This is an utilisation of the last-mentioned trick.
If the higher glass is placed behind the other, or even with its sides
parallel with its own, the latter could be behind an impenetrable wall
to shelter the observer, without the landscape being out of the scope of
vision.
Another plan is to place the upper mirror without a window above the
house-door, to reflect the part of the street beneath upon a mirror
within on the sill.
A third is in miniature, by which an opera-glass has an opening at the
side of the barrel, by which one’s neighbours are scrutinized while, to
all appearance, one is intently examining the performers. Happily the
frequenters of the playhouse of the present day are less sensitive than
our forefathers, and no such deceptions to avoid showing impoliteness
are in request.