The Iselin family archive, Basel
Finding a private archive – Comment
Unfortunately, you do not find the bequest of Isaak Iselin in the two catalogues. Only a single picture of his house in the St. Alban–Graben is filed in the Staatsarchiv. The name «Iselin» nevertheless appears a couple of times in the catalogues. This is not surprising, given that it was one of the most renowned and influential families in the history of Basel. Interestingly, the name «Isaak Iselin» appears more than once, implying that there was more than one person with this name.
The most famous of Isaak Iselin–Roulet's namesakes was a relative who lived from 1728 to 1782. As a philosopher and historian of the enlightenment, this Isaak Iselin is foremost known for his Geschichte der Menschheit (History of Mankind) (1764). Both the Staatsarchiv and the University Library hold primary materials concerning his life.
Asking the staff of the Staatsarchiv about materials concerning Isaak Iselin–Roulet (because this is always a good idea), you are told that a private archive of the Iselin family exists. You get the telephone number of the owner.