In the same way the right eye sees through the orange screen only a black picture on a red background; this black image consisting only of the blue portions of the picture.  Any other part of complementary colors than blue and orange, as for instance red and green, would serve the same purpose.

The principle on which the stereograph works may be demonstrated by a very simple experiment.  On white paper one makes a picture or mark with a red pencil.  Looking at this through a green glass it appears black on a green ground; looking at it through a red glass of exactly the same color as the picture, it, however, disappears fully.

Through the glass one will see only a regular surface of the color of the glass itself, and without any picture.  Through a red glass a green picture will appear black.

So with the stereograph; each eye sees a black picture representing one of the pictures given by the stereoscope; the only difference being that in the case of the stereograph the background for each eye is colored; while both eyes together see a white background.

In the pictures the red and the green lines and dots must not coincide; neither can they be very far apart in order to produce the desired result.  In order that the picture shall be “plastic,” which increases the sense of depth and shows the effect of distance in the picture, they must be a very trifle apart.  The arrangement of the two pictures can be so that one 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.