I think you may very well be correct
about the glazed fronts. I was confused by that, but not knowing what glass
presses looked like, thought the glass presses were being refered to in that
excerpt. There are glass presses in that room. Here is the passage where they
are mentioned as being in the same room as the mirror: At the farther
end, a flight of stairs mounted to a door covered with red baize; and through
this, Mr Utterson was at last received into the doctor's cabinet. It was a large
room, fitted round with glass presses, furnished, among other things, with a
cheval-glass and a business table, and looking out upon the court by three dusty
windows barred with iron.
So I am not sure what kind of
presses are the ones reflected in the mirror, but the glass presses are
certainly in the same room. I tried to find out about glass presses, but
other than that they press molded glass into shapes, was not very
successful.
Interestingly, Stevenson does use the metaphor of glass's
crystalline and malleable nature in describing the Jekyll/Hyde
transformations: The powders were neatly enough made up, but not with
the nicety of the dispensing chemist; so that it was plain they were of Jekyll's
private manufacture; and when I opened one of the wrappers, I found what seemed
to me a simple crystalline salt of a white colour. ... He [Hyde] thanked me and
with a smiling nod, measured out a few minims of the red tincture and added one
of the powders [cfmirror-image red wop]. The mixture which was at
first of a reddish hue, began, in proportion as the crystals melted, to brighten
in colour, to effervesce audibly, and to throw off small fumes of vapour.
Suddenly, and at the same moment, the ebullition ceased, and the compound
changed to a dark purple, which faded again more slowly to a watery green. My
visitor, who had watched these metamorphoses with a keen eye, smiled, set down
the glass upon the table, and then turned and looked upon me with an air of
scrutiny.
Of course when Hyde drinks the potion he begins the
horrible transformation: the features seemed to melt and alter.... as
glass would if heated. The coincidence of the colors red and green
and the persistent reference to glass and crystals do of course have echoes
in Pale Fire.