Tiny Microscopes

Antony van Leeuwenhoek's single-lens microscope was not available in a store. He had to design and make his own, right down to the screws and rivets. Each was a one-off, a unique product, a palm-sized magnifying glass according to the design Hooke described in 1665's Micrographia. There was no need for a tube to change the relative position of multiple lenses. Instead, he focussed by moving the specimen.

Leeuwenhoek was the world's first microscopist, not to be equalled 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
in review of Palm and Snelders Antoni van Leeuwenhoek (1982)
Med Hist. Oct 1983; 27(4): 445–447

In the long tradition of pioneers and innovators, Antony van Leeuwenhoek's discoveries were only as good as his tool. His tool, a single-lens microscope, was not available in a store. Leeuwenhoek had to design and make his own, right down to the screws and rivets. Each was a one-off, a unique product.

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. When he made his microscopes, he did not use the little-telescope design of Galileo and Hooke.

He used the single lens microscope, really a palm-sized magnifying glass, according to the design Hooke described in 1665's Micrographia. There was no need for a tube to change the relative position of multiple lenses. Instead, he focussed by moving the specimen.

Early in his career, van Leeuwenhoek settled on a "good enough" design for brass and silver devices to hold his tiny spherical lenses. It had to solve these problems:

• how to bring the specimen into focus
• how to hold both dry specimens and liquids
• how to illuminate the specimen

He referred to it as a "Vergroot-glas", a magnifying glass.

Telescope lenses and even early microscope lenses had diameters measured at the scale of inches. Leeuwenhoek's tiny lenses were less than 2 millimeters in diameter, less than a tenth of an inch. It did not need sliding cardboard tubes; it needed, more than anything, something that would keep lens and the specimen at an unchanging but adjustable distance from each other. Once he finally got focus, he didn't want to lose it!

Focus

With most microscope designs, as with most telescope designs, the specimen stays still and the lens moves closer or farther. If the specimen is dry, flat, and fixed on an immovable stage, as with a modern glass slide, it can move in only two dimensions, controlled by hand when placing the slide or shifted later. The other dimension is controlled by moving the lens closer or farther. You can't see the back side of the object on the slide unless you turn the slide over.

Leeuwenhoek, however, especially early in his career, was often observing three-dimensional objects that either were moving or could be rotated on an axis. It made more sense to keep the lens fixed and to move the specimen. His microscope had to accomplish three tasks: magnification, resolution (separating the details), and visibility to the human eye. In 1699, he wrote:

But to mount such small glasses well, requireth a far greater judgment, than to make them.

The parts

• lens
• two plates with matching holes
• rivets
• bracket
• pin (with swivel handle)
• block (for focussing screw)
• screws

The image comes from Giorgio Carboni's June 2012 excellent step-by-step instructions on how to make one. Leeuwenhoek mounted his lenses between palm-sized plates. The widest of the surviving plates is 28 millimeters or 1.1 inches, and the longest is 47 millimeters or 1.9 inches. The matching plates, riveted together, had matching holes (light blue in the diagram) slightly smaller than the lens that they held.

Behind the plates was a pin on a block controlled by three screws, one for each dimension. The pin itself could be swiveled by the little handle to rotate the specimen around its vertical axis. These parts are not interchangeable. Leeuwenhoek made them all by hand out of brass, copper, silver, and even gold. The metal was soft so that after use, the screw threads would wear. This is one reason that Leeuwenhoek made so many of these tiny magnifying glasses.

Martin Folkes, vice-president of the Royal Society, in the year after van Leeuwenhoek died, described the microscopes that he bequeathed to the Society:

... a very small double Convex-Glass, let into a Socket, between two Silver Plates rivetted together, and pierc'd with a small Hole:

The Object is placed on a Silver Point, or Needle, which, by Means of Screws of the same Metal, provided for that Purpose, may be turn'd about, rais'd, or depress'd, and brought nearer or put farther from the Glass, as the Eye of the Observer, the Nature of the Object, and the convenient Examination of its several Parts may require.

The pin was fine for dry or sliced specimens, but not for liquids. Leeuwenhoek drew liquids into a thin glass tube, which had to be held or clamped directly behind the lens, or broken into a small enough section to glue to the pin.

According to the description of the lot of hundreds of microscopes sold after the death of Leeuwenhoek's daughter Maria, a few of the microscopes were wider and had two or three single lenses mounted side by side. One of these, with three lenses, is clearly visible in the 1686 portrait by Verkolje. It looks as though it has only one specimen pin. Perhaps it was for wide specimens.

After he developed this design and adjusted to the trade-offs inherent in its differences with Hooke's design, he stayed with it for fifty years. He put his energy into improving the lenses.

The shape of the plates and the stage, the angle of the main screw, the composition of the plates, and the number and placement of rivets were a little different each time because each van Leeuwenhoek microscope is a one-off, a unique product.

Short focal length

The greater the magnification, the smaller the lens and thus the closer the focal length, that is, the distance from the lens where the object will be in perfect focus. The focal length of a spherical lens is just a little more than its radius but much less than its diameter.

Thus, for Leeuwenhoek's strongest lenses, the specimen had to be .9 millimeters, less than 4 hundredths of an inch, away from the lens. That led to Leeuwenhoek's final design problem: how to light the specimens enough to make the simple microscope useful.

Leeuwenhoek on ...