Isbell envelope

Dear categorists, One of the more folklorish constructions in category theory is that of the Isbell envelope. The folklorishness, in this case, seems to be so severe that I cannot find mention made of it in any published article at all (though there are several to the related notion of Isbell conjugacy). I am writing, therefore, in the hope that this is only due to my own poor knowledge of the literature, and that some other reader of this list may be able to put me to rights. Richard [For admin and other information see: http://www.mta.ca/~cat-dist/ ]