sees the
pictures either in front of or on the back of the card on which
they are printed. In order to make them appear before the card,
the left eye sees through a blue screen, but the red picture which
is seen by it is a black one, and lies to the right on the
picture; and the right eye sees the lefthand picture. The further
apart the pictures are, the further from the card will the
composite image appear.

In the manufacture of a stereoscope the difficulty is in the
proper arrangement of the prisms; with the stereograph, in the
proper choice of colors.

** Mercury Make-and-Break Connections for Induction Coils [187]

Induction coils operating on low voltage have a make-and-break
connection called the "buzzer" to increase the secondary
discharge. Two types of make-and-break connection are used, the
common "buzzer" operated by the magnetism of the core in the coil
and the mercury break operated by a small motor. The sketch
herewith shows how to make the motor-operated break. Two blocks of
wood are nailed together in the shape of an L and a small motor
fastened to the top of the vertical piece. The shaft of the motor
is bent about 18 in. in the shape of a crank, so that in turning
it will describe a circle 1/4 in. in diameter. A small connecting
bar is cut from a piece of brass 1/8 in. thick, 1/4 in. wide and 1
in. long and a hole drilled in each end; one hole to fit the motor
shaft and the other to slip on a No. 12 gauge wire. Two L-shaped
pieces of brass are fastened to the side of the block and drilled
with holes of such a size that a No. 12 gauge wire will slip
through snugly. Place a NO.12 gauge wire in these holes and bend
the top end at right angles.

[Illustration: Motor-Driven Make-and-Break]

Put the connecting brass bar on the motor shaft with washers
fitted tight on each side and slip the other end over the bent end
of the wire. Have the wire plenty long so it can be cut to the
proper length when the parts are all in place. A small round
bottle about 1/2 in. in diameter is now fitted in a hole that has
been previously bored into the middle of the bottom block and
close up to the vertical piece. This should only be bored about
half way through the block. The wire is now cut so at the length
of the stroke the end will come to about one-half the depth, or
the middle of the bottle.

Fill the bottle with mercury to a point so that when the motor is
running, the end of the wire will be in the mer