d the plate. But remember that a black and white
negative is wanted with as little detail in the features as
possible. The best plate to use is a very slow one, or what is
called a process plate.

[Illustration: Sample Silhouette]

In developing get all possible density in the high lights, without
detail in the face, and without fog. Printing is best done on
contrasty development paper with developer not too strong.

The ideal silhouette print is a perfectly black profile on a white
ground. With a piece of black paper, any shape in stopping off
print may be made as shown at C in the sketch.

** How to Make a Galvanoscope [68]

A galvanoscope for detecting small currents of electricity can be
made from a coil of wire, A; a glass tube, B, full of water; a
core, C; and a base, D, with binding posts as shown. The core C,
which is made of iron and cork, is a trifle lighter than the water
it displaces and will therefore normally remain in the top of the
tube; but as soon as a current of electricity passes through the
coil, the core is drawn down out of sight. The current required is
very small, as the core is so nearly balanced that the least
attraction will cause it to sink.

The glass tube may be a test tube, as shown in Fig. 2, or an empty
developer tube. If one has neither a test tube nor developer tube,
an empty pill bottle may be used. The washers at the ends of the
coil can be made of fiber, hard rubber, or wood; or can be taken
from an old magnet. The base may be made of wood or any other
insulating material and should have four short legs on the bottom.
Make the coil of single-covered wire about No. 18 and connect ends
to binding posts as shown in Fig. 2.

The core is made by pushing a small nail through a piece of cork.
It should be made so that it will rise slowly when placed under
water. Some filing may be necessary to get the weight just right,
but it should be remembered that the buoyancy of the core can be
adjusted after the parts are assembled, by pressing the cork in
the bottom of the test tube. This causes compression in the water
so that some is forced into the upper cork, reducing its
displacement and causing it to sink. The lower cork is then slowly
withdrawn, by twisting, until the core slowly rises.

[Illustration: Galvanoscope]

The instrument will then be adjusted ready for use. Connect the
binding posts to a single cell of battery--any kind will do, as a
slight current will answer. O