w to dry. From
a piece of thin

[Illustration: Detail of Lensless Microscope]

transparent celluloid or mica, cut out a small disk, B, and fasten
to the end having the enlarged bore, by means of brads. On the
other end glue a piece of thin black cardboard, C, and at the
center, D, make a small hole with the point of a fine needle. It
is very important that the hole D should be very small, otherwise
the image will be blurred.

To use this microscope, place a small object on the transparent
disk, which may be moistened to make the object adhere, and look
through the hole D. It is necessary to have a strong light to get
good results and, as in all microscopes of any power, the object
should be of a transparent nature.

The principle on which this instrument works is illustrated in
Fig. 2. The apparent diameter of an object is inversely
proportional to its distance from the eye, i. e., if the distance
is reduced to one-half, the diameter will appear twice as large;
if the distance is reduced to one-third, the diameter will appear
three times as large, and so on. As the nearest distance at which
the average person can see an object clearly is about 6 in., it
follows that the diameter of an object 3/4 in. from the eye would
appear 8 times the normal size. The object would then be magnified
8 diameters, or 64 times. (The area would appear 64 times as
large.) But an object 3/4-in. from the eye appears so blurred that
none of the details are discernible, and it is for this reason
that the pin-hole is employed.

Viewed through this microscope, a fly's wing appears as large as a
person's hand, held at arm's length, and has the general
appearance shown in Fig. 3. The mother of vinegar examined in the
same way is seen to be swarming with a mass of wriggling little
worms, and may possibly cause the observer to abstain from all
salads forever after. An innocent-looking drop of water, in which
hay has been soaking for several days, reveals hundreds of little
infusoria, darting across the field in every direction. These and
hundreds of other interesting objects may be observed in this
little instrument, which costs little or nothing to make.

** How to Make a Telegraph Key and Sounder [76]

The sounder, Fig. 1, is made from an old electric-bell magnet, D,
fastened to a wooden base. The lever, A, can be made of brass and
the armature, C, is made of iron. The pivot, E, is made from a
wire nail and is soldered to A. It shoul