Ned Block links to these two additions to _Philosophical Investigations_ on the subject of ‘fog’. Both of them are quite old, the Frayn apparently from the 1960s, and Fodor’s perhaps from around 1970. Anyway, for weekend amusement.

* “Michael Frayn’s additions”:http://stevepetersen.net/personal/wittgenstein-fog.html
* “Fodor after Frayn”:http://www.nyu.edu/gsas/dept/philo/faculty/block/miscellaneous/Fodor_Wittgenstein.png

The Fodor page is a graphic, and it might be easier to read zoomed in.