Fish viewers

In the lower left is what van Seters called a fish viewer (viskijker). The view on the left shows the two copper arms for adjusting a single lens. The view on the right shows the slimy fish wrapped and held by a copper strip against a plate of glass, the darker gray rectangle. The images below come from Uffenbach's travel journals.

In a letter of January 14, 1710 to the Royal Society (AB/CL 282; not yet available online), he wrote:

I intended to conclude here, but I thought fit to add the following: how I described previously a method for seeing the circulation on the blood in the eel, by putting the eel in a glass tube, a method which some years ago I rejected. And I have now had prepared pieces of a copper plate which are about one foot long and seven inches broad caused them to be bent at one end the breadth of one inch, and in those plates, close to the other ends, I had a quadrilateral hole cut out, which was five inches long and at least two inches broad. And in the cut-out copper I had glasses placed, of glass as clean and thin as I could obtain.

On one of those glasses I put an eel of the smallest kind, and even if it was one finger thick or more, if I could not get another, after I had first wrapped the greater part of the body of the eel in a cloth, so that it should not see anything; and it should therefore lie still the sooner on the glass and the copper. And I put it on the copper with the tail before the glass, which is achieved finally after much wriggling of the eel. And attached to the copper is also a copper spring, which is placed over the cloth in which the eel lies, in order that the eel should not drop off the copper.

Now while the eel is placed on one side of the glass, on the other side before the glass stands the magnifying glass, which can be put by means of copper springs and screws close to the glass, which one can move the magnifying glass as much up and down and to every side as one likes. And one must take care that the magnifying glass does not come into contact with the glass against which the eel lies, in order that the magnifying glass is not damaged. And by proceeding in that way one can discover more than if the eel lay in a glass tube.