The Street of Signs - Copenhagen 1930

In his letter to Anna, Oskar describes the atmosphere and culture of Niels Bohr's Institute, the "foster home" of the fledgling quantum mechanics. 

The Institute seems ordinary enough from the outside; there is nothing to distinguish it from its neighbors, nothing to warn a passerby of the madness that reigns within these walls. Often I think we should follow the custom of Copenhagen’s shopkeepers and hang up a metal sign above the front door; but what symbol would we employ? The pet store displays the silhouette of a parrot, grapes announce the liquor store, a pastry dangles above the bakery, and a watering can and kettle are suspended above the metalworker’s shop: each simple image announces its occupant’s vocation and business with clarity. Part of the trouble with quantum mechanics is that we don’t have such a symbol yet. The world that exists this side of the threshold is surreal, but it has a strange logic all its own—unexpected, but self-consistent.

There's a charming little video on YouTube (courtesy of British Pathe) that shows the signs Oskar is talking about. Click on the picture below to watch it.