SCIOPTICON MANUAL.
133
motions there is an advancement of the wave, while the
individual particles only rise and fall without advancing.
The slips of glass, mentioned above, can be con¬
veniently prepared for drawing diagrams, by coating one
side with plain collodion (gun cotton dissolved in equal
parts of alcohol and ether) ; when dry this surface takes
India-ink admirably, and diagrams can be traced, or
pictures copied in a rough way, by laying the glass plate
so prepared over the picture to be copied and tracing
its outline with a pen filled with good India-ink.
I would strongly advise any one using your lantern to
procure some of the comic slides, such as you illustrate
in Class XV of your catalogue of slides, and they can see
how to make similar ones to be used in illustrations
of scientific subjects. Thus with the wreck of one of
these three glass slides, picked up at some opticians and
purchased for a few cents, I improvised a slide which
answered better to illustrate the process of carbon print¬
ing in photography than the process itself would have
done in a lecture-room. One figure changed with
another by means of sliding glass plates is very useful
in many kinds of experiments or illustrations of facts
And processes.
The tank figured in your manual, in Chapter VII, on
Chemical Experiments, contributed by Prof. Morton,
can be made to do service in a long line of experiments
with electricity, by a very simple device. Thus, to illus¬
trate the decomposition of water, cut a slip) of segar-box
wood, of a size that will lay on the bottom of the tank
loosely, attach to this bit of wood copper wires, which
will extend up to the end of the tank and will not quite
meet at the centre of the bit of wood; to upturned ends
at this place, solder little slips of platina foil, f inch long
by i inch wide, they must stand vertically face to faco,