kind, should appear at a b, appears, on the contrary, at b c, the part
a b seeming to be entirely empty.
And if the bottle be inverted, and placed before the mirror, as in the
under part of the figure, its image will appear in its natural erect
position, but the water, which is in reality at b c, will appear at a b.
And if, while the bottle is inverted, it be uncorked, and the water
suffered to run gently out, it will appear, that while the part BC is
emptying, the part a b in the image is filling; and if, when the bottle
is partly empty, some drops of water fall from the bottom A, towards
BC, it seems in the image as if there were formed at the bottom of
the part a b bubbles of air arising from a to b, which is the part that
seems full.
The circumstances most remarkable in this experiment, are, first, not
only to see an object where it is not, but also where its image is not;
and, secondly, that of two objects, which are really in the same
place, as the surface of the bottle and the water it contains, the one
should be seen at one place, and the other at another; and also that
the bottle should be seen in the place of its image, and the water
where neither it nor its images are.
It is, however, to be noted, that if any coloured liquor be put into the
bottle instead of water, no such illusion will take place.
There is one phenomenon more of this kind, which ought not to be
omitted; for though it be common enough, it is also extremely
pleasing, and easy to be performed.
If you place yourself before a concave mirror, at a proper distance,
your figure will appear inverted; and if you stretch out your hand
towards the mirror, you will perceive another hand, which seems to
meet and join it, though imperceptible to the touch.
And if, instead of your hand, you make use of a drawn sword, and
present it in such a manner that its point may be directed towards
the focus of the rays reflected by the mirror, another sword will