A, the plain glass. B, card corresponding to that which the spectator
has been forced to draw. C, the front side of the second glass. D,
the other side, over which is pasted coloured paper.
]
_To prepare for performance_, fill the receptacle at the top part of the
frame with the sand dyed the same colour as the paper at the back, and
let it run down till it fills the space between the two panes of glass,
and consequently, conceals the card, and is itself unnoticeable, from
looking exactly like the paper.
After the handkerchief has covered the frame, and you take it into your
own hands, reverse it unseen, so that all the sand shall run down into
the receptacle.
On showing it now, the card will appear.
By turning the frame again so that the sand shall run out, and once more
hide the card, it becomes invisible, as at first. The trick can be
repeated at pleasure.