van Leeuwenhoek wrote to the Royal Society:

My method for seeing the very smallest animalcules and minute eels, I do not impart to others; nor how to see very many animalcules at one time. That I keep for myself alone.

Note that he says "method" and "very smallest". Nine years later, in 1685, Thomas Molyneux visited van Leeuwenhoek at the request of the Royal Society. His account, a letter to the Society's secretary Francis Aston, was read to the Society and recorded in Birch's History. Molyneaux was elected Fellow the following year.

Such were the microscopes, which I saw, and these are they that he shews to the curious that come and visit him: but besides these, he told me that he had another sort, which no man living had looked through setting aside himself; these he reserves for his own private observations wholly, and he assured me they performed far beyond any, that he had shewed me yet, but would not allow me a sight of them, so all I can do is barely to believe, for I can plead no experience in the matter.

Dobell gives these secret methods several pages of guarded speculation (pp. 330-332). Many short biographies of van Leeuwenhoek seem to mention the "secrets" as a dash of romantic flair or to portray him as a hopeless amateur too shy to communicate with real scientists.

There's a related question. Van Leeuwenhoek left hundreds of little microscopes, complete with mounted lenses, as well as many more mounts without lenses and many lenses without mounts. All but a few are now lost, but from Folkes's and Baker's descriptions after his death as well as descriptions from visitors during his life, it seems that van Leeuwenhoek made a microscope for every specimen, or at least those he wrote about. Why?

After he finished, he stored them, two to a box. Of course, with this method, he could refer to them again, he could show them to visitors, and he bequeathed more than two dozen to the Royal Society. But perhaps there was another reason.

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Cabinets of wonders and curiosities

With our industrial, high-tech, science-as-expensive-business perspective, we must make an effort to understand that the world van Leeuwenhoek was revealing was new and unprecedented. He had one foot in the world of spirits, crafts, guild secrets, and alchemical mysteries. And the other foot in the world of mechanical laws, mathematical proofs, and unbelievably large numbers. Galileo was also secretive about his methods, and his results were also hard to reproduce at first. Van Leeuwenhoek's contemporary Isaac Newton (1643 – 1727) was an alchemist.

Just as his countrymen collected cabinets of curiosities gathered from around the Earth, van Leeuwenhoek collected cabinets of curiosities gathered from his body, around his house, and from the canal in front of his house. Without the microscope accompanying it, his collection to the naked eye was just water in a thin tube or a bit of dried animal tissue, not very interesting in themselves. However, with the microscope, van Leeuwenhoek's cabinets of wonders were visible to anyone. Or at least those who had patience and good eyesight.

In one of his last letters (1722), he wrote:

Whenever I make any Discovery, which I apprehend will not easily meet with Credit, I suffer the Object to lie before the Microscope Day after Day, and sometimes for whole Years together, till it is eaten up by Insects. This I do with design to let it be seen by as many different Persons as possible.

There was no standardization in 1722. There were no boxes of interchangeable parts for van Leeuwenhoek to reach into. Each lens was different. A given lens might work well for one specimen yet be too strong or too weak or too aberrant to see what he wanted to see with the next specimen.

His lens-making skills were also evolving. He could only make them so small. Since he saw bacteria early in his career, and never saw anything smaller, he reached the size limit early on. However, by continuing to make lenses, dozens and dozens of them, he continued to make them better, which means clearer glass and more regular surfaces rather than stronger (smaller).

And finally, he kept evolving his methods of observation. It was his art, and there was no more reason to reveal it than for Vermeer to reveal the "secrets" of his use of perspective or his recipes for color pigments. But if you had van Leeuwenhoek's years of patient use, then the secrets would reveal themselves to you. In a letter published in 1720, he wrote:

Nor should I ever have attained thereto, but by continual Labour in the investigation of things, which are concealed from our naked Eyes, and towards which I have a much greater inclination, than what I observe in most other Men.

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Solutions to the problems caused by the constraints of his design

Van Leeuwenhoek never discussed his techniques at length. We have four sources to help us understand. See the Bibliography page for more information.

references to technique by van Leeuwenhoek scattered through the hundreds of letters

written comments by contemporaries, chiefly Robert Hooke

recent attempts by Dobell, Cohen, van Zeulen, and others to replicate van Leeuwenhoek's microscopes and techniques

recent analysis by Brian Ford of actual van Leeuwenhoek specimens

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Specimen preparation

Given the size of the surviving specimens, to say nothing of the tiny size of the microscopes, van Leeuwenhoek looked at bits of things: plant parts, pond water, and animals' internal organs. If they couldn't be impaled on the pin behind his lens, he stuck them there with spit or glue.

Other things, he would let dry or leave damp on a bit of glass, and then glue the glass to the pin.

These specimens were dead. But the smaller specimens, the animalcules that we now call protozoa and bacteria, were living and moving and reproducing and dying. AvL not only had to make them visible, he had to keep them visible long enough to observe their life cycles.

The smallest sort of which I shall here speak, I see alive and exhibit as plainly to my eye as one sees, with the naked eye, little flies or gnats sporting in the air, though they may be more than a hundred million times less than a coarse grain of sand; for not only do I observe their progression, both when they hurry, and when they slow down, but I see them turn about, and stand still, and in the end even die.

Some of van Leeuwenhoek's specimens were transparent or thin enough to pass or transmit light through them. They either came that way or he sliced a thin-enough section. The section that survives and Brian Ford examined is "sliced with a razor" to .2 mm thick, good even by today's standards.

For everything else, van Leeuwenhoek relied on light reflected off the specimen. Many of his specimens were living, moving creatures. Others were anatomical structures whose three-dimensionality would be destroyed by slicing into sections.

Late in his career, van Leeuwenhoek had some muscle tissue that he wanted to see better. He stained it with saffron, and was able to see details that he could not see before, but this does not seem to be a technique that he used before or after.

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Illumination

For van Leeuwenhoek, more light was not always better. Early in his career (June 1674), he wrote to Oldenburg:

You then hold the microscope towards the open sky, within doors, and out of the sunshine, as though you had a telescope and were trying to look at the stars in the sky through it.

And later:

The instrument may be held within doors and in the shade, yet held to the free Air, as is with a Telescope you would look upon the Stars in the Firmament.

A quarter-century later, in June 1699, he wrote:

Above all things you must have a care, not to make your view in the Sunshine, for if you do so, the Circumference of each Animal, will have almost as many Colours, as we see in the Rainbow

The complete quotation is in the right-hand column of this page.

Was the specimen transparent? Then he used transmitted light -- more light from behind and a smaller aperture.

Was the specimen opaque? Then he used reflected light -- more light from the sides and a larger aperture.

After van Leeuwenhoek's death, the Royal Society's Henry Baker wrote (p. 518):

Mr. Leeuwenhoek says ... that sometimes, to throw a greater Light upon his Objects, he used a small convex Metal Speculum.

Baker was probably referring to what van Leeuwenhoek wrote on June 9, 1699:

To have still more light, I use sometimes a metal Concave Looking glass.

He could have polished the bronze plate around the lens to direct light back onto his side of an opaque specimen. He could also have placed a thin mirror or white paper on the bronze plate to reflect light back on the specimen. Baker continues:

He had filed the Brass which was round his Microscope, as bright as he could, that the Light, while he was viewing Objects, might be reflected from it as much as possible.

He tried microscopes of silver perhaps because, polished, they would reflect light onto the specimen better than bronze. He made so few microscopes of gold perhaps because, not reflecting any better than bronze, they were not worth the expense.

Was the specimen hard to distinguish? To increase the contrast, he manipulated it against a dark background; later in his career, he once tried a saffron stain.

Both Dobell and Cohen speculate about the potential of dark-field or dark-ground illumination. Through decades of using his instrument, van Leeuwenhoek may have found methods to compensate for the transparency and low optical contrast of many of his specimens. Shining light on the specimen from the side while pointing the instrument toward a dark background would create a good-enough dark field illumination effect, as would slowly moving a finger or stick just off the centerline between the light source and the scope and then slowly moving it back towards the center. Think about how the dust in a room, dancing in the sunlight, can be made visible by viewing it from the side against a dark curtain or piece of furniture.

What about aberrations? The way van Leeuwenhoek used his microscopes, he created an aperture by mounting the sphere behind a slightly smaller hole in a metal plate. With an aperture smaller than the lens, the microscope had fewer problems with spherical aberrations. And with the eye so close to the lens, the chromatically aberrant edges were off to the periphery of his vision.

Thomas Molyneux's report to the Royal Society refers directly to van Leeuwenhoek's "reflective specula" as well as "secrets":

As for the microscopes I looked through, they do not magnify much, if any thing, more than several glasses I have seen, both in England, and Ireland: but in one particular, I must needs say, they far surpass them all, that is in their extreme clearness, and their representing all objects so extraordinary distinctly.

For I remember we were in a dark room with only one window, and the sun too was then off of that, yet the objects appeared more fair and clear, than any I have seen through microscopes, though the sun shone full upon them, or though they received more than ordinary light by help of reflective specula or otherwise:

So that I imagine ’tis chiefly, if not alone in this particular, that his glasses exceeds all others, which generally the more they magnify, the more obscure they represent the object; and his only secret, I believe, is making clearer glasses, and giving them a better polish than others can do.

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Additional magnification

While we always think of van Leeuwenhoek as working with a single magnifying lens rather than the system or two or more lenses in a microscope, his methods of using this lens could have introduced another lens.

In addition to the lenses themselves, he wrote in one of his first letters, September 1674, that he blew:

very slender hollow glass pipes, of which some were not thicker than a mans-hair; and the slenderer they are, the clearer will they make the red Globuls of the Blood appear.

The thin glass tube containing the liquid could further magnify the specimen, as could an air bubble, creating in just the right lateral light a dark background that van Leeuwenhoek compared to "sand-grains on black taffeta."

Birch's History of the Royal Society records three meetings, November 1 and 8, 1677, and March 14, 1678, when Robert Hooke, defending van Leeuwenhoek, suggested ways from his own experience that van Leeuwenhoek could have used to achieve results that members of the Royal Society doubted.

Hooke started with thin glass pipes, some ten times thinner than a human hair (my emphasis).

The discoveries, affirmed to be made by Mr. LEEWENHOECK, were made by help of viewing with a good microscope such small pipes containing the liquor or water, in which those multitudes of exceedingly small insects or animals wriggling among each other are discovered;

for that he alledged, that the said pipes being filled with liquors became themselves as it were magnifying glasses, augmenting such bodies, as swim in the said liquor, on those parts of the said pipes, which are farthest from the eye-g1ass;

for the pipes themselves being looked on by the help of a very good microscope, are made very large and conspicuous; and they again augmenting the opposite parts by the refraction on their cylindrical surfaces double the effect of a single microscope, as was very evident.

A week later:

Mr. HOOKE suggested same farther improvement of that instrument by making use of the convexity of the surface of the liquor itself (put upon the plates of Muscovy glass) for augmenting the body within the liquor.

At the March meeting, Hooke read part of what he would publish later that year in Microscopium about his own work with grains and seeds steeped in water (aka infusoria):

[Hooke] discovered the several ways and contrivances by which he made those observations; and therein shewed how easily and apt such persons are to be deceived by the appearances of these transparent bodies through a microscope, who are not aware of certain properties of transparent bodies, especially such as are peculiar to substances of such small bulk.

And for the avoiding and preventing all these inconveniences, he shewed several ways and expedients, without which no true discovery could be made, and by the help of them they were very easily made.

Some of those mentioned by him were glass plates, and plates of Muscovy glass, particular kinds of light, the immersing the bodies in waters and other liquors, the squeezing bodies between two glass plates, the stretching and squeezing others with a kind of tongues, etc. whilst they are looked upon in a convenient light by the eye.

Conclusion

On the other side of that lens, AvL held the microscope as close to his unblinking eye as he could, resting on his cheek or forehead if held vertically, or on his temple if held horizontally.

He angled himself into the light, and he concentrated. He turned a little left, a little right, carefully controlling the angle of illumination to optimize image contrast. He turned the bottom screw to raise the specimen, then the side screw to move it just a little farther away.

On the close inspection of 3 or 4 drops, I may indeed expend so much labor that the sweat breaks out on me.