Eel viewer - showcase for visitors

The pin on Leeuwenhoek's simple microscopes did not work for large living specimens. For example, Leeuwenhoek's showcase for visitors was a demonstration of the blood circulating through the capillaries in the tail of an eel. The eel had to be living if the blood were to circulate and a living eel would not stay still on the edge of a pin.

Leeuwenhoek modified the design of his microscope and referred to it as an aalkijker, or eel viewer. The photo below shows the only surviving example, now in the Boerhaave Museum in Leiden.

Very early in his career, March 23, 1677, Leeuwenhoek wrote about what he does "whenever I wish to show the animalcules to some curious person or other".

This spring system was for observing infusions. What about much larger animals? Leeuwenhoek wrote about a similar observation of tadpoles on September 7, 1688, by which time he had developed the eel viewer on the left that seems to be a successor to what he descibed in 1677:

A sight presented itself more delightful than any mine eyes had ever beheld; for here I discovered more than fifty circulations of the blood, in different places, while the animal lay quiet in the water, and I could bring it before my microscope to my wish.

For I saw not only that in many places the blood was conveyed through exceedingly minute vessels, from the middle of the tail towards the edges, but that each of the vessels had a curve or turning, and carried the blood back towards the middle of the tail, in order to be again conveyed to the heart.

For the curious who came to visit, this observation was easy to explain and, finally proving Harvey's famous conjectures, it was an important observation. In addition, it could be easily seen with a relatively low-powered lens without the patience and rigor needed for viewing protozoa. People could relate it to their own bodies without having to accept anything too unusual or too numerous.

Leeuwenhoek wrote what he may well have told his visitors:

If we now plainly perceive, that the passage of the blood from the arteries into the veins of the tadpole, is not performed in any other than those vessels, which are so minute as only to admit the passage of a single globule at a time, we may conclude that the same is performed in like manner in our own bodies, and in those of other animals.

Even though he wasn't seeing a capillary as a separate structure, he understood the function of transporting the same blood:

Hereby it plainly appeared to me that the blood-vessels which I now saw in the animal, and which bear the names of arteries and veins, are, in fact, one and the same; that is to say, that they are properly termed arteries so long as they convey the blood to the furtherest extremities of its vessels, and veins when they bring it back to the heart. And thus it appears that an artery and a vein are one and the same vessel prolonged or extended.

In January 1689, Leeuwenhoek wrote another letter to the Royal Society describint the eel viewer and its construction. All of the figures for that letter were on one plate, below.

Faced with the uncertainties of publication by the Royal Society and the fact that most of his letters were not getting published by them anymore, Leeuwenhoek began in 1684 to publish his own letters, first as pamphlets with several letters, then beginning in 1688 collecting 165 of them, most of them never published in Philosophical Transactions.

He had only only three letters printed as their own self-contained piece. The last of them was his letter to the Royal Society on September 7, 1688, quoted above and titled Den Waaragtigen Omloop des Bloeds, On the True Circulation of the Blood. The images accompanying it are below, illustrating the experiment that the aalkijker demonstrated for van Leeuwenhoek's visitors.

This letter was never published by the Royal Society. It was self-published by Leeuwenhoek in 1688. We do not know how many of them Leeuwenhoek printed and gave out to visitors.

The three images above are rotated 90-degrees from the original for display.	The two on the left are tadpoles and the one on the right is an unspecified fish.	They illustrated the showcase experiment with the aalkijker that van Leeuwenhoek presented to visitors.