Aquatic microscopes and lenses

Leeuwenhoek stuck dry specimens onto the pin behind the lens. He sometimes glued them. He noted that, as did Martin Folkes when he discussed the 26 microscopes that Leeuwenhoek bequeathed to the Royal Society.

But what about liquids?

He blew very thin tubes, called capillary tubes after 1710 (Google Ngram). Into them, he put infusions and fluids from plants and animals: sap, blood, semen, etc. He broke off a short section and glued that to the specimen pin, sometimes with turpentine.

What about live animals?

For example, in order to study the circulation of blood, Leeuwenhoek needed to view it while it was circulating, thus while the animal was alive and its heart still pumping. An oyster or an eel or a fish would not fit into a capillary tube. The tube had to contain all or almost all of the specimen, which would make it far too large and heavy to glue to the specimen pin.

Not only that, but he needed to be viewing a part of the specimen that light could shine through, like the tail. Finally, it had to be a part where the blood made the turn from the arteries through the capillaries to the veins. Again, the tail met that need.

The specimen thus went head first into the tube, as shown in the image on the right. The lens needed to be near the top of the tube. The tube and the lens needed to be at a distance that was very close as well as finely adjustable. Apparently not one to keep secrets, Leeuwenhoek published detailed instructions about how to make this device along with a detailed drawing (below). The instructions were in his letter to the Royal Society of January 12, 1689 (AB/CL 113), which was a follow-up to the letter of September 7, 1688 (AB/CL 110), also to the Royal Society. The Royal Society published neither of them but Leeuwenhoek did, the earlier letter in 1688 as a separate pamphlet and the later letter in 1689 in his second volume of collected letters.

An excerpt from the instructions:

I further prepared a small piece of brass that was more than twice as thick as the aforementioned brass, with the intention that I should screw this brass firmly on to the foregoing instrument, as well as attaching also the magnifying glass to it, as is shown here by fig:+ 5. NOPQRST.

I further drilled two holes V. and W. in it, in such a way that they were so far from each other that they corresponded to the holes in fig: 2. with O. and N. In these holes V. and W. I made a screw, or fixing, to be able to screw the said brass piece on to fig: 2.

In addition I drilled a third hole in the same brass piece, in which I also made a screw, and which hole would only serve, by means of a screw that would go into it, to set the magnifying glass at its proper distance, as indicated by X.

By January, Leeuwenhoek had already published the September letter himself as a separate pamphlet, Den Waaragtigen Omloop des Bloeds, describing what he saw that demonstrated the circulation of the blood. The follow-up in January was also sent to the Royal Society even though they had not published new numbers of Philosophical Transactions since 1686 and Leeuwenhoek had already begun his self-publishing program.

At some point in the mid-1680's, the ever-resourceful Leeuwenhoek found some solutions to his problem of viewing live specimens. He built a metal frame to hold the tube. To hold the tube tight, he fashioned a two-part C-shaped spring with a groove on each end to slip partway around the tube. Opposite it, the frame held a smaller frame into which he could slide a lens in a brass holder. The smaller frame, near E and F in fig. 9 below, held the lens very close to the larger tube. For focus, the screw at H made that distance adjustable. Because the tube had to extend through the bottom of the frame, it is not clear how he kept his hands free.

It turned out that he needed two versions of these viewers because he needed a range of larger tubes.

Eel viewers and oyster viewers

Following the auction catalogue description, van Seters called them oyster viewers (oesterkijkers) and eel viewers (aalkijkers). The C-spring that would hold an oyster viewer would be too strong for an eel viewer and might crack it. The spring that would comfortably hold the eel viewer would not secure the oyster viewer.

The only screw on these viewers is near the top right of the photo on the right. It looks as though it could easily be replaced when the relatively soft brass or silver threads wore down.

The one on the right, currently in the Boerhaave Museum in Leiden, was probably not made by Leeuwenhoek. The frame has two vertical supports, not one; the C-clamp is in one piece; the focusing screw is on the other side of the tube. But it was certainly made according to the design in his letter of January 12, 1689. Leeuwenhoek did not invent the microscope, but he did invent this device to view aquatic specimens.

The auction catalogue listed 11 oyster viewers; Leeuwenhoek used them for observations recorded in half a dozen letters between 1695 and 1702. The catalogue listed ten of the larger eel viewers.

The lens went into the empty circle at the top left of the photo of the Boerhaave eel viewer.

The catalogue lists 172 of these lenses. The five picture below (click to enlarge) fit the Boerhaave eel viewer and came in a red leather case. Yes, they were all unique and Leeuwenhoek did not, as far as we know, calibrate them. So he must have used a trial and error method match lens with specimen. The question remains:

Why did Leeuwenhoek need so many?