The parts

The classic Leeuwenhoek microscope has a dozen or more parts: a lens, two rectangular plates, an L-bracket, a mount (or stage), three screws, one threaded specimen pin, a nut for the braking screw, and up to six rivets to hold the body plates together.

The brass microscopes all are copper with about thirty percent zinc in it, which was the standard for brass before the mid-18th century.

For the silver microscopes, however, the purity is different for the plates, mount, and screws on any given microscope. The figure on the right shows the silver purity as measured by Tiemen Cocquyt and his curating team at the Boerhaave Museum of three of the extant microscopes: 248x, 80x, and 68x.

None of the gold microscopes has survived for us to test.

These differences suggest that Leeuwenhoek did not make them one microscope at a time. He made a batch of plates, a batch of screws, and a batch of mounts.

The plates

Instead of a single plate and wax to hold the lens, as suggested by Hooke, Leeuwenhoek used two plates of the same size riveted together.

The figure on the right shows the ranges and averages of the eleven surviving microscopes. The all have rounded corners and several taper as they get closer to the L-bracket end.

The plates had matching holes for the rivets that held them together. The rivets are almost as small as the lenses, about 1.5 mm to 2 mm. Every microscope has two rivets in the corners closest to the lens. Several have two in the middle on either edge. Some even have two in the corners closer to the L-bracket. Since the main purpose of the rivets was to keep the lens tight without breaking it, the two rivets in the bottom corners seem two too many, expecially since the braking screw had a nut to secure the plates in that area.

Four of the plates have extra holes such as the one on the 68x silver lens, the microscope on the far right of the three in the display of silver content above.

The mount and bracket

The exploded diagram on the right was adapted from Hans Loncke's 2007 excellent step-by-step instructions on how to make a replica.

Behind the plates was a mount, a pin screwed into a block of silver or brass. This mount was not attached directly to the plates. Its movement was 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.

The positioning screw could be screwed in and out for one direction. It could also be moved back and forth in the other direction by pivoting around a loosened braking screw.

The L-bracket held the mount and screws to the body via a screw with a nut that when tightened also served to freeze the L-bracket and thus the specimen's position behind the lens.

The screws

The microscopes had four single-thread screws:

• a longer positioning screw

• a shorter focusing screw

• a slightly shorter pin screw with a knob

• a very short braking screw with a thin nut

Good enough design

The design was unique. The parts are not interchangeable. Leeuwenhoek did not invent the microscope, as is often claimed. He did, however, invent this positioning system. It was the result of a trial-and-error process, of course, and all we see is the finished product.

Leeuwenhoek made them all by hand out of brass, copper, silver, and even gold. In his letter of January 12, 1689 (AV/CL 113) addressed to the Royal Society, he wrote (my emphasis):

I have said heretofore how I composed my instruments, which some people would have made far finer and more accurate. ... I have for many years made the tools that I needed for several matters. And that is why what I required for my use was only made a bit roughly by myself.

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. The design was good enough that Leeuwenhoek used it for over half a century of observations.

Martin Folkes (right; click to enlarge), vice-president of the Royal Society, in the year after 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.