ill soon happen if a person does
not get out of the way before the composition explodes.

_Money augmented by an Optical Illusion._

In a large drinking-glass of a conical shape, (small at the bottom and
wide at the top,) put a shilling, and let the glass be half full of
water; then place a plate on the top of it, and turn it quickly over,
that the water may not escape. You will see on the plate a piece of
coin of the size of half-a-crown; and a little higher up another the
size of a shilling.

It will add to the amusement this experiment affords, by giving the
glass to any one in company, (but who, of course, has not witnessed
your operations,) and, desiring him to throw away the water, but save
the pieces, he will not be a little surprised at finding only one.

_Three objects discernible only with both Eyes._

If you fix three pieces of paper against the wall of a room at equal
distances, at the height of your eye, placing yourself directly before
them, at a few yards' distance, and close your right eye, and look at
them with your left, you will see only two of them, suppose the first
and second; alter the position of your eye, and you will see the first
and third: alter your position a second time, you will see the second
and third, but never the whole three together; by which it appears,
that a person who has only one eye can never see three objects placed
in this position, nor all the parts of one object of the same extent,
without altering his situation.

_To construct the Camera Obscura._

Make a circular hole in the shutter of a window, from whence there is
a prospect of some distance; in this hole place a magnifying glass,
either double or single, whose focus is at the distance of five or six
feet; no light must enter the room but through this glass. At a
distance from it, equal to its focus, place a very white pasteboard,
(what is called a Bristol board, if you can procure one large enough,
will answer extremely well;) this board must be two feet and a half
long, and eighteen or twenty inches high, with a black border round
it: bend the length of it inward to the form of part of a circle,
whose diameter is equal to double the focal distance of the glass. Fix
it on a frame of the same figure, and put it on a moveable foot, that
it may be easily placed at that distance from the glass, where the
objects appear to the greatest perfection. When it is thus placed, all
the objects in front of the window